Showing papers in "American Mineralogist in 1971"

The crystal chemistry of the silicate garnets

Abstract: Refined structures of eight natural garnets [Cr-pyrope, almandine, spessartine, Mngrossular, grossular, uvarovite, goldmanite, and andradite] are compared with that of synthetic pyrope to determine the effects of the substituent cations on polyhedral interactions,bond lengths, and angles. The Si-O bonds in these garnets constitute two populations whichcan be related to (r!X!), the mean radius of the X dodecahedra] cation. If (r{X}) is lessthan 1.0 A, then Si-O= 1.635 :1:0.005A, but if (r!X\) exceeds 1.0 A, then Si-O = 1.650 to.005 A. However, the X-O and Y-O bond lengths and the O-X-O, O-Y-O, and O-Si-O anglesare linearly dependent on (r{X!) and on (r[Y]) (where (r[Y])= the mean radius of the Y octahedral cation). A multiple linear regression analysis indicates the positional parameters of these garnets to be related to (1'{X} ) and (1'[ V]) using Shannon and Prewitt's effectiveradii as follows: x = 0.006 + 0.022(r{X} > + 0.014(r[Y]) y = 0.051 - 0.023(r{X}) + 0.037(r[Y]) z = 0.643 - 0.009(r{X}) + 0.034(r[Y]). The positional parameters of Fe-pyrope calculated with these equations [x=0.0336; y =0.0491; z=0.6530] are in statistical agreement with the observed [x=0.0339(5); y =0.0491(6);z=0.6535(6)] (Euler and Bruce, 1965). Furthermore, using a cell edge calculated from an equation obtained by regression analysis of 56 well characterized silicate garnets [a=9.04+1.61(r{XD+1.89(r[YJ)], the predicted positional parameters give bond lengthsand angles that are statistically identical with those observed. These equations were used to predict the structural details of over 200 hypothetical cubic silicate garnet compounds by assigning (r{XI) values between 0.80 and 1.50 A and (r[Y]) values between 0.50 and 1.15 A (at 0.05 A intervals). Using criteria based on reasonable 0-0, Si-O, X-O, and Y-O distances, a diagram was prepared in which the structural "stability" field of silicate garnets is delineated as a function of (r{X}) and (r[Y]).

Refinement of The Crystal Structures of Epidote, Allanite and Hancockite

Abstract: Complete, three-dimensional crystal structure studies, including site-occupancy refinement, of a high-iron epidote, allanite, and hancockite have yielded cation distributions Cal. ooCal.oo(Alo. 93F eo.05) Ah. 00(Alo.24F eo.76) Si30'3H for epidote, Ca,. oo(REo.74Ca0.26) (Alo...FeO.34)Ah.oo(Alo.17Feo. 83) ShO'3H for allanite, and Cal.oo(Pbo.5Sro.25Cao.'5) (Alo.86FeO.14)Al,.00(Alo.16Feo.84)Si30,3H for hancockite. These results when combined with those obtained in previous epidote-group refinements establish group-wide distribution trends in both the octahedral sites and the large-cation sites. Polyhedral expansion or contraction occurs at those sites involved in composition change but a simple mechanism, involving mainly rigid rotation of polyhedra, allows all other polyhedra to retain their same geometries in all the structures examined.

The effect of cation substitutions on the physical properties of trioctahedra micas.

Abstract: Thesis (MS)--Massachusetts Institute of Technology, Dept of Earth and Planetary Sciences, 1971

Mg2+ -Fe2+ ORDER-DISORDER AND THE THERMODYNAMICS OF THE ORTHO­ PYROXENE CRYSTALLINE SOLUTION

Abstract: The distribution of Mg2+ and Fe"+ between the Ml and M2 sites in orthopyroxene (Fe, Mg).Sh06 at 500, 600, 700, and 800 °C, determined by Mossbauer resonance spectro­ scopy, can be interpreted in terms of Guggenheim's (19 67) "simple mixture" model for the individual sites. The standard free energy change !:J.G0 for the F e"-Mg2 exchange between the two sites and the adjustable energy constants W for the Fe"+-Mg2+ mixing on the Ml and M2 sites vary linearly with the inverse of the absolute temperature between 600 and 800°C. Fe"+-MgH mixing at the Ml site is more non-ideal than that at the M2 site, both approach­ ing ideality with increasing temperature. "Partial" excess thermodynamic functions of mix­ ing of Mg2+-Fe"+ on the individual sites are calculated using the "simple mixture" model. The activity-composi tion relation in orthopyroxene is derived from the "partial" activ­ ities of Mg2+ and FeH at the two sites at 500, 600, 700, and 800°C. The excess free energy of mixing of the Fe and Mg components in orthopyroxene at various temperatures is ex­ pressed as a polynomial in the mole fraction, using Guggenheim's equation with three con­ stants A o, A1, and A2, all of which decrease systematically as the temperature increases from 600 to 800°C .

Infrared characterization of water and hydroxyl ion in the basic magnesium carbonate minerals

Abstract: Infrared spectra in the 0.5-50 m range have been measured on nesquehonite (MgCO 3 .3H 2 O, artinite (MgCO 3 .Mg(OH) 2 .3H 2 O) and hydromagnesite (4MgCO 3 .Mg(OH) 2 .4H 2 O). The CO 3 2- internal vibrations are sharp in hydromagnesite and unexpectedly broad and diffuse in artinite and nesquehonite. The OH - stretching region contains a sharp band indicative of a weak, well defined hydrogen bond in all three minerals. There is also a broad band characteristic of moderately strong and less well defined hydrogen bonds in all three. In addition hydromagnesite has two sharp intense bands not present in the other two minerals. The artinite spectrum is interpretable by Jagodzinski's structure and nesquehonite appears to be similar. Hydromagnesite should have a distinctly different structure. There is no evidence for bicarbonate groups in nesquehonite.

Vibrational spectra of the common silicates: I. The garnets

Abstract: Infrared spectra have been measured on 22 specimens of silicate garnet and Raman spectra on 6 specimens. Seventeen IR modes were found as predicted by a factor group analysis. Not all of the predicted 25 Raman lines were found. The factor group splitting of the tetrahedral vibrations correlates with the occupancies of the octahedral and cubal sites. The site group splitting was used to measure the distortion of the garnet tetrahedral site.

Dissolution of clay minerals in dilute organic acids at room temperature

Abstract: TWO GRAMS EACH OF REPRESENTATIVE CLAY MINERALS WERE SHAKEN AT ROOM TEMPERATURE IN DISTILLED WATER, AND IN 0.01M ASPARTIC, CITRIC, SALICYLIC, TARTARIC, AND TANNIC ACIDS (REPRESENTATIVE OF COMPONENTS OF HUMIC ACID). ALIQUOTS OF CENTRIFUGED SOLUTION, UP TO 102 DAYS DISSOLUTION, WERE ANALYZED FOR SILICON, IRON, ALUMINUM, IRON, MAGNESIUM, CALCIUM, SODIUM, AND POTASSIUM. THE TOTAL WEIGHT OF CLAY MINERALS DISSOLVED BY STRONGLY COMPLEXING ORGANIC ACIDS EXCEEDS THAT DISSOLVED BY DISTILLED WATER BY FACTORS 5 TO 75. DISSOLVED SILICON IN STRONGLY COMPLEXING ACID SOLUTIONS EXCEEDED 2-35 TIMES ITS CONCENTRATION IN DISTILLED WATER; DISSOLVED ALUMINUM, 3-500 TIMES. ASPARTIC, AN AMINO-ACID, DISSOLVED CALCUM AND MAGNESIUM BEST. CLAY MINERALS DISSOLVE INCONGRUENTLY IN DISTILLLED WATER, SILICON PREFERENTIALLY TO A ALUMINUM. IN ALL ACIDS USED, SILICON INCONGRUENTLY EXCEEDS ALUMINUM DISSOLVED FROM ARIZONA AND WYOMING SMECTITES, SEMI-PLASTIC REFRACTORY CLAY, ALSO FOR ILLITES IN SALICYLIC ACID, AND KEOKUK KAOLINITE IN CITRIC AND TANNIC ACID. CONGRUENT DISSOLUTION, OR ALUMINUM INCONGRUENTLY EXCEEDING SILICON, 1.3-1.6:1, OCCURRED WITH BUEKER FLINT CLAY, GEORGIA KAOLINITE, AND ILLITES IN CITRIC, TARTARIC, AND TANNIC ACIDS. PREFERENTIAL SOLUBILITIES OF EITHER SILICON OR A ALUMINUM FROM CLAY MINERALS INDICATE GENETIC MECHANISMS FOR POSSOLS, LATERITES, KAOLIN (ESPECIALLY MOORVERWITTERUNG TYPE), AND BAUXITES. GEOLOGIC AND PEDOLOGIC EXAMPLES ARE CITED. /AUTHOR/

Analysis of the Infrared Absorption Spectrum of Diopside

Abstract: Frequencies of the normal modes of vibration for a long chain (Si:Oo)" molecule in both infrared and far infrared absorption spectra of diopside were calculated on the basis of the Urey-Bradley force field and compared withthose measured. The wave numbers calculated were found to be in good agreement with those measured. The force constants obtained for the Si:Oo moiecule were Ka 5.0, K" 4.5, I1o 0.4, .F-narsis;y 0.4, Fas166; 0.1, 4,0.1, Fn" 0.05, Faa -0.9 and Fa, -0.9 md /A.

Conpositional sextor-zoning in clinopyroxene from the narce area, Italy

Abstract: An electron microprobe study of sector-zoning in clinopyroxenes from the Narce area, Italy shows that the relative order of enrichment of the pyroxene components with trivalent or quadrivalent cations in the Ml sites in the four cr1'5fs.ll6graphically distinct sectors, (100), (110), (010), (T11), is (100)>(110)>(010)>(111). For the crystal showing the largest amount of sector-zoning, the (100) sector has about 50/6 more CaAl:SiOr, Ar/s rr'ore CaFes+AlSiO6, llsmore NaTiAlSiOe, and 116 more CaTiAlzOo than the (T11) sector. In order to interpret the sector-zoning in clinopyroxene, the model developed to interpret sector-zoning in staurolite (Hollister, 1970) is expanded to include efiects of the atomic configurations on the surfaces of growth steps and the properties of the material added to a growing crystal. It is concluded that there are at least four controlling factors involved in deveioping compositional sector-zoning: size and composition of ionic complexes added to the crystal as it grows, rate of addition of material, rate of equilibration of the new material with the matrix at the surfaces of growth steps, and rate of re-equilibration of surface layers with the matrix by exchange of ions perpendicular to the crystal Iaces. The fact that py'roxene from a lava flow can be sector-zoned in the component CaAl:SiOo suggests that care should be exercised in interpreting the presence of this component in a crystal as indicative of crystallization in a high pressure environment, especially in the absence of co-crystallization of anorthite.

Orientation of Exsolution Lamellae in Clinopyroxenes and Clinoamphiboles: Consideration of Optimal Phase Boundaries: Erratum

Abstract: Microscopic observations of clinopy'roxene and clinoamphibole exsolution lamellae in clinopy'roxene and clinoamphibole hosts, referred to space grotps C2fc and lZfm respectively, show the Iamellae are not usually oriented paraliel to (001) and (100) as comrronly believed, even though X-ray single crystal photographs usually suggest host and lamellae share a common (001) or (100) Iattice plane. Coexisting magnesioarfvedsonite and manganoan cummingtonite, described in detail here, show an extreme example with lamellae 16\" from (001) in the acute angle B and 60 from (100) in the obtuse angle B. Two-dimensional evaluation in (010) of measured Iattices of exsolved clinopyroxenes and clinoamphiboles sho*'s that the lamellae are parallel to generally irrational planes of dimensional best fit between the Iattices at the time exsolution began. Best fit is achieved by slight relative rotation of the lattices; but in most natural examples this rotation is less than 10' and not easily observed on X-ray photographs. Using augite and pigeonite as examples, the relative a and c dimensions of the lattice with the smaller B angle (AUG) and the lattice with the larger B angle (PIG) determine the positions of the lamellae as foilows: \"001\" lamellae @auc ) aprc inacuteangleB 6tuc ) cprc

Structural and textural relationships of amphibole and phlogopite in peridotite inclusions, Dish Hill, California

Abstract: Peridotite inclusions in basanite at Dish Hill and Siberia craters near Ludlow, California, contain abundant amphibole, smaller quantities of phlogopite and apatite, and rare plagioclase. These minerals occur as veins transecting the peridotite and as interstitial components. Veins of undeformed amphibole cut across wehrlite-lherzolite layering and metamorphic foliation in therzolite and offset kink bands in olivine, indicating that the amphibole postdates both anhydrous mineral layering and plastic deformation of the peridotite. Gradations in textural behavior of arirphibole frorn veins to interstitial modes of occurrence suggest that all of the amphibole and associated minerals are post-consolidation additions to the peridotite, unrelated to its original crystallization. This conclusion is supported by the presence of amphibole with the same textural relations in all types of peridotite, irrespective of their modal composition or degree of deformation and recrystallization. The source of the secondary hydrous minerals is not known. They may predate the basanite and have contributed to its formation, or they may have been derived from the basanite or related magma. The absence of amphibole phenocrysts in the basanite, as well as similar occurrences of amphibole in alpine-t1pe peridotites that were not carried into the crust by mafic alkaline magma, support the former view. Both alternatives are currently under investigation.

The crystal structures of yavapaiite, KFe(SO4)2, and goldichite, KFe(SO4)2.4H2O

Abstract: Yavapaiite [KFe(SO+):l crystallizes in space group C2/m with two formula weights in a unit cell of dimensions a:8.152, b:5.153, c:7.877 A, B:O+.Of . The structure was solved by a three-dimensional Patterson synthesis and refined by the frill-matrix leastsquares method to R:3.27o. Goldichite [KFe(SOr)z.4HzO] in space group P21f c contains foui formrila weights in a unit cell with a:10.387,b:10.486, c:9.086 A,0:101.68". The structure was solved by hand application of the s;'rnbolic addition procedure for phase determination and refined by least squares to R:3.37o. In yavapaiite, ferric sulfate sheets of composition zlFe(Sor)glare linked to coplanar potassium ions; this arrangement accounts for the perfect [001] cleavage. Coordination polyhedra of iron and sulfur are distorted. In goldichite, corrugated sheets of composition n[Fe(SOr)z.2HzOlare linked to potassium and water molecules; this spatial arrangement of the distorted coordination polyhedra accounts for the excellent { 100} cleavage. Details of the observed polyhedral distortions are explained by the extended electrostatic valence rule. A plausible hydrogen bonding scheme is presented for goldichite and krausite