Showing papers in "American Mineralogist in 1979"

Magmas and volatile components

Abstract: The trivial quantities of CO_2 and H_2O (or reduced combination of C-H-O-S) in the upper mantle have little effect on the abundant basalts, but H_2O influences magmas generated in peridotite overlying subducted, hydrated oceanic crust, and CO_2 causes the generation of alkalic subsilicic magmas from peridotite beneath continental shields. PT sections through the (partly schematic) phase diagram for peridotite-CO_2-H_2O with low (CO_2 + H_2O) and selected CO_2/H_2O, compared with isotherms, illustrate the petrological structure of the upper mantle for different tectonic environments. If vapor is present, the cooler the geotherm, the higher is H_2O/CO_2: only in regions of upwelling can CO_2-rich vapor exist. The solidus surface for peridotite-CO_2-H_2O (P, T, X^V) has been mapped with tentative boundaries marking changes in normative compositions of near-solidus magmas. The restricted area for quartz-normative magmas suggests that if these are to be generated from mantle in subduction zones, there must be active asthenospheric convection carrying hot mantle to shallow levels above dehydration fronts in subducted oceanic crust. Changes in the relative positions of dehydration fronts and solidus boundaries for warm and cool subduction models are illustrated in new diagrams, unencumbered by the need for precise temperatures and depths. Tests for processes require knowledge of the phase relationships in the system basalt-andesite-dacite-rhyolite-H_2O from magma sources at depth to the surface. Data from scattered sources have been synthesized in a PTX(SiO_2)X(H_2O) framework to 35 kbar, and illustrated in sections and projections, including liquidus surfaces for the rock series to 35 kbar, dry, with 5 percent H_2O, and with excess H_2O (saturated). The volatile components become prominent in residual magmas: pegmatites from granitic magmas are enriched in H_2O, and carbonatites from alkalic magmas are extraordinarily enriched in CO_2. The behavior of another volatile component is illustrated by experimental data in NaAlSi_3O_8-H_2O-HF at 2.75 kbar. The vapor-saturated liquidus field boundary extends from 808°C-8.5 percent H_2O to 777°C-(9.2 percent H_2O + 1.0 percent HF), and three-phase boundaries demonstrat1i strong partition of HF into liquid compared with vapor. Details remain uncertain for the sources of volatile components, in mantle reservoirs or recycled through subduction, and of oxygen fugacity and temperature variation with depth in different tectonic environments.

The tetravalent manganese oxides: identification, hydration, and structural relationships by infrared spectroscopy

Abstract: A compilation of the infrared powder absorption spectra of most naturally occurring tetravalent and trivalent manganese oxides is presented which is intended to serve as a basis for the spectroscopic identification of these minerals in both ordered and disordered varieties, including those too disordered for X-ray diffraction studies. A variety of synthetic manganese oxides are also included for comparison to the natural phases. The samples include: aurorite, birnessite, braunite, buserite, chalcophanite, coronadite, cryptomelane, groutite, hausmannite, hollandite, lithiophorite, maoganite, manganese(Ill) manganate(IV), manganosite, manjiroite, marokite, nsutite, partridgeite, pyrolusite, quenselite, rancieite, ramsdellite, romanechite, sodium manganese(Il,III) manganate(IV), todorokite, and woodruffite. The spectra indicate that well-ordered water occurs in ramsdellite, chalcophanite, and most romanechites. Disordered water is observed in the spectra of nsutite, hollandites, birnessite, todorokite, buserite, and rancieite. The infrared spectra of well-ordered todorokite, birnessite and rancieite differ which indicates that they possess different structures and should be regarded as distinct mineral species. Much variation is observed in the spectra of pyrolusites, nsutites, birnessites, and todorokites which is interpreted as arising from structural disorder. Spectral trends suggest that todorokite, birnessite and rancieite have layered structures.

A crystal-chemical study of Mg,Al and Ni,Al hydroxy-perchlorates and hydroxy-carbonates

Abstract: Synthetic products of the form M,_,A1.(OH)rR;|" . fHrO, with M : Mg,Ni and R : COr and/or ClOo have been prepared, with x ranging from about 0.33 to 0.20, by precipitation from mixed perchlorate solutions by addition of NaOH; the carbonate ion is present owing to solution of atmospheric CO2. The two kinds of anions show a strong tend€ncy to segregate into separate crystals or, under particular conditions, into separate layers within the same crystals, forming an alternating sequenoe of carbonate and perchlorate layers. Pure carbonate and pure perchlorate phases were obtained by anion exchange reactions. The parameter C, layer thickness, diminished with increase in Al3* ions due to increase in electrostatic attraction between positive layers and negative interlayers. The parameter a for a hexagonal cell also diminished with increase of Al3* ions because of their smaller size. The observed change Aa/L,x agrees semi-quantitatively with calculated values. Reasons are suggested for the range of Al3* ions which can substitute in mixed hydroxide phases and for the commsaly observed value of x = 0.25 in natural minerals, based on possible supercell arrangements of the Al3* and M2* ions.

Geothermometry, geobarometry, and fluid compositions of metamorphosed calc-silicates and pelites, Mica Creek, British Columbia

Abstract: Temperature, pressure, and fluid compositions that prevailed during metamorphism of pelites and calc-silicates from staurolite and kyanite zone rocks of the Mica Creek area, British Columbia, have been estimated from an electron microprobe study of mineral equilibria. Temperatures estimated from Fe-Mg distribution between garnet and biotite range from 570o to 640oC and are generally consistent with temperatures estimated from coexisting calcite and dolomite in nearby rocks. Pressure estimates based on garnet-plagioclase-kyanitequartz equilibria range systematically from 6 to 8.5 kbar. These estimates are consistent with the geological cross section, which suggests an increase in the depth of structural level exposed from south to north in the study area. Fluid compositions estimated from paragonite-quartzalbite-kyanite equilibria from the metapelites are dominated by HzO, and fluid compositions estimated for calc-silicates have XCO2 > XH2O. Calc-silicates formed at carbonate-pelite contacts appear to have equilibrated at temperatures near 610oC and XCO"near 0.25.

Mineralogy of manganese dendrites and coatings

Abstract: Infrared spectroscopic study of desert varnish, manganese dendrites, river deposits, and other manganese oxide concentrations of the terrestrial weathering environment has shown that the morphological distinctions among them have a sound basis in differences in their mineralogy. The manganese oxide in manganese dendrites collected in surface exposures is either romanechite or a hollandite-group mineral. These are mixed with varying amounts of silicate minerals, which are a passive substrate for the oxide deposition. Dendrites collected in underground mine workings are todorokite. Manganese stream deposits are generally birnessite with minor amounts of silicate minerals; one nsutite stream deposit has been identified. Crack deposit mineralogy resembles that of manganese dendrites. Cave and subglacial deposit mineralogy resembles that of manganese stream deposits. Although dendrites have long been considered to be pyrolusite, no example of pyrolusite mineralogy has been identified.

Entropy of mixing in sanidine

Abstract: The entropy of mixing in high-temperature alkali feldspars has been re-evaluated using recently-published heat-of-solution data in conjunction with the results of earlier equilibrium experiments. We find that the excess entropy of mixing is distinctly positive over most of the K-Na composition range and most strongly so for the more potassic compositions. The excess entropies at the sadie end of the series are much smaller and may even be slightly negative for compositions near the N a end. The asymmetry in the excess entropy appears to be the principal reason for the asymmetry of the alkali feldspar solvus. Short-range K-Na ordering can only cause a negative excess entropy, hence is either negligible or overwhelmed by other effects of opposite sign. A likely explanation is that there is a positive excess (probably best seen at very low temperatures) in the vibrational heat capacity, and that this effect is greatest for the more K-rich compositions. This asymmetry is what would be expected if it is assumed that a small atom on a large-atom site would have greater vibrational freedom than a large atom on a small-atom site.

High-temperature crystal chemistry of hydrous Mg- and Fe-cordierites

Abstract: Structural refinements have been completed for a Mg-rich cordierite using data recorded at 24",375o,775o and 24"C (after heating to 775") and for an Fe-rich cordierite at 24" and 375'C. The mean T-O bond lengths in both cordierites remain unchanged but the mean octahedral bonds (M-O) lengthen upon heating. The unusually low thermal expansion of the Mg-cordierite is the result of its relatively "rigid" tetrahedral framework and the anisotropic expansion ofoctahedra isolated from each other. This anisotropic expansion leads to a slight rotation of the six-membered rings, a concomitant collapse of the structure parallel to c, and an expansion parallel to a and 6. In the Fe-cordierite, the octahedron is more flattened, resulting in c being smaller and a and b being larger than the cell dimensions of the Mgcordierite. Upon heating Fe-cordierite, there is no evidence for a rotation of the rings, and a, b, and c increase as the M-O bonds expand. X-ray Ap maps calculated for the Mg-cordierite showed approximate positions and relative amounts ofchannel constituents. The peak ascribed to the alkali and other atoms that centers the six-membered rings becomes elongated parallel to c upon heating through 375.c. However, the peak ascribed to the oxygen associated with HrO in the 24' and 375' maps is absent in the 775'C maps. It reappears in maps computed from the 24" (after heating) data. In both cordierites, small amounts of hematite were produced during heating (prematurely halting data collection on the Fe-cordierite), and apparently formed by combination of octahedral and channel iron with oxygen from the channel water molecules. A re-examination of the water-orientation the channels o[ the Mg-cordierite using neutron and X-ray Ap maps does not clearly show either type I [H-o-H in the (100) plane with the H-H vector parallel to cl or type II [H-o-H in the (100) plane with the H-H vector parallel to b] water, as previously suggested by spectroscopic studies. Instead, our ap maps indicate that the water molecule lies in a plane tilted -29o from (100) and that the H-H vector is tilted -l9o from c.

Zeolite facies metamorphism of basaltic rocks from the East Taiwan Ophiolite

Abstract: The basaltic rocks of the fragmented East Taiwan Ophiolite have been subjected to \"oceanfloor\" zeolite facies metamorphism. Depending on the bulk composition and mode of occurrence, various mineral assemblages occur: thomsonite + analcime * chabazite; pumpellyite * chlorite * laumontite in veins of the pillow cores; and pumpellyite * chlorite * K-feldspar; pumpellyite * laumontite * thomsonite; and prehnite (about 5 weight percent FeOr) * hematite in veins of the pillow matrices. Plagioclase phenocrysts were replaced by albite * pumpellyite * Ca-zeolites, pumpellyite * K-feldspar, analcime * chabazite * thomsonite, or by K-feldspar alone, the olivine phenocrysts by brown chlorite * serpentine i pumpellyite, but the pyroxenes are well preserved. Except for local palagonitization along fractures and rims, the pillowed glassy rims are perfectly fresh. The variations in mineral association among pillow cores, rims, and matrices are believed to have resulted from local variations in compositions of the circulating fluid, in the extent of recrystallization, and in temperature. The occurrence of potash feldspar suggests that the basaltic rocks have been subjected to local metasomatic exchange with a hydrothermal solution under zeolite facies conditions. Pumpellyites characteristically contain up to 25 weight percent total Fe as FeO, higher than most reported pumpellyite (except julgoldite). Calculation of their structural formulas indicates that they may contain iron dominantly in the ferric state. Substitution of Fe8+ for Al in this phase evidently enlarges the pumpellyite P-T stability field relative to the zeolite facies assemblages under oxidizing conditions. Pumpellyite probably crystallized directly from palagonitized basaltic glass. The basaltic rocks of the East Taiwan Ophiolite were hydrothermally metamorphosed at I = 150-250'C and depths of 0.6 to 1.6 km, probably at some distance away from the ridge.

Raman study of the coordination of aluminum in jadeite melts as a function of pressure

Abstract: The coordination of Al3* in glasses of jadeite composition, synthesized at 1450" and 1550"C and over the pressure range 0.fi)l-40 kbar, has been determined using laser Raman spectroscopy. The Raman spectrum of glass ofjadeite composition at I atm resembles that of SiO2 glass, except that the Si-O stretching bands occur at lower frequencies than in SiO, glass and are weakly polarized in contrast to the depolarized bands in SiO, glass. The shift of the Si-O bands in the jadeite glass to lower frequencies is due to Si(Al)-O coupled modes. The weakly polarized stretching bands indicate that the local ordering in jadeite glass may resemble that of c-cristobalite. The spectra ofjadeite glasses formed from liquids quenched at high pressure are similar to that of glass formed from liquid quenched at I atm, except that the S(AI)-O stretching bands become strongly polarized with increasing pressure and are shifted to lower frequencies. There is also a decrease in the intensity of the Rayleigh tail. The major spectral differences betweon the l-atm and high-pressure glasses are explained in terms of differences in the symmetry of the local ordering of the network structure and a systematic decrease in the bond angle at the oxygen atom shared by two adjacent silicon atoms. Overall, the effect ofpressure on the structure of NaAlSirOu melts is to increase the degree of local ordering and to change the local network structure to a coesite-type structure. Al3* remains tetrahedrally coordinated over the pressure range investigated; there is no support for the hypothesis that a pressure-induced coordination change ofAl3* from fourfold to sixfold is responsible for the observed decrease in viscosity of melts ofjadeite composition at elevated pressures.

Chain-width order and disorder in biopyriboles

Abstract: High-resolution c-axis images of the biopyribole minerals anthophyllite [(Mg,Fe)' Si8Orr(OH)rl, jimthompsonite [(Mg,Fe),oSi,rOrr(OH)o], and chesterite [(Mg'Fe),rSiro O'4(OH)61 have been produced in the transmission electron microscope. Interpretation of the images has been confirmed by dynamical electron di-ffraction and imaging calculations. In material from Chester, Vermont, the chain width of areas corresponding to more than one million amphibole chains in the b direction has been imaged and examined. In addition to perfectly ordered phases, parts of some crystals exhibit chain-width disorder ranging from minor faulting of otherwise ordered material to extreme disorder in the chain sequence. Individual silicate chains with widths corresponding to as many as 333 pyroxene chains have been observed. Several new ordered mixed-chain pyriboles have been found to occur in limited but statistically signifrcant amounts. These new ordered structures have primitive unitcell chain sequences (2233), (233), (232233), (222333), (2332323), (2333), (433323), (2234), and (43332343332423). A method for distinguishing statistically-significant ordered structures from those that arise simply from random combination of structural elements is described. If the minerals chesterite and jimthompsonite are stable, their stability ranges are probably between those of anthophyllite and talc. The ordered minerals may, however, be produced metastably during reaction, as the result of constraints imposed by the reaction mechanisms. The occurrence of chesterite appears to be controlled by the chemical potentials of Fe, Ca, and Mg. The newly observed long-range ordered structures are probably the result of some non-random reaction or growth mechanism, rather than being thermodynamically stabilized.

Barian-titanian biotites in nephelinites from Oahu, Hawaii

Abstract: Late-magmatic biotites in nephelinites from Oahu, Hawaii contain unusually high amounts of BaO (up to 20 weight percent) and TiO, (up to 14 weight percent) and are markedly low in SiO, (21 to 28 weight percent) and KzO (2to 6 weight percent). As BaO content increases, SiOr, KrO, and MgO decrease and FeO and AIzOa increase. Atomic substitution schemes indicate that Ba substitutes entirely for K, Na, and Ca in interlayer sites; Al compensates for tetrahedral Si deficiencies; and Ti is probably limited to octahedral substitution for Mg and Fe. Tetrahedral occupancy by Ti appears unlikely, although undetermined Fe3+ may compensate for tetrahedral deficiencies. A substitution scheme of the type K + 3(Mg,Fe) + 3Si : Ba + 2Ti + 3Al is suggested.

The system NaAlSi 3 O 8 -H 2 O-CO 2 to 20 kbar pressure; I, Compositional and thermodynamic relations of liquids and vapors coexisting with albite

Abstract: The distribution of HrO and CO2 between liquid and vapor, in equilibrium with albite crystals, has been determined in eight isothermal, isobaric sections at pressures from 3 to 20 kbar. These data project into P-?space as melting curves at particular vapor compositions. The HrO contents of liquids that were determined experimentally and those calculated from the thermodynamic model of Burnham and Davis are in agreement, even at pressures to 20 kbar. Activity coefficients for HrO in the COr-HrO vapor [y(H'O)'], calculated over the range 3-20 kbar and 900o-l l00oc, are everywhere >1. Because of nonideal mixing in the vapor, melting curves appear at lower temperatures i^ P-T-X(H2O)" projection than in P-Ta(HrO)^ projection (as calculated by Burnham). The mixing is least ideal [y(HrO)" is greatest] in the pressure range 5-10 kbar, resulting in a pronounced trough of temperature minima on the melting curves.

Compression of pyrope

Abstract: Unit-cell parameters have been determined on a single crystal of pyrope mounted in a diamond-anvil cell at pressures from one atm to 50 kbar. From a least-squares fit ofthese data to a third-order Birch-Murnaghan equation of state, a value for the bulk modulus of pyrope (Kr: 1.75 Mbar) and its pressure derivative (K'r:4.5) were calculated. The bulk modulus is in good agreement with two other determinations using cubic-anvil and acoustic techniques but does not agree with another determination using single-crystal diamond-anvil techniques. It appears possible to obtain reasonable values and errors for K'r wrth diamond-anvil techniques at low pressures (<50 kbar), even for minerals with large values of K., when complementary acoustic data are available.

The structure of vanadium-bearing tourmaline and its implications regarding tourmaline solid solutions

Abstract: The structure of an alkali cation-deflcient, vanadium tourmaline has been refined (R : 0.Ml for 2727 rntensity data) in order to evaluate the effects of tourmaline composition on structural distortion. V-tourmaline, (Nao*Ca.r"Mgo.,rIGor)(Mg,rrVlj"Ct'olrFe2o:r8) (Al5s6 V3*38Tit*o6XSi563Abj7XB2e8o space group R3m. Despite compositional di-fferences the structure is very similar to those of other members of the tourmaline group, most notably a recently refined aluminous dravite. Analysis of tourmaline-group structural data reveals (l) a negative correlation between tetrahedral bond angle variance and mean "alkali'(3a)-oxygen bond length; (2) negative correlations between both 9b and l8c octahedral angle variances and mean 9b-O bond length; (3) a negative correlation between ditrigonality and 9b octahedral angle variance; (4) a positive correlation between weighted mean octahedral bond length and cell volume; and (5) a positive correlation between Na occupancy in the 3a site and mean 3a-O bond length. These observations demonstrate a systematic flexibility of the structure in response to diverse cation substitution. The lack of a distinct coupling between the sizes of the 9b and l8c octahedra in refined tourmaline structures and the extensive and possibly complete substitution of Al3* in the 9b site in tourmalines suggest that the presence of cations that can vary in size (e.g., ""2+' r+ ) in order to create a compatible edge may not be a prerequisite for dravite-elbaite solid solution. In view of the structural flexibility of tourmaline and the ease of proton exchange to maintain charge balance, the apparent immiscibility of dravite and elbaite is thought to reflect extreme fractionation of Mg and Li during petrogenesis and by the tourmaline structure, due to the large difference in the field strengths of these cations. Other major features of tourmaline substitutional chemistry are also rationalized on the basis of cation field strength.

Determination of liquid compositions in high-pressure melting of peridotite

Abstract: Experimental problems in the determination of liquid compositions from the partial melting of peridotite are examined in the light of data obtained in an experimental study of the anhydrous melting of peridotite. A scanning electron microscope coupled with an energy-dispersive microprobe has been employed to examine the nature of quench phases and their effect on the melt composition, and to examine the efect of iron loss on crystal-melt equilibria. In most cases the problems of iron loss, non-equilibrium, and quench modi-fication of the melt composition preclude direct determination of the composition of partial melts, even under anhydrous conditions. Provided the modal proportions and compositions of residual phases are known, and due allowance made for preferential adjustment of residual phases to iron loss, the compositions of equilibrium partial melts may be obtained by mass-balance calculations.