Showing papers in "American Mineralogist in 1978"

Biopyriboles and polysomatic series

Abstract: Many crystals may be regarded as made of layer modules that are chemically distinct and that in different crystals may occur in different pioportions, giving rise to polysomatic series. The humites constitute a classic example, as do certain phyll"osilicates and hydroxides. Amphiboles, in particular,_ may be factored by (010) cuts inio layers that are alternately pyroxene and trioctahedral mica (or talc). The pyroxenes, amphiboles, and trioctahedral micas thus form several such series and we shall designate these minerals, collectively, as the biopyriboles, following Johannsen (l9ll). Each such series shows interesting relationships connecting the unit cells and space groups of successive members. The existence of these series also leads to useful chemographic relations in composition space.

Mineral parageneses of eclogitic rocks and related mafic schists of the Piemonte ophiolite nappe, Breuil-St. Jacques area, Italian Western Alps

Abstract: The principal unit exposed along the southern flank of the Matterhorn consists predominantly of Mesozoic calc-schists, greenstones, and serpentinized peridotites of the Piemonte ophiolite nappe. This complex is structurally underlain by the Pennine Monte Rosa nappe, and overlain by the Austroalpine Dent Blanche * Sesia-Lanzo nappe. Juxtaposed representatives of two petrotectonic units occur within the Piemonte ophiolitic complex: the structurally lower Zermatt-Saas unit, and the structurally higher Combin unit. The former represents metamorphosed oceanic crust + uppermost mantle of the Tethyan lithospheric plate, whereas the latter reflects a lithologic transition to the European continental margin. Although both tectonic entities have been subjected to pervasive greenschist-facies tallization, mainly of Eocene-early Oligocene age (the Lepontine event), the Zermatt-Saas unit in addition retains abundant relict Late Cretaceous (Early Alpine) eclogitic and blueschistic phase assemblages. The chief minerals of the eclogites and related mafic schists, including 10 metabasalts and three metagabbros from the Zermatt-Saas unit as well as two greenschists and two metagabbros from the Combin unit, have been analyzed employing electron microprobe techniques. Chemical data are presented for l3 garnets, 1l omphacites, l2 sodic amphiboles,24 calcic amphiboles, 19 epidotes, 12 white micas, 8 chlorites, 10 sodic plagioclases, and 6 sphenes. Textural relations presented previously indicate the prograde development of highest grade garnet * omphacite * rutile rocks, partly or completely converted to schists which display the successive newly-generated retrograde minerals glaucophane, epidote, barroisite, sphene, albite, chlorite * white micas, and actinolite. Only the culmination of the prograde relatively high-pressure metamorphism has been preserved, but a continuum of phase compatibilities marks the retrograde event. Employing experimentally-determined phase equilibrium and element fractionation data, and by analogy with oxygen isotope geothermometry of analogous parageneses, the following physical conditions have been estimated for the BreuilSt. Jacques area mineral assemblages: eclogites, 470 + 50'C, l0 + 2 kbar, a".6 very low; blueschists,450 + 50"C, )7 kbar, ag,6 low; greenschists (prasinites),400 + 50oC,3 + 2 kbar, aH,s high. Barroisitic amphibolites appear to have formed over a range of P-T conditions intermediate between those quoted above for blueschists and greenschists and at moderate HrO activities. The presumed high-pressure Early Alpine prograde path is characteristic of subductionzone metamorphism, whereas the better preserved retrograde P-T trajectory may represent nearly adiabatic decompression-evidently accompanying buoyant return of the subducted complex towards the surface after its detachment from the downgoing Tethyan lithospheric slab. The greenschist-facies which according to isotopic age measurements is connected chiefly with the Lepontine metamorphic event, seems to be related to a postcollisional thermal reequilibration of the pile of nappes.

The origins of color in minerals

Abstract: Four formalisms are outlined. Crystal field theory explains the color as well as the fluorescence in transition-metal-containing minerals such as azurite and ruby. The trap concept, as part of crystal field theory, explains the varying stability of electron and hole color centers with respect to light or heat bleaching, as well as phenomena such as thermoluminescence. The molecular orbital formalism explains the color of charge transfer minerals such as blue sapphire and crocoite involving metals, as well as the nonmetal-involving colors in lazurite, graphite and organically colored minerals. Band theory explains the colors of metallic minerals; the color range black-red-orangeyellow-colorless in minerals such as galena, proustite, greenockite, diamond, as well as the impurity-caused yellow and blue colors in diamond. Lastly, there are the well-known pseudochromatic colors explained by physical optics involving dispersion, scattering, interference, and diffraction.

A uranium- and thorium-rich monazite from a South-Alpine pegmatite at Piona, Italy

Abstract: A green pegmatite mineral from Piona, northern Italy, originally described as a phosphate of lanthanum and cerium, has unit-cell parameters a = 6.78, b = 6.96, c = 6.48A, ..beta.. = 103/sup 0/54', in agreement with monazite. However, the unusual character of this mineral with respect to ''ordinary'' monazite is evident in some physical properties, such as the absorption spectrum in the visible region, which shows bands with their maxima at 640 to 680, 555, and 510 nm, with only weak absorption at 570 to 590 and 522 nm. The refractive indices (..cap alpha.. approximately equal to ..beta.. = 1.78, ..gamma.. = 1.82) are slightly lower than for most monazite specimens. Electron microprobe analysis indicates a content of uranium (16% UO/sub 2/) which exceeds any previously-reported figure for members of the monazite group, and a moderately high content of thorium (11% ThO/sub 2/). A relevant quantity of CaO (4.4%) is also present; silica is nearly absent (0.16%). From these data, relationships to cheralite are evident, and the mineral might be considered as a uranium-rich cheralite, or as its uranium-bearing equivalent; the name monazite is used on acount of the ratio (Ca + U + Th)/(Ce + La + Pr +more » Nd), which is below unity (0.91 against 1.47 for type cheralite). The rare-earth distribution shows considerable enrichment of La/sub 2/O/sub 3/ (40% of the total rare-earth oxides), and depletion of Nd/sub 2/O/sub 3/ (7.75%) and Sm/sub 2/O/sub 3/ ( .. P is a relatively minor factor in reestablishing the charge balance when thorium or uranium are present.« less

The low-temperature stability of andradite in C-O-H fluids

Abstract: The low-temperature stability of andradite was experimentally investigated as a function of temperature, Xs6,andf6, at constant P.,ra of2000 bars. Experimental results indicate that the reaction3quartz* 3calcite*Tqhemat itelVzmagnetite* lsOz:andradite*3 COroccurs atT:550 + lO'C at Xqs\":0.22,596 + 8\"C atX\"o,:0.5, and640 + l0'C atXc6,:1.0. From these experimental data, the standard (298\"K, I bar) Gibbs free energy ol formation of andradite (G?,ea) is -1293.44 + l.2kcal/gfw, and the enthalpy (H?.oo)is -1377.48 I1.2 kcal/gfw. These values are slightly less negative than those for Q,oo and Hi oo calculated from data of Gustafson (1974). Mean values for G?oo and Hio6 derived from Gustafson's experiments and the present results are, respectively, -1297.80 kcal/gfw and -1382.13 kcal/gfw. The standard entropy of formation of andradite (Sl.ea) is -282 + 4 gb/gfw, and the Third Law entropy (S0) is 68.2 I 3 gb/gfw, which is close to the oxide sum estimate of 69.0 gblgfw. The experimental data for the low-temperature stability of andradite plus other pertinent data on the stabilities ofwollastonite, hedenbergite (calculated), clinozoisite (zoisite), epidote, and grossular provide T-Xqs,-f6,-P1,,,o relations which delineate physical-chemical conditions for Ca-Fe-Al-Si skarn formation. Relative to the stability field of grossular in C-O-H fluids (Gordon and Greenwood, 197 I ), andradite is stable with fluids richer in CO, at a given temperature and pressure for all values of/6,, although the temperatures of reactions which delineate the stability field of andradite aresensitivetosl ightchangesineit her X\"o,,-fo,, orboth.Likegross ular,andraditeis stableto lower temperatures with HrO-rich fluids. Addition of Fe3+ to grossular extends the thermal stability limits of grandite plus quartz to both higher and lower temperatures. In natural systems, simple retrograde carbonation of grandite may not occur if the fluid is sufficiently HzO-rich to stabilize epidote.

Natural hydration and ion exchange of obsidian: an electron microprobe study

Abstract: The electron microprobe can be successfully used in the analysis of hydrated glass (perlite), and its ability to analyze very small volumes makes it an ideal tool for studying processes of hydration and ion exchange. Water content, although not measured directly, may be estimated quite accurately by difference-of-sum of oxide components between associated nonhydrated glass (obsidian) and its hydrated equivalent (perlite). Analyses of four obsidian-perlite pairs with a wide electron beam (lOOprm in diameter) are very similar to wet-chemical analyses of the samples. Analyses of massive perlite using a narrow (5prm) beam detected about 3 weight percent HrO, but revealed only a very small amount of ion exchange, mainly a slight loss in NarO. Analyses made along fine fractures in the perlite, however, show slightly to significantly higher water contents accompanied by appreciably higher KrO and appreciably lower NazO contents. The amount of water of hydration in a glass is probably governed by the availability of openings in the polymerized glass structure. A higher degree of hydration in a thin layer along fractures is accompanied by more intense ion exchange. This higher degree of hydration and ion exchange, associated with further weakening and disruption of the glass structure, is an early stage in the eventual formation of secondary argillic or zeolitic assemblages. During the time when the material still retains the optical properties of glass the ion-exchange processes appear to be largely governed by the composition of the glass.

Phase compositions through crystallization intervals in basalt-andesite-H_2O at 30 kbar with implications for subduction zone magmas

Abstract: The melting relationships of a gabbro (olivine tholeiite, quartz eclogite composition) and tonalite (andesite composition) with varying amounts of H_2O added were determined at 30 kbar pressure using piston-cylinder apparatus. With 5 percent H_2O, clinopyroxene and garnet are the liquid us minerals for the basalt, and they occur together throughout the crystallization interval from 1260° to 760°C. With 5 percent H_2O, garnet is the liquidus phase for andesite at 1200°C and it crystallizes alone through 100°C; garnet and clinopyroxene occur together through the rest of the crystallization interval from 1100° to 740°C. Compositions of garnets, clinopyroxenes, and glasses were measured with electron microprobe at the following temperatures: tonalite, 1175° (Ga,Gl), 1100° (Ga,Cpx,Gl), 1000° (Ga,Cpx,Gl), 900° (Cpx,Gl); gabbro, 1200° (Ga,Cpx), 1100°C (Ga,Cpx). Published data by T. H. Green and Ringwood on hydrous and anhydrous calc-alkaline rock compositions, combined with these new data, define compositional trends for minerals through the upper parts of crystallization intervals. From these trends and thermodynamic constraints for mineral pairs, the compositions of minerals through the complete crystallization intervals were calculated. Using calculated mineral compositions and estimated mineral proportions, the compositions of equilibrium liquid paths were calculated through the crystallization intervals of hydrous and anhydrous basalts, basaltic andesite, and andesite. Compositions of quenched liquids in the crystallization interval of andesite with 5 percent H_2O measured with the electron microprobe agree well with calculated compositions, supporting the validity of the calculated liquid paths. Equilibrium liquid paths for basalts and andesites at 30 kbar diverge from the average chemical variation trend of typical calc-alkaline rocks (basalt-andesite-dacite-rhyolite). With decreasing temperature and increasing SiO_2 content, the liquid paths increase in Ca/(Mg+ Fe) compared with the average trend, more so for hydrous compositions than for anhydrous. With equilibrium partial fusion of quartz eclogite, the first liquids are richer in Si02 than average andesites, significantly so if H_2O is present. Liquids with SiO_2 content corresponding to andesites occupy no distinctive position such as a thermal valley; they are situated within a continuous sequence of liquid compositions. Partial melting of subducted ocean crust at 100 km depth produces liquids with a range of intermediate SiO_2 contents, but these must be modified by fractionation at shallower depths if they are to reach the surface with chemistry corresponding to average calc-alkaline lavas.

Hydrothermal reactivity of smectite

Abstract: Hydrothermal experiments were conducted to explore the effects of changing chemistry, temperature, and pressure on the reaction of smectite, run I : I with pure water, to mixed-layer clay. At high temperatures trioctahedral smectites are more stable than dioctahedral smectites' For dioctahedral smectites, hydrothermal stabil ity is enhanced by saturation with interlayer cations of greater hydration energy than potassium, and by increased water pressute. Introduction Many of the unusual physical and chemical properties of the smectite group of clay minerals are related to the expandable interlayer region. These properties, which include swell ing, cation exchange capacity, catalytic activity, and thixotropy, are sometimes diminished or lost during hydrothermal treatment as the smectite reacts to form mixed-layer clay. It is of interest, therefore, from both a geologic and an industrial point of view, to determine the conditions under which smectites are least l ikely to react. Our experiments explore the effects of chemical composition, temperature, and pressure on the hydrothermal alteration of smectite to mixed-layer clay. Experimental techniques A direct comparison between the reactivity of dioctahedral and trioctahedral smectites was made by saturating each type of clay with various interlayer cations and then subjecting them to identical hydrothermal conditions. Trioctahedral starting materials included a natural saponite from Karolihof, Switzerland (<2 micron size fraction) saturated with K, Na, Ca, or Mg; u natural saponite from the Amargosa Valley, Nevada ((2 micron fraction); and gels of potassium and magnesium saponi te composi t ion, Mg.Sir..rAlo3aOr0(OH)rxilrr, where X+ is the exchangeable cation. Dioctahedral starting materials were the Wyoming bentonite ((2 micron fraction, Mol l e l a l . , 1975) saturated wi th K, Na, Ca, or Mg, and a gel of potassium montmor i l lon i te composi t ion, Alrsia 6?4.10 $Oro(OH)rKo.re. Gels were prepared by the method of Hami l ton and Henderson (1968). oo03-004x/78/0304-0401$02 00 401 Hydrothermal runs were prepared by introducing 30 mg of clay or gel and 30 pl of pure water into gold tubes (20 mm long, 2.5 mm I.D.) which were then welded shut. These charges were heated in large hotseal autoclave reaction vessels or in small cold-seal reaction vessels. The former generated maximum pressures of approximately 0.3 kbar, and the latter were maintained at 0.5 or 2 kbar by equil ibration with a large reservoir (see Eberl and Hower, 1976). All vessels were heated in resistance furnaces, and temperatures were controlled by on-off regulators attached to thermocouples located in wells near or in the base of the vessels. Run products were oriented on glass slides, glycolated, and X-rayed with a Norelco diffractometer, using Ni-fi l tered CuKa radiation. The expandabil ity of mixed-layer i l l i te/smectite was determined by comparison with the calculated patterns of Reynolds and Hower (1970). Experimental results The difference in reactivity between dioctahedral and trioctahedral smectite is clearly shown in Tables I and 2 and in F igures 1,2, and 3. At 400'C and autoclave pressure, the K-montmoril lonite gel (run I ) and the natural montmoril lonites saturated with K, Na, Ca, and Mg (runs 2-5) reacted extensively to lbrm regularly interstratif ied mixed-layer clay. The sraponites run under equivalent conditions did not react (runs l0-18). The effect of interlayer chemistry on the reaction of dioctahedral clays is also shown in Table l. Montmoril lonite with interlayer potassium (run 6) reacted at a lower temDerature than did mont402 EBERL ET AL.' REACTIVITY OF SMECTITE Table I Montmor i l loni te autoclave runs Run n o . Start ing mater i .a l Ternp. ( oc ) Time (days ) Run produc ts

Phase equilibria in fluid inclusions in ultramafic xenoliths

Abstract: Over 200 fluid inclusions in five dunite, peridotite, and pyroxenite xenoliths associated with basaltic rocks from Arizona, Hawaii, and Germany were examined using a petrographic microscope equipped for cooling to - 140.C. The temperatures of phase changes observed in individual fluid inclusions are interpreted according to published, experimentally determined phase equilibria as follows: (l) Most inclusions contain nearly pure co, and, in some cases, a small amount (on the order of 0.05 to 0.10 mole fraction) of Sor, HrS, or cos. only one inclusion contains a possible aqueous phase. (2) The CO, fluid densities range from 0.34 to l.l4 gm/cms. Assuming a temperature of entrapment of 1200"C, this implies confining pressures of more than l0 kilobars at the time of entrapment of the densest inclusions. The presence ofglass linings on some ofthe inclusion walls suggests coexistence at depth of a COr-rich volatile phase with a melt phase at the time of entrapment of the two fluids in ihe host minerals. Compositions of the glass linings of two samples, Dreiser Wehier, Germany, and Red Hill, Arizona, most closely match high-alumina andesite and high-alumina basalt, respectively.

Low-temperature heat capacities and entropies of feldspar glasses and of anorthite

Abstract: The heat capacities of glasses near the feldspar compositions KAlSisO8, NaAlSiaOs, and CaAlzSizOr, and of crystalline anorthite were measured between | 2 and 380 K by means of an adiabatic calorimeter. Difference plots, C!(glass) - C!(crystals), show pronounced maxima at 30 K for KAlSisOs, at 50 K for NaAlSirOr, and at 35 K for CaAlrSirO., similar to maxima previously observed for other inorganic glass-crystal pairs (e.g., AsrO, and SiOr). The entropy changes, S?08 - S?, for KAlSirOr, NaAlSirOs, and CaALSizO, glasses and for anorthite based onourmeasurement sare224.3 +0.3,213.8+0.3, 198.7t0.3,and199 '3+ 0.3Jl(mol 'K)' respectively. Approximate values for the zero-point entropies, S?, of NaAlSisOB, KAlSi3O. and CaAl, SirO, glasses were calculated from our heat-capacity data combined with (l) the high-temperature Hi - Hor", data for these glasses and their crystals; (2) the low-temperature heat capacities of analbite and high sanidine; and (3) the enthalpy changes, obtained by HF(aq) solution calorimetry, for the transformation crystals + glass, LHo"ry Our calculated values for the entropies of KAlSirO, glass, NaAlSieOs glass, and CaAlzSizOa glass at zero Kelvin, that is, the residual or zero-point entropies, are37.3 + 2.5, 38.1 + 1.5' and 38.6 I 2.2 J/(mol. K), respectively. The entropies of fusion at the melting points of high sanidine. analbite. and anorthite are 33.8, 42.6,and44.3 J/(mol' K)' respectively' The Debye temperature, do, of CaAlrSirO. glass is 210 + l0 K, and that of anorthite is 227 + l5 K.