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Showing papers in "American Mineralogist in 1980"



Journal Article
TL;DR: In this paper, unit cells and crystal structures were determined on a single crystal of quartz at seven pressures from I atm to 61.4 kbar by fitting the P-V data to a Birch-Murnaghan equation of state.
Abstract: Unit cells and crystal structures were determined on a single crystal of quartz at seven pressures from I atm to 61.4 kbar. Unit-cell parameters are a:4'916(l) and c : 5.a054(4)A at I atm, and a: 4.7O22(3) and c : 5.2561Q)A at 61.4 kbar. Structural changes observed over this pressure range include a decrease in the Si-O-Si angle ftom 143.73(7)" to 134.2(l)"' a decrease in the average Si-O bond distance from 1.6092(7) to 1.605(l)A, and an increase in distortion ofthe silicate tetrahedron. Several O-O distances show very large changes (ll7a) that can be related to the unit-cell-edge compression. As pressure is increased, the geometry of the SiO, (quartz) structure approaches that of the low-pressure GeO, (quartz) structure. The structural changes that take place with increased temperature are not the inverses of those that occur with increased pressure; changes in the Si-O-Si angle and the tetrahedral tilt angle control thermal expansion, whereas smaller changes in the Si-O-Si angle and tetrahedral distortion control isothermal compression. By constraining the zero-pressure bulk modulus to be equal to that calculated from acoustic data [K, : 0.371(2) Mbar], the pressure derivative of the bulk modulus at Tnro pressure tKi : 6.2(l)l has been calculated by fitting the P-V data to a Birch-Murnaghan equation of state. The anomalously low value of Poisson's ratio in quartz can be explai-ned by the low ratio of the off-diagonal shear moduli to the pure-shear moduli. This small ratio reflects the easily expandin! or contracting spirals of tetrahedra that behave like coiled springs.

577 citations




Journal Article
TL;DR: In this paper, the authors used a Philips PW ll00 single-crystal di-ffractometer to collect X-ray diffraction data, collected on a single crystal di-fractometer.
Abstract: Whewellite, CaCrOo'HrO, and weddellite, CaCzO+' (2 + x) HrO where x = 0'5, occur in sediments, in plants, and in urinary stones' Their crystal structures have been refined to R : 0.033 and R : 0.032 respectively, using new sets of X-ray diffraction data, collected on a single-crystal di-ffractometer. Refined cell parameterc are'. Y2,/c, a:6.29O(l), D: l4'583(l), c: 10'116(l)A' B: 109.46(2)\", Z : 8 for whewellite; I4/m, a : 12.371(3), c :7'35't(2)A, Z : 8 for weddellite' During refinement of whewellite, three out of four H atoms could be located, and split positions, p\".tiutty occupied, for the two independent water molecules were found. In weddellite refinement, it was possible to locate all the H atoms, and a split position for the \"zeolitic\" water was found: a maximum water content of 2'5 HrO was confirmed' The comparison of the structures explains the relationships existing between some repeats of the two minerals and shows the differences between the Ca coordination polyhedra. A possible correlation between the structural features and the mechanism of formation of the two mineral species is suggested. The symmetry and planarity of the oxalate groups are discussed' Introduction Whewellite and weddellite are hydrated oxalates of calcium, respect ively CaCrOo' H'O and CaCrOo'(2+x)HrO (with x = 0.5), found naturally in plant tissues, in sediments as a mineral of organic origin, and in urinary stones. About 70 percent of human urinary stones contain whewellite and/or weddellite, either singly or mixed with other components, mostly phosphates, uric acids, or urates. The frequency with which the two minerals are found in association in natural sources suggested a re-examination and a comparison of their crystal structure as the first stage of an investigation on their genetic relationshiPs. Previous structure analyses were carried out by Cocco (1961) and Cocco and Sabelli (1962) for whewellite (two-dimensional photographic data, R : 0.14) and by Sterling (1965) for weddellite (three-dimensional photographic data, R : 0.13). Experimental Single crystals of whewellite and weddellite were obtained from urinary calculi. A Philips PW ll00 single-crystal di.ffractometer was used to collect the X-ray diffraction data. Table I presents the experimenial details. On the crystals of both species three standard reflections monitored at three-hour intervals showed less than 3.5 percent intensity variation of figures by Germain et al. (1971), S'u5ing ef a/' (1g62),and Johnson (1965) were employed' The scatiering curves for neutral atoms given by the International Tablesfor X-ray Crystallography (1974) were used. High-precision methods using an co scan of four intense reflections from each of several lattice rows provided accurate data for unit-cell parameter computations. Results of the structure refinements VVhewellite The orientation and the dimensions of the cell axes (Table l) are consistent with one of the two alternaiive orientations proposed by Arnott et al' (1965) 0np3-ffi4x / 80 / 0304-0327$o2.oo 32'.1 328 TAZZOLI AND DOMENEGHETTI: Table l Crystal and diffraction data WHEWELLITE AND WEDDELLITE from them). The occupancy of these positions W(10) and W(20) and, in turn, that of the main positions W(l) and W(2) were refined with other cycles of least squares, together with the coordinates and the anisotropic thermal factors of the non-hydrogen atoms. Fixed isotropic temperature factors were used for the atoms H(ll), H(21), H(22), W(10), and W(20). The final atomic parameters are shown in Tables 2 and 3; bond lengths and angles in Table 4. The observed and computed structure factors are compared in Table 5.' The final value of the discrepancy index was R : 0.033 for the 2864 observed reflections. lleddellite The values of the cell parameters (Table l) agree well with those of Sterling (1965). The atomic coordinates of this author, with the exclusion of those relative to oxygen of the \"zeolitic\" water, were used to start the refinement. During the first cycles, carried out using isotropic thermal factors, the examination of the difference Fourier map confirmed the presence of zeolitic water, with incomplete occupancy, in the channel running along the four-fold axis, with coordinates similar to those given by Sterling. In the last stages of refinement, difference Fourier maps allowed us to locate the hydrogen atoms of the two molecules of non-zeolitic water and to find a split po-, sition W(30) for the oxygen of the zeolitic water, the occupilncy of which was refined alternately with that of the main position W(3) (he distance between the two positions was 0.574). Fixed isotropic temperature factors were used for the atoms H(5), H(6), and W(30). The final atomic parameters are given in Tables 6 and 7, lengths and bond angles in Table 8. The observed and computed structure factors are compared in Table 9.'The final value of the discrepancy index was R : 0.032 for the 889 observed reflections. Discussion and comparison of the structures lMhewellite In whewellite (Fig. l) the coordination polyhedra of the pseudo-equivalent atoms Ca(l) and Ca(2) are distorted square antiprisms: in each of them seven of the oxygens belong to five oxalic groups and one to a water molecule. Each Ca polyhedron shares three edges with three adjacent Ca polyhedra. In this way I To receive a copy of Tables 5 and 9, order Document AM-?9116 from the Business Office, Mineralogical Society of America, 2000 Florida Avenue, NW, Washington, DC 20009. please remit $1.00 in advance for the microfiche. P n o p e r t y W h e w e l l i t e W e d d e l l i t e

183 citations


Journal Article
TL;DR: In this article, the early 1950s adoption of the X-ray powder diffractometer for clay studies helped to solve some of the problems in charactetization in nomenclature.
Abstract: Because of their small particle sizes and v~riable degrees of crystal perfection, it is not surprisi4g that clay minerals proved extremely difficult to characterize adequately prior to the development of ~odem analytical techniques. Problems in charactetization led quite naturally to problems in nomenclatute, undoubtedly more so than for the macroscopic~ more crystalline minerals. The popular adoption ~ the early 1950s of the X-ray powder diffractometer for clay studies helped to solve some of the probl

176 citations



Journal Article
TL;DR: In this article, high-resolution transmission electron microscopy of ferromagnesian chain and sheet silicates from a metamorphosed ultramafic body at Chester, Vermont has revealed a wide variety of structural defects.
Abstract: High-resolution transmission electron microscopy of ferromagnesian chain and sheet silicates from a metamorphosed ultramafic body at Chester, Vermont has revealed a wide variety of structural defects. Images of these microstructures have been interpreted by analogy with images of ordered chain silicates and with the aid of dynamical diflraction and imaging calculations. Most of the defects are concentrated in regions of chain-width disorder; they apparently result from retrograde reaction ofanthophyllite to talc and from deformation of the ultramafic body. The most common defects are the terminations of (010) slabs having a given chain width. These slabs, referred to as "zippers," in most cases terminate coherently, with no displacive planar defects associated with the termination. Two theoretically-derived rules must be obeyed for coherent termination. Rule l: the terminating zipper must have the same number of subchains as the material it replaces. Rule 2: the numbers of silicate chains in the zipper and in the material it replaces must both be even, or they must both be odd. Where these rules are disobeyed, zipper terminations are usually associated with planar faults having displacements projected on (001) of either % [010] or t/+ [fi)], referred to an anthophyllite cell. In rare cases, violation of the replacement rules results in severe structural distortion, rather than the creation of displacive planar faults. These replacement rules hold for all pyriboles and may [s s6ahqlling factors not only in reactions involving amFhib,ole, but in pyroxene hydration reactions as well. Other observed microstructures in the Chester chain silicates include cooperative terminations of many zippers; terminations of individual silicate chains; column defects parallel to the pyribole c axis; nanow lamellae of monoclinic pyribole; and extended displacive faults and rotational faults that apparently result from physical deformation. Two types of talc have 6"sa disfin8uished by their orientation relationships with the pyriboles. In "coherent talc," the relationship is D,.llDo' ar"llco, cnlla,,. Structural considerations and high-resolution images suggest that this type of talc possesses 20 stacking.In "(210)-talc" there are no rigid orientation relationships with the intergrown pyribole, but the sheets of the talc tend to be parallel or subparallel to (210) ofthe pyribole. This type oftalc displays extreme stacking disorder. Textural evidence suggests that the wide-chain silicates and fibrous talc at Chester formed by retrograde hydration reaction of anthophyllite. This conclusion is supported by the microstructures observed with electron microscopy. These microstructures suggest that there were several mechanisms for the reaction: (a) anthophyllite and wide-chain pyribole were replaced along grain boundaries and fractures by talc, and (b) anthophyllite was replaced by a far more complex mechanism involving the reaction sequence anthophyllite -r disordered chain silicate --+ chesterite ---) more disordered chain silicate --+ jimthomFsonite + talc or disordered chain silicate with very wide chains. The disordering steps of this sequence apparently occurred by the nucleation and growth of wide-chain zippers. When a region of crystal achieved a chemical composition close to that of one of the ordered phases, reordering occurred by the passage of en echelon termination faults through the structure in the a direction. Much of the chemical transport necessary for these reactions may have taken place by "ultrafast" difusion along large structural tunnels associated with zipper terminations.

135 citations


Journal Article
Abstract: The influence of TiO, on the structure of silicate melts has been determined at I atm pressure by laser Raman spectroscopy. The following melt compositions were used: CaSiOr, CaMgSirOu, NarSiOr, NaAlSi3Os, CaAl2OE, and NarTiOr. Titanium is in four-fold coordination in all melts studied, as there are no Raman bands below 700 cm-r that can be assigned to Tia* in six-fold coordination. In melts with nonbridging oxygen (NBo), solution of rio, results in a decrease in NBO/T (nonbridging oxyg€ns per tetiahedrally coordinated cation). In metasilicate melts, monomers, chains, and sheets of Sio* and Tio* occur as discrete units. The proportion of the Ti-rich structwal units increas€s relative to the Si-rich units with increasing TiO, content of the system. The overall NBO/T of the melt decreases with increasing TiO, content which results in an increase of chain and sheet units relative to monomers. Titanium substitutes for silicon in quenched Ab and An melts to form tlree-dimensional (3D) aluminotitatrate complexes. The aluminosilicate complexes (3D) in these melts become depleted in Al as a result of this solution mechanism of Ti. Discrete, tfuee-dimensional units of TiO, composition occur together with three-dimensional SiO, units in quenched melts in the system SiO2-TiO2. The position of liquidus boundaries between minerals of different degree of polymerization shifts toward the silica-deficient portions of the appropriate systems with the addition of Tior. This behavior is a result of the decrease in NBO/T in the melts as Tio2 is added. Crystal-liquid partition coefficients for a variety of transition metals and incompatible trace elements are likely to increase with increasing TiO, content of the melt as a result of the decrease of NBO/T of melts with increasing TiO, content. The viscosity of depolymerized silicate melts depends on NBO/T. Inasmuch as this ratio decreases with increasing TiOr, the viscosity of such melts probably increases with increasing titanium content. The viscosity of fully polymerized melts depends on the strength of Al-GSi and Si-O-Si bonds. By substitution of Ti for some of the Si the bonds probably become weaker. The viscosity of such melts will decrease, therefore, with increasing TiO2 content.

132 citations


Journal Article
TL;DR: In this article, it was shown that trace-element crystal-liquid partition coefficients involving highly polymerized melts will decrease with increasing water content because of the formation of the less polymerized chain units in the melt.
Abstract: Raman spectroscopy has been employed to determine the solubility mechanisms of HrO in silicate melts. In melts that have a threp-dimensional network structure (e.g., melts on the join NarO-AlrOr-SiOr), water reacts with bridging oxygens to form two OH groups per broken oxygen bond. At the same time some of the three-dimensional network is broken down to chain units, accompanied by the expulsion of Al3* from tetrahedral coordination. In melts that have nonbridging oxygen (NBO), water reacts with both nonbridging oxygen and network modifiers (e.9., Na*) to form Si-OH bonds and M(OH) or M(OH), complexes. The anhydrous portion of the network becomes more polymerized. The formation of chain units at the expense of three-dimensional network units in melts implies that the liquidus boundaries involving pyroxenes and silica minerals or feldspar minerals shift to higher silica contents. Liquidus fields of silica minerals or feldspar minerals are depressed relative to those of pyroxene minerals. This prediction is supported by published observations of phase relations in hydrous basalt and andesite systems. Similar logic can be used to explain the formation of partial melts of andesitic bulk composition from hydrous peridotite in the upper mantle. We propose that trace-element crystal-liquid partition coefficients involving highly polymerized melts will decrease with increasing water content because of the formation of the less polymerized chain units in the melt. Partition coefficients involving less polymerized melts (e.9., picrite and komatiite) may increase because the degree of polymerization of the melt is increased as a result of dissolved water.

129 citations



Journal Article
TL;DR: In this article, the anionic structure of the silicate network of the melts has been determined with laser Raman spectroscopy, and it was interpreted to suggest that quenched metasilicate melts on the join CaSiO3-MgSiO 3 predominantly consist of three structural unitsseparate Sioxtetrahedra, Siroland Sirofunits.
Abstract: Redox equilibria and coordination states of iron in silicate melts have been determined with Mrissbauer spectroscopy. The anionic structure of the silicate network of the melts has been determined with laser Raman spectroscopy. Mcissbauer spectroscopic data indicate that in all quenched melts in the system NazOFeO-FerOr-SiO, at I atm pressure, ferric iron is in tetrahedral coordination, probably as an NaFeOz complex. For example, melt of NaFe3*SirOu composition quenched from 1400'C has a three-dimensional network structure. In all melts, the Fe2* is octahedrally coordinated with oxygen. Both Fe3* and Fe2* are network modifiers in the system CaO-MgO-FeO-FerO:-SiOz, probably because CaFerOo and MgFe2Oo complexes are not stable in molten silicates. Only alkali metals can stabilize ferric iron in tetrahedral coordination. Quenched melts along joins such as NarSiOr-NaFeSirOu become progressively more polymerized as the acmite content of the system increases. Underl(Or) conditions more reducing than that of air, Fe2*/)Fe of the melts also increases with increasing acmite content (increasing degree of polymerization) of the melt. At fxed bulk composition iron-bearing melts of alkali silicates become depolymerized with decreasingl(O) because NaFeO2 complexes in the silicate network are transformed to Fe2*O:ooctahedra, whereby ferrous iron becomes a network modi-fier. The Raman spectra are interpreted to suggest hat the anionic structure of quenched metasilicate melts on the join CaSiO3-MgSiO3 predominantly consists of three structural unitsseparate Sioxtetrahedra, Siroland Sirofunits. The proportions of sioxand si'olunits increase relative to that of SirO?units as the Ca/(Ca + Mg) of the melt increases. In melts on this join with 5 mole 7o FeSiO3 component added, the Fez*/I,Fe decreases with increasing Ca/(Ca + Mg).This decrease is related to the increased degree of proportionation of the SirO:units to SiOland SirOlunits with increasineCa/(Ca + MC). Provided that sufficient alkali metal is available in rock-forming silicate melts for local charge balance of Fe3*, the ferric iron will be in the network. A progressive decrease of M* / Ivf, of a magma (as a result of fractional crystallization, for example) will result in ferric iron shifting from IV to VI coordination, thus changing the anionic structure of the melt. This change will also change some properties of the melt such as viscosity and density and the activitv coeftcients of trace elements.

Journal Article
TL;DR: The degree of alteration in clinopyroxenes from eclogitic inclusions in South African kimberlites is directly related to the deviation from stoichiometry of remnant unaltered areas in individual grains.
Abstract: The degree of alteration in clinopyroxenes from eclogitic inclusions in South African kimberlites is directly related to the deviation from stoichiometry of remnant unaltered areas in individual grains. Deviations from stoichiometry are reconciled by allowing up to 9% vacancy in the M2 site. The breakdown reaction accounting for the apparent alteration is: CaAl/sub 2/Si/sub 4/O/sub 12/..-->..CaAl/sub 2/SiO/sub 6/ + 3SiO/sub 2/. The products of this reaction are observed in X-ray precession photographs as a second Ca- Tschermak's pyroxene intergrown with the host omphacite plus quartz powder rings. I infer that a vacancy-containing pyroxene is stabilized by pressure but is highly unstable at lower pressures. A general FORTRAN program which breaks pyroxene compositions into end-members has been written to include a vacancy-containing end-member.


Journal Article
TL;DR: In this article, single-step and multistep undercooling experiments using both Fe,Mg-free and Fe,mgbearing model granitic compositions were conducted to investigate the influence of mafic components on the crystallization of granitic melts.
Abstract: Single-step and multistep undercooling experiments using both Fe,Mg-free and Fe,Mgbearing model granitic compositions were conducted to investigate the influence of mafic components on the crystallization of granitic melts. Crystallization of granite and granodiorite compositions in the system NaAlSi3O6-KAlSirOr-CaAlzSi2Os-SiOr-H2O produces assemblages containing one or more of the following phases: plagioclase, alkali feldspar, qloartz, silicate liquid, and vapor. The observed phase assemblages are generally in good agreement with equilibrium data reported in the literature on the same bulk compositions. With the addition of Fe and Mg to these bulk compositions six new phases participate in the equilibria (orthopyroxene, clinopyroxene, biotite, hornblende, epidote, and magnetite). However, crystalline assemblages produced in phase equilibrium and crystal grofih experiments brought to the same fnal P-Z-X6,. conditions are in general not equivalent. In crystal-growth experiments, nucleation of the feldspars and quartz is greatly inhibited in the presence of Fe and Mg. Indeed, plagioclase is the only tectosilicate to nucleate in the granodiorite composition. Mafic phases nucleate and grow outside of their thermal stability fields as defined by the equilibrium phase diagrams. This contrast in mineral assemblages between the equilibrium and crystal growth experiments is in marked contrast to the results obtained for Fe- and Mg-free compositions. Perhaps the addition of Fe and Mg has caused a breakdown of the Si-O framework in the melt, thereby promoting the more rapid nucleation of the inoand phyllosilicates rather than the framework silicates. Border zones of granitic plutons, com-only rich in mafic minerals, may result from the more rapid nucleation of mafic phases from the silicate liquid. These zones are thought to develop by early crystallization along the walls of the pluton. Our results suggest the mafic phases should nucleate more quickly than the feldspars and quartz and thus should enrich the early crystallization products in ferromagnesian minerals.

Journal Article
TL;DR: In the Tula Mountains and Amundsen Bay areas of Antarctica, the proportion of Al to total cations in orthopyroxene ranges from 0.072 to 0.117, increases linearly with FeSiO3 content, and is extrapolated to be 0.048 as discussed by the authors.
Abstract: Quartzites and granulites containing the sapphirine-quartz association are found in the pyroxene-granulite-facies Archean Napier complex in the Tula Mountains and Amundsen Bay areas of Enderby Land, Antarctica (50'30' to 53'E; 67\"5). Quartzites from the Tula Mountains consist of SiOr, AlrOr, MgO, and FeO and contain virtually no alkalis or CaO. At the peak of metamorphism, the mineral assemblage in these quartzites is interpreted to be quartz-sapphirine-orthopyroxene-rutile+plagioclase. Cordierite is present, mostly as overgrowths on sapphirine, and appears to have formed from the reaction between sapphirine and quartz after the peak of metamorphism. Sapphirines are somewhat more siliceous than the TMgO ' 9AlrO3 . 3SiO, composition and the proportion of the iron end member ranges from 6.8 to 23.5 mole Vo. Fe3+ contents do not exceed lSVo of the total Fe in sapphirine. The proportion of Al to total cations in orthopyroxene ranges from 0.072 to 0.117, increases linearly with FeSiO3 content, and is extrapolated to be 0.048 in a hypothetical iron-free orthopyroxene. On the basis of the compositions of coexisting pyroxenes, the Al content of hypothetical iron-free orthopyroxene associated with sapphirine, and the compositions of coexisting plagioclase and garnet, metamorphic temperatures and pressures are estimated to be 900'a30'C and 7+l kbar. Colorless sapphirine (2.40wtVo FeO) with overgrowths of sillimanite is associated with orthopyroxene, q\\aftt, biotite, and plagioclase in a magnesian quartzite (MgO : 4.24 wt%o; Fe2O, : O.5l wtVo) at Reference Peak, an exposuie south of the Tula Mountains-Amundsen Bay areas. This association may represent the univariant reaction sapphirine + quartz : sillimanite + orthopyroxene in the iron-free system and may have crystallized at lithostatic pressures greater than 7 kbar and temperatures near 90ooc.

Journal Article
TL;DR: In this paper, Stewart's value of charge *1.7 was shown to be 1.604, which is about 0.20 a less than the sum of the I formulated the electronegativity scale in 1932, single-bond radii of the two atoms.
Abstract: Donnay and Donnay (1978) have stated that an electron-density determination carried out on low-quartz by R. F. Stewart (personal communication), which assigns charges -0.5 to the oxygen atoms and +1.0 to the silicon atoms, shows the silicon-oxygen bond to be only 25 percent ionic, and hence that \"the 50/50 description ofthe past fifty years, which was based on the electronegativity difference between Si and O, is incorrect.\" This conclusion, however, ignores the evidence that each silicon-oxygen bond has about 55 percent double-bond character, the covalence of silicon being 6.2, with transfer of 2.2 valenc,e lectrons from oxygen to silicon. Stewart's value of charge *1.0 on silicon then leads to 52 percent ionic character of the bonds, in excellent agreement with the value 5l percent given by the electronegativity scale. Donnay and Donnay (1978) have recently referred each of the four surrounding oxygen atoms, this obto an \"absolute\" electron-density determination carservation leads to the conclusion that the amount of ried out on low-quartz by R. F. Stewart, and have ionic character in the silicon-oxygsa 5ingle bond is stated that \"[t shows the Si-O bond to be 75 percent 25 percent. It is, however, not justified to make this covalent and only 25 percent ionic; the 50/50 deassumption. scription of the past fifty years, which was based on In the first edition of my book (1939) I pointed out the electronegativity difference between Si and O, is that the observed Si-O bond length in silicates, given incorrect.\" as 1.604, is about 0.20A less than the sum of the I formulated the electronegativity scale in 1932, single-bond radii of the two atoms (p. 222), and I and in 1939, in the first edition of my book The Nawrote a valence-bond structure representing all four ture of the Chemical Bond,p.74, I wrote that \"The atoms as attached to the central atom by double Si-O bond is of especial interest because of its imporbonds G,.224). Some years later (1948) I formulated tance in the silicates. It is seen to have 50 percent the electroneutrality rule. This rule is based on the ionic character, the value of xo x\", being 1.7.\" Here argument that a large amount of energy is needed to x is the electronegativity, 1.8 for silicon and 3.5 for remove a second electron from a singly-charged catoxygen. The value 50 percent comes from a curve (p. ion, and a singly-charged anion has zero or negative 70) relating the amount of ionic character of a bond electron affinity. It states that in most stable moleto the difference in electronegativity of the atoms, the cules and crystals each atom is close to being electriprincipal experimental basis for the curve being the cally neutral, with its residual charge lying within the observed values of the electric dipole moment of limits+land-1. Ifweaccepttheassignmentof 50 molecules. The curve in fact gives 5l percent, which percent partial ionic character to the Si-O bonds, the does not differ significantly from 50 percent. electroneutrality principle rules out a structure for Stewart (private communication) has shown by a tetrahedral silicate in which there are single bonds careful determination of the electron density in lowbetween the silicon atom and each of the four surq:uatlcz that a reasonable assignment of electrons to rounding oxygen atoms, because this structure places silicon and oxygen leads to the resultant charge +1.0 a charge *2 on the silicon atom. In the third edition on each silicon atom and -0.5 on each oxygen atom. of my book (1960) a more thorough discussion of the If we assume that there is a covalent bond with parstructure of tetrahedral silicates is given, beginning tial ionic character between each silicon atom and on p. 320 and based upon an equation (p. 241) for 0003-004x/80/0304-032 l$ 2.00 321 322 PAULING: SILICON-OXYGEN BONDS bond length of bonds in which a single bond is in resonance with multi-bonded structures involving both the p, and p\" orbitals. The observed Si-O bond length was taken as l.614, in agreement with the value 1.610(9),{ given by Baur (1978) as the average for 64 compounds in which the sihcate group shares oxygen atoms with other tetrahedral Al, B, Ga, P, or Si atoms. The result of this consideration of the observed bond length is that the bond number for each silicon-oxygen interaction is placed at 1.55, the covalence of silicon being 6.2, with transfe r of 2.2 valence electrons from the oxygen atoms to each silicon atom. Stewart's value +1.0 for the resultant charge on the silicon atom then leads to 51.6 percent ionic character of the bonds lQ + 2.2)/6.21, in excellent agreement with the value 5l percent expected from the electronegativity difference. For the oxygen atom, as the resonance occurs between single and double bonds, the bond orbitals for the structures with two single bonds formed by the oxygen atom, and a single bond and a double bond have mostly p character, whereas those for the structure with two double bonds have sp3 character. For the silicon atom the bonds may be considered to be essentially of the sp'& type, permitting each silicon atom to form two single bonds and two double bonds, which resonate among the four positions determined by the four surrounding oxygen atoms. Support for this description of the bonds is given by a calculation of the value of the Si-O-Si bond angle. The angles corresponding to the greatest stability for the structures with two single bonds formed by the oxygen atom, a single bond and a double bond, a double bond and a single bond, and two double bonds are 108\", 114\",114\", and 180o, respectively. If we allow the single and double bonds of the two adjacent silicon atom to resonate independently into the two positions and accept 55 percent of double-bond in each position, the four structures for the oxygen atom contribute 0.2025, 0.2475, 0.2475, and 0.3025, respectively. With the assumption of Hooke's law for the dependence of the strain energy on the deviation of the angle fron the most stable values for the individual structures. and with the Hooke's-law constants in the ratios l:2:3, the equilibrium angle is calculated to be 141.9\", in good agreement with the observed value, 143.68' (Le Page and Donnay,1976). The calculated value is not very sensitive to the assumptions; thus for constants in the ratios I :2:4 it is 146.6\". The bond angle Si-O-X depends upon the values of the bond number n for both Si-O and O-X. the value l4l.9o being that for n: 1.55 for each. When n is I for O-X, as in Si(OCHr)o, the calculated value of the bond angle is I11.3\", in good agreement with the experimental value, ll3!2\" (Pauling, 1960, p. 322). The deviation of the bond number 1.55 from the val,rre 3/2 may not be significant. Accordingly a simple approximate description of binding in lowquartz and similar silicate crystals may be given. Each of the two oxygen atoms donates an electron to a silicon atom, permitting it to form six covalent bonds (two double bonds and two single bonds) with its four oxygen neighbors. These bonds have about 50 percent ionic character, leading to acceptable values of the resultant charges, -0.5 for oxygen and

Journal Article
TL;DR: In this article, at least squares refinements of the structure of low albite from Amelia, Virginia with threedimensional neutron and X-ray diffraction intensity data sets yield weighted R factors of 0.024 and 0.035, respectively, with anisotropic thermal models.
Abstract: Least-squares refinements of the structure of low albite from Amelia, Virginia with threedimensional neutron and X-ray diffraction intensity data sets yield weighted R factors of 0.024 and 0.035, respectively, with anisotropic thermal models. The two methods result in essentially identical positional parameters, though slight differences in thermal parameters may be due to the variation in relative scattering powers. Direct refinement of Allsi occupancy of tetrahedral sites was possible for the neutron data but not for the X-ray data, resulting in the following values of (T:O) Gn A) and Al occupancy (associated errors in parentheses): TrO r.743Q)L,0.97(2) Al; T,m 1.609(l)A, O.U(z) Al; T,O 1.614(l)A, 0.0 Al; T,m 1.616(l)A, 0.0 Al. The diference between the refned T,O occupancy and an Al-filled T,O site is signi-ficant at a 9Wo confidence. When these data are combined with other neutron di-ffraction estimates of order/disorder for a low sanidine and Himalaya orthoclase, a non-linear plot of (T-O) vs. Al content results, which is best fit with a 3rd-order polynomial. This curve differs from the linear plot by as much as 0.016A in the high-Al region. However, direct comparison of average T-O bond lengths between ordered and disordered tetrahedra includes an apparent shortening in disordered feldspars (comparable to a thermal motion etrect) which may amount to 0.003A. Analysis of the apparent-thermal-motion anisotropy of the Na site with displacive split-site models yields a splitting distance of 0.39A for both data sets, but a singleNa, anisotropic thermal model is preferred.

Journal Article
TL;DR: In this article, carbon dioxide solubility in melts of CaMgSi2Ou and NaCaAlSirO, composition at high pressure and temperature have been combined with Raman spectroscopic measurements of quenched melts to determine the structural role of CO, in silicate melts.
Abstract: Determinations of carbon dioxide solubility in melts of CaMgSirOu and NaCaAlSirO, composition at high pressure and temperature have been combined with Raman spectroscopic measurements of quenched melts to determine the structural role of CO, in silicate melts. Carbon dioxide solubility increases with increasing temperature under isobaric conditions between 1450o and 1700'C and decreases as a function of increasing temperature below 1450" and above l700oC. We suggest that these contrasting temperature dependences are a result of similar contrasting behavior of the fugacity of COr. Carbon dioxide dissolves in silicate melts predominantly as CO3-. This anion is most likely closely associated with one or more network-modifying cations. In the present melts we suggest that (CaCO3)o complexes are formed. The CO, contents of the quenched melts are systematically correlated with structural changes of the network of the melts. The anionic structure of alkaline earth metasilicate melts is predominantly a combination of structural units that, on the average, have 4, 2, and I nonbridging oxygens per silicon (NBO/Si). These structures are referred to as monomers, chains, and sheets. The formation of carbonate is associated with a lowering of the proportion of monomers in the melt and an increase of the proportions of chain and sheet units. The principles of CO, solubility mechanisms derived from melt of CaMgSi2Ou composition also apply to melt of NaCaAlSirO, composition. The solubility mechanisms in the two types of melts differ in detail, however, as the latter melt contains a large proportion of aluminum in tetrahedral coordination, which combined with the silicon results in a higher degree of polymerization of the melt. The NaCaAlSirO, melt consists of structural units that, on the average have 2, l, and,0 nonbridging oxygens per tetrahedrally coordinated cation (NBO/T). These units are referred to as chains. sheets. and three-dimensional network structures. respectively. When CO, is dissolved in such melt, the three-dimensional network units become more dominant at the expense of chain units. We suggest that the NaCaAlSirO, melt composition is a better model for natural basalt magma than CaMgSirOu melt. On the basis of our observations, we conclude, however, that the predictions of CO2 solubility behavior in natural basalt melt based on studies with binary silicate systems provide an accurate basis for understanding the role of CO, in silicate melts.

Journal Article
TL;DR: The thermal-compositional dependence of Fe2*-Mg distributions between coexisting gar-net and pyroxene in diverse metamorphic lithologies from two small areas in the Ruby Range, southwestern Montana, has been studied by electron microprobe and multiple linear regression techniques as mentioned in this paper.
Abstract: The thermal-compositional dependence of Fe2*-Mg distributions between coexisting gar- net and pyroxene in diverse metamorphic lithologies from two small areas in the Ruby Range, southwestern Montana, has been studied by electron microprobe and multiple linear regression techniques. For coexisting garnet-clinopyroxene, this dependence, which is in- ferred from indepindent metamorphic P-Testimates for two areas and thirteen mineral-pair analyses, is surnrnarized by the equation Rf h KD : Q482!845) + (1509i1392)(X."-X'Jo"


Journal Article
TL;DR: The Au Sable biotite is a 2M, mica with space group symmetry C2/c as discussed by the authors, which is the same as the 2M-mica of Takeda and Ross.
Abstract: The crystal structure of a metamorphic biotite from Au Sable Forks, New York (northeast Adirondacks) has been refined to an unweighted R value of 4.2Vo. The Au Sable biotite is a 2M, mica with space group symmetry C2/c. Although the biotite equilibrated near and cooled slowly from Z: 7fi)ot5OoC, octahedral and tetrahedral cations are apparently disordered. There are few, if any, octahedral and interlayer cation site deficiencies. Average bond lengths are Ml-O 2.106, M2-O 2.086, Tl-O 1.657,T2-O 1.661, inner K-O 3.046, ourer K-O 3.289A. The tetrahedral rotation angle (a) is 5.3o. The Au Sable biotite structure is nearly identical to theigreous 2M, biotite of similar composition refned by Takeda and Ross (le7s). The Au Sable biotite coexists with magnetite (usp ,r), ilmenite (Hmr), perthite (orur), and plagioclase (Anrr). This assemblage fxes f 000"C), Io,(10-t12y, and /;r,o (lG-103) for metamorphic pressure of 7tl kbar. Metamorphism of orthogneisses in the Au Sable area of the Adirondacks took place under quite dry (p""o ( p*ua) conditions.


Journal Article
TL;DR: In this article, the authors used a combination of imprecise laboratory calibration of mineral equilibria and to conditions of p6,6 (P,o-r) during metamorphism.
Abstract: Pelitic schists from a Buchan-type metamorphic terrain contain eight lndependent geothermometer/geobarometers. Three pairs of fluid-independent geothermometer,/geobarometers record lithostatic pressures consistent with 3500i200 bars. Fluid-independent geothermometers are mutually consistent and record a N-S temperature gradient in the area ranging from a mean of 460"C in the garnet zone to a mean of 550"C in the sillimanite zone. Fluid-dependent geothermometers record temperatures that are systematically higher than those recorded by fluid-independent geothermometers. The discrepancy is due to a combination of imprecise laboratory calibration of mineral equilibria and to conditions of p6,6 ( P,o-r during metamorphism. The mole fraction of HrO in the metamorphic fluid was variable, and generally ranged between values of -0.5 and l. Temperature and pressure estimates based on mineral equilibria in pelitic schists are in good agreement with temperature and pfessure estimates based on mineral equilibria in metamorphosed carbonate rocks from the same area.

Journal Article
TL;DR: In this article, the authors report results of X-ray and neutron structure refnements of four labradorites An 62-An 66 with di-fferent geological histories and different superstructures.
Abstract: The paper reports results of X-ray and neutron structure refnements of four labradorites An 62-An 66 with di-fferent geological histories and di-fferent superstructures. A rapidly quenched crystal collected during the eruption of Surtsey (Iceland) is essentially disordered. Labradorite from massive lava flows of Lake County (Oregon) shows D reflections, and two metamorphic labradorites from the Central Alps display an intermediate plagioclase structure with e reflections. The average structure (c : 7 A) of all four samples converging at R factors of 3-6Vo are virtually identical with well-defined Si/Al and smeared-out oxygen positions. We interpret this as evidence that the di-fferent superstructures consist of periodic stackings of similar basic units probably lssepfling those in 1l anorthite but with diflerent stacking orders. The main intensity contribution to superstructure reflections comes from displacements of oxygens in the superstructures, probably caused by Al-Si ordering. Phase relations are illustrated in a hypothetical TTT diagram in which we summarize results from structure refinements, microstructural observations and changes during experimental heating. At high temperature Qittle undercoqling below the critical temperature) ordering takes place by a nucleation and growth mechanism which produced 1l domains separated by curved APB's (antiphase boundaries) (Lake County). At larger undercooling continuous ordering prevails, giving rise to periodic APB's and satellites in the diffraction pattern.


Journal Article
TL;DR: The S shape of the molar volume curve for binary silicate solid solutions is Sshaped, rather than linear or near-linear as discussed by the authors, and the region of negative deviation from linearity is always close to the small-volume end-member, with positive departures near the large volume end.
Abstract: The characteristic form of the molar volume curve for binary silicate solid solutions is Sshaped, rather than linear or near-linear. The region of negative deviation from linearity is always close to the small-volume end-member, with positive departures near the large-volume end. A Non-Equivalent Site (NS) type of volume behavior is a broad sigmoid that arises from the presence of some crystallographic sites in a structure larger than others capable of accepting the same large substituting cation(s). Initial substitution of the large-volume component into the smaller-volume end-member has only a small efect on the unit-cell volume until the large "easy" sites are saturated, which occurs at a rational mole fraction. Then a region of positive excess volumes follows more or less abruptly. Examples are (Na,K) nephelines and (Mg,Fe'?*) amphiboles. An Equivalent Sr'te (ES) volume behavior occurs in systems where, because of nearly identical sites accepting a certain substituting cation, there are no preferred "easy" sites. The fust small substitution of the larger-volume component produces only local deformations without expansion of the structure as a whole, giving rise to a sharply-curving region of negative excess volume, usually within ten mole percent of the small-volume end-member. Examples are the (Fe'?*,Ca) and (Mg,Ca) garnets, and, probably, the alkali feldspars and olivines. The volume sigmoid is less common among non-silicates. It is shown by some oxide systems, e.g. (Mg,Fe'z*) ilmenites (ES) and magnetite-ulv

Journal Article
TL;DR: In this paper, phase relations of peridotite + CO 2 have been determined at 15 and 30 kbar using rtiW to monitor the degree of melting, and it was shown that the principal phase relations applied to simple systems such as CaOMgO-SiOr-CO, apply to melting of natural peridotsite.
Abstract: Phase relations of peridotite * CO, have been determined at 15 and 30 kbar using rtiW to monitor the degree of melting. At 15 kbar the initial 5 percent melt (-1400'C), where the melting (to -20 percent melt) is nearly invariant. In this latter melting interval, the melt coexisting with olivine, orthopyroxene, clinopyroxene, and vapor is tholeiitic. Melting coillmences more than 200oC lower at 30 kbar than at 15 kbar for both CO2-saturated and COr-undersaturated conditions. Again, the exact nature of the melting interval from the solidus to about 5 percent melting is uncertain because of the presence of small amounts of HrO in the experimental charges. Dolomite is a solidus phase together with olivine, orthopyroxene, and garnet, and the initial melt is carbonatitic, as evidenced by quenching of the melt to carbonate. With increasing degree of partial melting the melt compositions become richer in silicate components, as evidenced by quenching of the melt to mixtures of silicate glass and carbonate minerals. These results indicate that the principal phase relations i1 simple systems such as CaOMgO-SiOr-CO, apply to melting of natural peridotite. Published experimental results in this system show a great depression ofthe solidus temperature (-150"C) as the pressure is raised above that of the carbonation reaction. Di+Fo+VCEn*Dol At the same time, the liquid composition changes from haplobasalt to haplocarbonatite and therefore serves as a suitable model for liquids produced in the system peridotite + CO2.

Journal Article
TL;DR: In this paper, the authors showed that the size, distribution, and morphology of the synthetic fluid inclusions show a remarkable similarity to fluid-inclusions in quartz from metamorphic terranes, suggesting a preferential solution and precipitation phenomenon where the inclusions developed.
Abstract: Planes of water-rich fluid inclusions were created in natural quartz by partially annealing laboratory-induced fractures in a hydrothermal apparatus at 6(X)"C and 2 kbar in the presence of water. The size, distribution, and morphology of the synthetic fluid inclusions show a remarkable similarity to fluid inclusions in quartz from metamorphic terranes. Fluid inclusion trains appear to be localized along former fracture-surface step litreations; their size varies proportionally and abundance inversely with the spacing of step lineations on the original fracture surface. Thin island-like inclusions grade to elongate ellipsoids with minimum di.mensions considerably greater than the original crack width and appear to have developed at the expense of the immediate crack walls. The surface of the unclosed portion of the crack appears unchanged, suggesting a preferential solution and precipitation phenomenon where the inclusions developed. Fluid density of the inclusions, determined by microthermometric measurement of the homogenization temperature, is in agreement with the fluid density under the experimental 4a1s4ling conditions. These observations suggest that the total density of the trapped fluid in the inclusions is the same as that of the homogeneous fluid phase present at the time of fluid inclusion formation.

Journal Article
TL;DR: In this paper, the distortion index of cordierites was investigated and it was shown that the cell-edge variations which cause variations in the distortion A more likely represent di-fferences in composition than in AllSi ordering.
Abstract: For natural cordierites (except for indialites), the cell-edge variations which cause variations in the distortion index A more likely represent di-fferences in composition than in AllSi ordering. Back-reflection Weissenberg data from ll cordierite grains representing atomic ratios (Fe + Mn)/(Fe * Mn * Mg) from 0.(x6 to 0.898 reveal that, with increase in R" (the average radius ofthe ions occupying the octahedral sites), cell edges a and b increase linearly whereas c decreases until R" exceeds ca.0.765, then apparently increases. Statistical analyses on data for unheated grains reveal A to decrease markedly as x (mole fraction Na) increases, iacrease moderately as Ro increases, but seem relatively independent of Hro, probably because the strong correlation between Na and HrO content in unheated cordierites partially masked the efect of HrO on A. In contrast, A increased signifcantly when four cordierites were heated at 800'C with HrO being driven off. Inclusion of these data in the regression analysis revealed HrO content, as suggested by Stout (1975), to have a signifcant effect on A. For a natural Na,Be cordierite, Schreyer (1979) confirmed this by changing its A value from 0.12" to 0.26" by dehydration, then returning it to 0.11" by rehydration. The optical data, remeasured for l0 grains after their partial dehydration at 800"C for six hours in a slowly flushing H, atmosphere, showed reduction of all refractive indices and yielded remarkably regular curves relating the indices a, B, ard y to (Fe + Mn)/(Fe * Mn * Mg) ratio. Exceptions were two specimens high in Na and Be whose indices, even after heating, exceed the general trends. As for A, heating increased 2v* (x :666 nm) except for the two optically (+) grains which show slight decreases. Prior to heating, 2V^cotelates inversely with HrO whereas, after partial dehydration at 800oC, 2V^ correlates inversely with Na content and, less certainly, with R..