American Mineralogist

Published by Mineralogical Society of America
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Spectral reflectance in the visible and near-infrared portion of the spectrum (0.35-2.55 mu m) offers a rapid, inexpensive, non-destructive technique for determining mineralogy and providing some information on the minor-element chemistry of the hard-to-discriminate carbonate minerals. Spectra may be obtained from samples in any form, including powders, sands and broken, sawn, or polished rock surfaces.-J.A.Z.
 
The transformation of an andalusite (Al2O3·SiO2 = A) single-crystal to 3:2-mullite (3Al2O3·2SiO2 = M) and non-crystalline silica (SiO2) was investigated at the (100)(A) and (010)(A) faces using electron microscopy. The transformation starts topotactically at the surfaces producing a dense layer of mullite and vitreous silica on the surface and underneath the mullite layer. Next, the reaction proceeds by dissolution of andalusite in the vitreous silica and by diffusional transport of alumina to mullite crystals occasionally nucleated at the dissolving andalusite. Transformation along [100](A) proceeds twice as fast as along [010](A), which is explained by the higher stability of the (010)(A) face against dissolution. Transformation by dissolution and precipitation is one order of magnitude slower than the topotactic transformation along [001](A). The transformation reaction of andalusite to mullite and silica is highly anisotropic. Combining the results along the directions [100](A), [010](A), and [001](A) predicts the type of transformation which will take place at an arbitrary andalusite face. Only if the c(A) axis is oriented more than 80°off the surface normal, will the reaction proceed by the dissolution-precipitation process, whereas for all other orientations andalusite transforms by the fast topotactic reaction along the c(A) axis.
 
The transformation of 2/1-mullite single crystals into stable 3/2-mullite was investigated using reaction couples of 2/1-mullite cut parallel to (001) and silica glass at 1650 °C. A thin layer of 3/2-mullite formed at the surface of the parent 2/1-mullite crystal. Between 3/2- and 2/1-mullite a gradual transition layer of about 1 μm exists. This gradual transition zone and the lack of any interface between the substrate 2/1-mullite and the newly formed 3/2-mullite implies a topotactic transformation mechanism controlled by Al, Si, and O interdiffusion with preservation of the basic crystal structure, especially the AlO6 octahedral chains. The number of O atom vacancies is reduced going from 2/1- to 3/2-mullite, as shown by a gradual decrease of the corresponding superlattice intensities in electron diffraction patterns. As the transformation requires diffusion of Al out of and of Si and O atoms into the 2/1-mullite crystal, a coexisting silicate melt is required as an Al sink and as a source of Si and O atoms.
 
A thin section of lunar rock 78235 is examined optically as well as with an electron microprobe and is shown to be a highly-shocked coarse norite. The rock is found to consist of approximately 30% low-Ca pyroxene, 55% Ca-rich plagioclase, and 15% glass veins. The composition of the glass veins is analyzed, and it is suggested that they resulted from shock melting of the norite. Comparisons with other noritic rocks indicate that the pyroxene in rock 78235 has P2 sub 1 ca symmetry. Based on this symmetry plus the coarse grain size, it is concluded that rock 78235 was formed under plutonic conditions. The origin of the minor phases in this rock is briefly considered.
 
All previously published libraries of infrared mineral spectra are in the form of transmitance. Reflectance spectra are, however, more useful for remote sensing and some potential laboratory applications, such as the use of an infrared microscope for mineral identification on polished sections. This note points out that construction of a new library of infrared (2.1-25.0 microns) mineral spectra is in progress. Both transmittance and reflectance measurements of a selection of 63 different, well-characterized minerals have been published to date. These data are available in both hard copy and digital form.
 
XRD results
NMR parameters
Mullite is an aluminosilicate of the composition Al 2 (Al (sub 2+2x) Si (sub 2-2x) )O (sub 10-x) with x generally ranging between 0.2 and 0.5. XRD and 29 Si and 27 Al nuclear magnetic resonance (NMR) have been used to investigate the structure of various mullite compositions (x = 0.26, 0.36, 0.42, and 0.69) as well as sillimanite (x = 0) and gamma Al 2 O 3 (x = 1). 27 Al magic angle spinning (MAS) NMR of the central and satellite transitions have been used to determine the chemical shifts and quadrupole interaction parameters for the various AlO 4 and AlO 6 units. The isotropic chemical shifts of the various units are 5.9 ppm for AlO 6 , 69.1 ppm for AlO 4 (T), 55.0 ppm for AlO 4 (T'), and 45.7 ppm for AlO 4 (T*) where AlO 4 (T) denotes the aluminum tetrahedra in the double chains, and AlO 4 (T') and AlO 4 (T*) those next to the oxygen Oc*. Quantitative numbers of the aluminum occupancy of these sites have been determined for the various powder samples. These results are in good agreement with the average structure model of mullites. NMR and XRD proved the presence of impurities of kyanite in natural sillimanite and of gamma -Al 2 O 3 in the mullites with x = 0.42 and 0.69.
 
Phase relations involving genthelvite are studied on the basis of physical-chemical principles and natural occurrences. Stability diagrams and exchange operators are considered as well as an extended system, natural associations, and solid state relations. It is noted that the extremely high chalcophilicity of Zn explains, in part, why genthelvite is such as rare mineral. It is only somewhat more stable than willemite and is therefore restricted to the extremely low-S environments provided by alkaline granites and syenites.
 
First principles linear response calculations are used to investigate the lattice dynamics of what is thought to be the third most abundant phase in the lower mantle, CaSiO_3 perovskite. The commonly assumed cubic structure (Pm3m) is found to be dynamically unstable at all pressures, exhibiting unstable modes along the Brillouin zone edge from the M-point to the R-point. Based on these results, we predict that the ground state structure of CaSiO_3 perovskite is a distorted phase with lower than cubic symmetry. We predict that a phase transition occurs in CaSiO_3 perovskite within the earth's lower mantle from the low temperature distorted phase to the cubic phase at high temperature. The predicted phase transition provides a possible explanation of some of the seismological observations of reflective features within the lower mantle.
 
The spectral data of Dowty and Clark for the titaniferous pyroxene in the Allende meteorite have been reinterpreted. Arguments are presented in support of an alternative assignment of the two absorption bands at 21,000 cm-'and 16,500 cm-'as both being due to crystal field transitions in Ti3* ions. Not only are two such bands expected for Ti"* ions in distorted Ml coordination sites of clinopyroxenes, but the derived spectral parameters a and A. are consistent with other Ti(III) compounds. The occurrence of Ti'* ions in the clinopyroxene structure is attributed to the enhanced crystal field stabilization energy ac- quired in the Ml site.
 
The accuracy of equation-of-state formulations is compared for theoretical total energies or experimental pressure-volume measurements for H2, Ne, Pt, and Ta. This spans the entire range of compression found for minerals and volatiles in the Earth. The Vinet equation is found to be most accurate. The origin of the behavior of different equation-of-state formulations is discussed. It is shown that subtle phase transitions can be detected by examining the residuals from an equation-of-state fit. A change in the electronic structure of Ta is found at high pressures using this procedure, and a possible new transition in H2.
 
Photomicrographs illustrating progressive recrystallization of primary pyroxenes (crossed nicols). (A): Primary inverted pigeonite surrounded by primary augite. Coarse augite lamellae (white) are inclined to the (001) direction of host orthopyroxene (at extinction). A later set of fine (100) augite lamellae (nearly vertical) is also present. Length of photograph: I mm. (B): Partially recrystallized inverted pigeonite illustrating the migration and coalesccence of coarse augite lamellae at grain boundaries. Length of photograph: 1.5 mm. (C): Granular mosaic of augite and orthopyroxene illustrating final result of recrystallizatron. Length of photograph: 2mm. 
Point counted modes of mafic rich dikes from the Marcy massif 
(A) Primary pyroxene compositions as determined using broad beam energy dispersive techniques, and olivine compositions as determined using wavelength dispersive analyses. Solid tie lines: mafic cumulate layers. Dashed tie lines: mafic dikes. Skaergaard trend after Wager and Brown (1968). (B) Compositions ofprimary igneouS pyroxene and pyroxene-olivine pairs (short dashed lines) and reconstituted metamorphic pairs (solid lines) plotted with respect to the augite-pigeonite solvus (dashed contours) and the augite orthopyroxene solvus (solid contours) of Ross and Huebner (1975). 
Representative.microprobe analyses of pyroxene and olivine from sample 5683d 
Representative WD microprobe analyses of Fe-Ti oxides 
At this locality, the mafic rocks occur as conformable segregations interpreted as cumulate layers in border zones and dykes throughout the massif. The dykes apparently crystallized from residual liquids at various stages of differentiation. Textures suggest the crystallization sequence: plagioclase, pigeonite + augite, hemo-ilmenite + magnetite, and apatite. Fe-rich olivine replaced pigeonite in the last stage.-K.A.R.
 
Mullite-type alkali aluminates Rietveld refinements of six members of the solid solution series (y = 0.0, 0.2, ..., 1.0), together with Fourier syntheses and grid search analyses show that the Na and K atoms reside in the vacant Oc sites, with K at 1/2, 0, 1/2 and Na on a split site off the special position. The number of alkali atoms is restricted to 2/3 atoms per unit cell due to crystal chemical constraints. Consequently, unlike the aluminosilicate mullites, alkali mullites do not form a solid solution series with varying oxygen composition. All compounds studied here crystallize in space group Pbam with lattice constants ranging from a = 7.6819(4) Å, b = 7.6810(4) Å, c = 2.91842(8) Å for the Na aluminate to a = 7.6934(3) Å, b = 7.6727(3) Å, c = 2.93231(7) Å for the K aluminate mullite.
 
Fe ion, Mg order-disorder in natural orthopyroxenes as function of temperature
 
Tie lines between coexisting immiscible sulfide and silicate melts at 1420" C and 12.5 and 30 kbar. Melt compositions are projected onto the FeS-FeO-SiO2 surface from the other comDonents.
When the sulfur content of an Fe-bearing magma exceeds the saturation limit for the bulk composition, an immiscible iron sulfide melt fraction separates. For an understanding of the geochemistry of sulfur-bearing magmatic systems, more information is needed regarding the solubility of metal sulfide in silicate melt at its source and the solubility changes as a function of changing intensive and extensive variables. In the present investigation, the sulfur saturation surface is determined for the pressure range from 12.5 to 30 kbar and the temperature range from 1300 to 1460 C for three silicate melt compositions representing a range of SiO2 and FeO compositions.
 
The petrogenesis of the topaz-rich ongonite and topazite dikes in the Flying W ranch area (Tonto basin, Arizona) was investigated, and the possible origin of the topaz in the Flying W rhyolite is discussed. The field, textural geochemical, and mineralogical characteristics of the topaz-bearing dikes in the Flying W ranch area all were found to support the genesis of the topaz-bearing bodies from a magma. The evidence obtained contradicts an origin for these rock types by greisenization or veining.
 
Fourier transform of the EXAFS data (solid line) and fit model (dots) for Cr-doped 
(Color online) Portion of the garnet structure. The oxygen atoms are labeled 
(color online) Cr K-edge XANES spectra in Mg 3 Al 2 Si 3 O 12 : Cr 3+ . The experimental 
The structural environment of substitutional Cr3+ ion in a natural pyrope Mg3Al2Si3O12 has been investigated by Cr K-edge extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) coupled with first-principles computations. The Cr-O distance is close to that in knorringite Mg3Cr2Si3O12, indicating a nearly full relaxation of the first neighbors. The local C3i symmetry of the octahedral Y site is retained during Cr-Al substitution. The second and third shells of neighbors (Mg and Si) relax only partially. Site relaxation is accommodated by strain-induced bond buckling, with angular tilts of the Si-centered tetrahedra around the Cr-centered octahedron, and by a deformation of the Mg-centered dodecahedra.
 
A model is described for the melting of a simple granulite assemblage, in the presence of CO2-rich fluid phases, which can occur between 750 and 1000 C at crustal pressures and is therefore within the range estimated for such regional metamorphism as that of the Adirondacks. For melting which occurs at about 750 C in the presence of both H2O and CO2, pressures corresponding to the deep crust are required to generate a melt enriched in pyroxene and feldspar components, while melting the precense of pure CO2 at about 1000 C generates analogous melt compositions at lower pressures. These experimental reactions are in keeping with observations constraining charnockite occurrences: (1) pressure and temperature constraints; (2) mineralogical constraints; and (3) constraints on the compositions of volatiles associated with peak conditions of charnockite formation.
 
Previous studies have demonstrated the usefulness of major and minor elements in silicate phases to understand differences among basaltic systems and the influence of different planetary environments on basalt chemistry (e.g., Papike [1]). Intriguing data displays presented by Papike [1] include a plot of Mn vs. Fe (atoms per formula unit, afu) for pyroxene and olivine and a plot showing the anorthite content of plagioclase from different planetary basalts. Here we combine portions of these plots (Fig. 4) and provide all new data for olivine and plagioclase.
 
This is a comparative planetary mineralogy study emphasizing the valence-state partitioning of Cr, Fe, Ti, and V over crystallographic sites in olivine, pyroxene, and spinel from planetary basalts. The sites that accommodate these cations are the M2 site (6 to 8-coordinated) and M1 site (6-coordinated) in pyroxene, the M2 site (6- to 8-coordinated) and M1 (6-coordinated site) in olivine, and the tetrahedral and octahedral sites in spinel. The samples we studied are basalts from Earth, Moon, and Mars, and range in oxygen fugacity conditions from IW-2 (Moon) to IW+6 (Earth), with Mars somewhere in between (IW to IW+2). The significant elemental valence-states at these fO2 conditions are (from low to high fO2): Ti 4+, V3+, Fe2+, Cr2+, Cr3+, V4+, and Fe3+. V2+ and Ti3+ play a minor role in the phases considered for the Moon, and are found in very low concentrations. V5+ plays a minor role in these phases in oxidized terrestrial basalts because it is probably lower in abundance than V4+, and has an ionic radius that is so small (0.054 nm, 6-coordinated), that it is almost at the lower limit for octahedral coordination, and can even be tetrahedrally coordinated. The role of Cr2+ in the Moon is significant, as Cr2+ predominates in basaltic melts at fO2 less than IW-1. Lunar olivine has been found to contain mostly Cr2+, whereas coexisting pyroxene contains mostly Cr3+. Fe3+ is very important in Earth, less so in Mars, and nonexistent in the Moon. The importance of the Fe2+ to Fe3+ transition cannot be overstated and, indeed, their crystal-chemical differences, in terms of behavior (based on size and charge), are similar to the differences between Mg and Al. We note that for pyroxene in six of the seven terrestrial suites we studied, Fe3+ (in the M1 site) coupled with Al (in the tetrahedral site) is one of the two most important charge-balance substitutions. This substitution is of lesser importance in Mars and does not exist in lunar basalts.
 
The suitability of boron nitride for use as a crucible material in silicate and oxygen-bearing metal sulfide systems has been investigated. Boron nitride is unsatisfactory for use with many silicate systems because its presence in combination with a source of oxygen establishes the oxygen fugacity at values below that of the assemblage quartz + fayalite + iron, reducing transition metal ions such as Ni(2+) and Fe(2+) to the metallic state. B2O3, resulting from the oxidation of BN, acts as a flux to promote formation of melt.
 
A regular-solution model of Ca-rich carbonate melts is developed, using the fact that they are ionic liquids, and can be treated as interpenetrating regular solutions of cations and of anions. The regular-solution model of carbonate melts can be applied to problems of carbonatite magma + vapor equilibria and of extrapolating liquidus equilibria to unstudied systems. Calcite and anhydrite may be present on the surface of Venus, but they would not be molten at ambient surface temperature (660-760 K). The Venus atmosphere contains 5 ppb HF, which implies that the anion solution of a carbonate-rich magma in equilibrium with the atmosphere would contain a F- mole fraction of ~7 × 10-3, or about 0.1 wt%. Although this proportion of F is much enriched compared with the atmosphere, it would have little effect on phase relations of the carbonatite. -from Author
 
Electronic structure calculations are carried out to estimate the enthalpies of formation of rare-earth orthophosphates from their oxides. The calculated enthalpies of formation are systematically less exothermic than the measured values. The discrepancy is almost entirely in the electronic total energy calculated from density functional theory, and appears to be intrinsic to the generalized-gradient exchange-correlation functional used. However, comparison with electronic structure calculation of the enthalpies of formation of alkaline earth oxyacid carbonates, silicates, and sulfates suggests that near chemical accuracy can be obtained for the enthalpies of formation of most of the compounds in the phosphate system by applying a scaling factor obtained from the simpler alkaline earth oxyacids. The increasingly exothermic ΔHfox with increasing ionic radius (i.e., LaPO4 is more exothermic than ScPO4) results from the higher charge localization on the oxide anion (O2−) relative to the phosphate anion (PO43−), making it more favorable, in a relative sense, to pair the smaller cation with the oxide anion than with the phosphate anion. This effect is also manifested in ΔHfox of the other oxyacids, such as carbonates, silicates, and sulfates.
 
Chondritic ilmenite has the compositional range TiO//2 51. 0 to 54. 4, FeO 35. 4 to 44. 6, MgO 1. 5 to 6. 6, MnO 0. 6 to 9. 8, Cr//2O//3 0. 03 to 1. 40 wt %. Ilmenite composition shows a relation to chondrite classification-for '%'equilibrated'%' chondrites, Fe content of ilmenite tends to increase as one passes from H-to L- to LL-group chondrites, whereas Mg and Mn have the reverse tendency. Because the Fe and Mg variation is the same as observed in pyroxene, olivine and chromite of '%'equilibrated'%' chondrites, it is suggested that ilmenite is in equilibrium with these minerals.
 
Chromite and ilmenite analysis in pallasites, mesosiderites, achondrites and meteorites with electron microprobe
 
Phase stability and elasticity data have been used to calculate the Gibbs free energy, enthalpy, and entropy changes at 298 K and 1 bar associated with the quartz-coesite and coesite-stishovite transformations in the system SiO2. For the quartz-coesite transformation, these changes disagree by a factor of two or three with those obtained by calorimetric techniques. The phase boundary for this transformation appears to be well determined by experiment; the discrepancy, therefore, suggests that the calorimetric data for coesite are in error. Although the calorimetric and phase stability data for the coesite-stishovite transformation yield the same transition pressure at 298 K, the phase-boundary slopes disagree by a factor of two. At present, it is not possible to determine which of the data are in error. Thus serious inconsistencies exist in the thermodynamic data for the polymorphic transformations of silica.
 
Rare earth element partitioning data
(continued)
' Orthopyroxene/melt partition coemcients for Ce, Sm, and Tm at2.3,8,20, atd37.7Vo partial melting. Error bars are tlo. Also shown are data ofMysen (1978a) in the system plagioclase62-forsteritel5-silica2 at 20 kbar and 1025.C. melting, D.^ increases from 0.030+0.002 zt 2.3Vo melting to 0.070+0.006 at37.7Vo melting, and Dr-increases from 0.037t0.001 at 2.3Vo melting to 0.046i0.002 at 37 .7Vo melting. The values of D^r, are close to those determined by Mysen (1978a). For 20 and 37.7Vo melt, DREE are substantiarlly highsl than those determined by Mysen (1978a). As discussed above, Mysen's data are obtained at 20 kbar compared with 35 kbar in this study, and the crystals coexist with hydrous melts (20 wt.Vo HrO). The data obtained at low pressure are reviewed by lrving (1978) and Mysen (1978a), and phenocryst-matrix data are summarized by Cullers et al. (1973). The values determined in this study are among ft6 highest of any olivine-basaltic melt REE partition coefficients and are also determined under the highsst pressure and temperature conditions.  
Fig. l. Melting curves of nodule PHN 16ll at 35 kbar pressure (anhydrous). Upper curve determined by Mysen and Kushiro (197?) with no control on iron loss to noble-metal capsules. Lower curve determined in this investigation with graphite capsules. Size of symbols includes uncertainties in temperature (tl0'C) and determination of percentage of liquid (+lo). Arrows indicate conditions selected for partitioning experiments.  
Partition coefficients for Ce, Sm, and Tm between garnet, clinopyroxene, orthopyroxene, olivine, and melt are determined at 35 kbar for 2.3, 8, 20, and 37.7% melting of a garnet lherzolite nodule with chondritic REE abundances. Partition coefficients are found to increase as the degree of partial melting increases. From 2.3 to 8% melting, this increase is for the most part a consequence of non-Henry's law behavior of REE in minerals.
 
The vector method for representing mineral compositions of amphibole and mica groups is applied to the tourmaline group. Consideration is given to the methods for drawing the relevant vector diagrams, relating the exchange vectors to one another, and contouring the diagrams for constant values of Na, Ca, Li, Fe, Mg, Al, Si, and OH. The method is used to depict a wide range of possible tourmaline end-member compositions and solid solutions, starting from a single point. In addition to vector depictions of multicomponent natural tourmalines, vectors are presented for simpler systems such as (Na,Al)-tourmalines, alkali-free tourmalines, and elbaites.
 
This study explores the controls of temperature, pressure, and silicate melt composition on S solubility in silicate liquids. The solubility of S in FeO-containing silicate melts in equilibrium with metal sulfide increases significantly with increasing temperature but decreases with increasing pressure. The silicate melt structure also exercises a control on S solubility. Increasing the degree of polymerization of the silicate melt structure lowers the S solubility in the silicate liquid. The new set of experimental data is used to expand the model of Mavrogenes and O Neill (1999) for S solubility in silicate liquids by incorporating the influence of the silicate melt structure. The expected S solubility in the ascending magma is calculated using the expanded model. Because the negative pressure dependence of S solubility is more influential than the positive temperature dependence, decompression and adiabatic ascent of a formerly S-saturated silicate magma will lead to S undersaturation. A primitive magma that is S-saturated in its source region will, therefore, become S-undersaturated as it ascends to shallower depth. In order to precipitate magmatic sulfides, the magma must first cool and undergo fractional crystallization to reach S saturation. The S content in a metallic liquid that is in equilibrium with a magma ocean that contains -200 ppm S (Le., Earth s bulk mantle S content) ranges from 5.5 to 12 wt% S. This range of S values encompasses the amount of S (9 to 12 wt%) that would be present in the outer core if S is the light element. Thus, the Earth s proto-mantle could be in equilibrium (in terms of the preserved S abundance) with a core-forming metallic phase.
 
A Rietveld refinement of Cr-doped mullite, Cr0.5Al3.92Si1.58O9.79, orthorhombic, space group Pbam [a = 7.56712(6) Å, b = 7.70909(6) Å, c = 2.90211(2) Å, V = 169.30 Å3] revealed Cr to reside predominantly in the octahedrally coordinated M1 site. The mean M1-O distance of 1.935 Å is that expected from Al3+ and Cr3+ molar fractions obtained from the chemical analysis and structure refinement. The small displacement factors of the T* and Oc* sites indicate deficiencies in the electron densities which could be compensated by assuming additional Cr atoms. The predominant Cr incorporation into the M1 site causes an expansion of the octahedral bonds which is directly related to the observed lengthening of the c edge. The strong expansion of the long and elastic octahedral M1-Od bond in Cr-doped mullite, which would affect the a and b edges, is partly absorbed by a shortening of tetrahedral bonds.
 
Raman features of chromite, ulvospinel, magnetite, ilmenite, hematite, and some of their solid solutions are presented. Although most Fe-Ti-Cr oxides produce relatively weak Raman signals compared to oxyanionic minerals, sufficient information can be extracted from their spectra to identify the end-member mineral phases as well as some information about compositional variations in solid solution. Correlations between Raman spectral features and mineral chemistry are used to interpret the Raman data of Fe-Ti-Cr oxides found during Raman point-count measurements on rock chips of EETA 79001, as an analog to Mars on-surface planetary investigations. In general, ulvospinel, magnetite, and chromite end-members are readily distinguished by their Raman spectral patterns, as are ilmenite and hematite. In the low signal-to-noise (S/N) spectra generally obtained from the point count procedure, the position and shape of the strongest peak of Fe-Ti-Cr oxides in the region 660-680 cm-1 (A1g mode) is the most useful for discriminating Fe3+-Ti-Cr-Al substitutions in the magnetite-ulvospinel, ulvospinel-chromite, and chromite-spinel series, but mionr peaks in the range 300-600 cm-1 also assist in descrimination. These spectral features are useful for investigating the variablity among Fe-Ti-Cr-Al oxide solid solutions in natural samples. In EETA79001, a martian basaltic meteorite, most of the oxide grains (as measured with the electron microprobe) are ulvospinel, chromian ulvospinel, and chromite, but ilmenite, titanian chromite, and titanochromite are also observed. The Fe-Ti-Cr oxides identified by Raman point-count include end-member ilmenite, low-Al chromite-spinel solid solutions, ulvospinel-magnetite solid solutions, and more complex chromite-spinel-ulvospinel-magnetite solid solutions; the latter exhibit a wide range of main peak positions and broadened peak widths that may reflect structural disorder as well as specific cation contents. One Raman spectrum suggests end-member magnetite, and one spectrum from a different rock chip appears to be that if non-terrestrial hematite, reflecting local oxidizing alteration which has not been observed previously in this meteorite. These results show that analyses done in an automated mode on the surface of an unprepared Martian rock sample can provide useful constraints on the Fe-Ti-Cr oxide mineralogy present and on compositional varitions withiin those minerals, including an indication of oxygen fugacity.
 
The distribution of bombardment energy in the moon's crust and the types of phase transformation resulting from this energy input are investigated. It is seen that the major significance of impact melts in the period of intense cratering is that they transfer most of the high-temperature heat following cratering and thus take part in many of the phase transformations on planetary surfaces. Energy partitioning studies suggest that of the projectiles' total kinetic energy, 23 to 35% is converted into heat by passage of the shock fronts, 43 to 53% transports ejecta and then is dissipated at modest temperature increases over a broad area, 8 to 24% is consumed comminuting country rock, and less than 1% becomes seismic and radiant energy. Subsequent to its formation, the melt is violently mixed with much colder debris to form the sheets of impact melt and the breccias.
 
The lunar Mg suite, which includes dunites, troctolites, and norites, could make up 20-30% of the Moon's crust down to a depth of 60 km. The remainder is largely anorthositic. This report focuses on norites because we have found that the chemical characteristics of orthopyroxene are effective recorders of their parental melt compositions. Many of the samples representing the Mg suite are small and unrepresentative. In addition, they are cumulates and thus are difficult to study by whole-rock techniques. Therefore, we decided to study these rocks by SIMS techniques to analyze a suite of trace elements in orthopyroxene. The 12 norite samples were selected from a recent compilation by Warren who attempted to select the best candidate samples from the standpoint of their pristine character. Our present database includes greater than 300 superior Electromagnetic Pulse (EMP) analyses and greater than 50 scanning ion mass spectroscopy (SIMS) analyses for 8 Rare Earth Elements (REE), Zr, Y, and Sr. The Mg#s for the parental melts calculated from Mg#s in orthopyroxene show that most melts have Mg#s in the range of 0.36-0.60. This compares with a range of Mg#s for lunar volcanic picritic glass beads of 0.4-0.68. Therefore, although the cumulate whole-rock compositions of the Mg suite can be extremely magnesian, the calculated parental melts are not anomalously high in Mg. A chemical characteristic of the Mg-suite norites that is more difficult to explain is the high KREEP content of the calculated parental melts. The REE contents for the calculated norite parental melts have REE that match or exceed the high-K KREEP component of Warren. Therefore, mixing of a KREEP component and a picritic melt cannot, by itself, explain the high estimated REE contents of the melts parental to norites. Advanced crystallization following KREEP incorporation, especially of plagiclase, may also be required.
 
A thin platinum wire loop is an effective way to hold silicate melts during experimentation in a gas-mixing furnace. This method results in a minimum of physical and chemical interaction between the sample and container but maximum interaction between sample and gas mixture. However, volatilization of sodium occurs while the silicate is molten. By minimizing the chance of heterogeneous nucleation, the method is ideal for experimental investigation of the origin of rock textures.
 
Mullite has been synthesized in a sol-gel process yielding an alumina-rich phase with 89 mol% Al2O3 corresponding to Al5.65Si0.35O9.175, with 0.825 O vacancies, space group Pbam, a = 7.7391(6), b = 7.6108(5), c = 2.9180(1) Å, and Z = 1. The correlation of lattice parameters with alumina content shows a strictly linear increase of a beyond the crossover point of a and b at 78 mol% Al2O3. This clearly proves that the average crystal structure remains orthorhombic and does not transform to a tetragonal form. The crystal structure has been analyzed by the Rietveld method on the basis of X-ray powder diffraction data. -from Authors
 
We have determined major (Si, Zr, Hf), minor (Al, Y, Fe, P), and trace element (Ca, Sc, Ti, Ba, REE, Th, U) concentrations and Raman spectra of a zoned, 200 microns zircon grain in lunar sample 14161,7069, a quartz monzodiorite breccia collected at the Apollo 14 site. Analyses were obtained on a thin section in situ with an ion microprobe, an electron microprobe, and a laser Raman microprobe. The zircon grain is optically zoned in birefringence, a reflection of variable (incomplete) metamictization resulting from zo- nation in U and Th concentrations. Variations in the concentrations of U and Th correlate strongly with those of other high-field-strength trace elements and with changes in Raman spectral parameters. Concentrations of U and Th range from 21 to 55 ppm and 6 to 31 ppm, respectively, and correlate with lower Raman peak intensities, wider Raman peaks, and shifted Si-O peak positions. Concentrations of heavy rare earth elements range over a factor of three to four and correlate with intensities of fluorescence peaks. Correlated variations in trace element concentrations reflect the original magmatic differentiation of the parental melt approx. 4 b.y. ago. Degradation of the zircon structure, as reflected by the observed Raman spectral parameters, has occurred in this sample over a range of alpha-decay event dose from approx. 5.2 x 10(exp 14) to 1.4 x 10(exp 15) decay events per milligram of zircon, as calculated from the U and Th concentrations. This dose is well below the approx. 10(exp 16) events per milligram cumulative dose that causes complete metamictization and indicates that laser Raman microprobe spectroscopy is an analytical technique that is very sensitive to the radiation-induced damage in zircon.
 
Tourmaline, a mineral often overlooked in petrologic studies, is shown to be a useful petrogenetic indicator mineral. Despite the large number of possible substitutions in tourmaline, generalizations relating tourmaline composition and rock type are made. Based on available literature data, distinct regions are defined within Al-Fe(tot)-Mg and Ca- Fe(tot)-Mg diagrams for tourmaline from different rock types. As an example of the utility of tourmaline in petrologic studies, the accessory tourmaline from staurolite-grade pelitic schists from NW Maine is investigated. These tourmaline grains typically display three general styles of chemical zoning: (1) a lack of zoning, (2) a continuous core-to-rim zonation attributed to growth during progressive metamorphism, and (3) a zonation marked by a distinct discontinuity apparently representing detrital tourmaline grains surrounded by metamorphic tourmaline overgrowths. All of the samples in this study contain si-, Al-, and Ti-saturating phases (quartz, staurolite, and ilmenite, respectively). Consequently, the substitutional complexities of tourmaline in equilibrium with the matrix are minimized. Systematic element partitioning suggests that chemical equilibrium has been attained among the rims of the tourmaline grains and matrix minerals. Tourmaline rims have the highest Mg/Fe and Na./Ca ratios of any of the phases in the metapelites. The compositions of tourmaline cores that are presumed to be detrital in origin are optically and chemically distinct from their tourmaline overgrowths. In the Al-Fe(totl Mg and Ca-Fe(totfMg diagrams, the compositions of the detrital cores fall in the fields of rock types that are commonly found in the presumed source region for the pelitic sediments.
 
The focusing geometry of an electron microprobe has been used to measure the wavelength shifts of sulfur X-rays from hydrous experimental melts synthesized at oxygen fugacities that range from near the iron-wustite buffer to the magnetite-hermatite buffer. It is found that the proportion of dissolved sulfur which is present as sulfate increases with increasing oxygen fugacity. It is noted that in natural melts that have equilibrated at or below fayalite-magnetite-quartz values of +1, sulfur is probably present mainly as S(2-).
 
Megaspherulites occur in the middle zone of a thick sequence of rhyolitic vitrophyre that occupies a small, late Eocene to early Oligocene volcanic-tectonic basin near Silver Cliff, Custer County, Colorado. Diameters of the megaspherulites range from 0.3 m to over 3.66 m, including a clay envelope. The megaspherulites are compound spherulites. consisting of an extremely large number (3.8 x 10(exp 9) to 9.9 x 10(exp 9)) of individual growth cones averaging 3 mm long by 1.25 mm wide at their termination. They are holocrystalline, very fine- to fine-grained, composed of disordered to ordered sanidine (orthoclase) and quartz, and surrounded by a thin K-feldspar, quartz rich rind, an inner clay layer with mordenite, and an outer clay layer composed wholly of 15 A montmorillonite. Whole rock analyses of the megaspherulites show a restricted composition from their core to their outer edge, with an average analyses of 76.3% SiO2, 0.34% CaO, 2.17% Na2O, 6.92% K2O, 0.83% H2O+ compared to the rhyolitic vitrophyre from which they crystallize with 71.07% SiO2, 0.57% CaO, 4.06% Na2O,4.l0% K2O, and 6.40% H2O+. The remaining oxides of Fe2O3 (total Fe), A12O3, MnO,MgO, TiO2, P2O5, Cr2O3, and trace elements show uniform distribution between the megaspherulites and the rhyolitic vitrophyre. Megaspherulite crystallization began soon after the rhyolitic lava ceased to flow as the result of sparse heterogeneous nucleation, under nonequilibrium conditions, due to a high degree of undercooling, delta T. The crystals grow with a fibrous habit which is favored by a large delta T ranging between 245 C and 295 C, despite lowered viscosity, and enhanced diffusion due to the high H2O content, ranging between 5% and 7%. Therefore, megaspherulite growth proceeded in a diffusion controlled manner, where the diffusion, rate lags behind the crystal growth rate at the crystal-liquid interface, restricting fibril lengths and diameters to the 10 micron to 15 micron and 3 micron and 8 micron ranges respectively. Once diffusion reestablishes itself at the crystallization front, a new nucleation event occurs at the terminated tips of the fibril cones and a new cone begins to develop with a similar orientation (small angle branching) to the earlier cones. During crystallization, these fibril cones impinge upon each other, resulting in fibril cone-free areas. These cone-free areas consist of coarser, fine-grained phases, dominated by quartz, which crystallized from the melt as it accumulated between the crystallizing K-feldspar fibrils of the cones. The anhydrous nature of the disordered to ordered sanidine (orthoclase) and quartz, suggests that water in the vitrophyre moved ahead of the crystallization front, resulting in a water rich fluid being enriched in Si, K, Na, Mg, Ca, Sr, Ba, and Y. The clay layers associated with the megaspherulites are therefore, the result of the deuteric alteration between the fractionated water and the vitrophyre, as indicated by the presence of the minerals mordenite and montmorillonite. This silica rich fluid also resulted in the total silicification of the megaspherulites within the upper 3 m of the vitrophyre.
 
All the possible space groups for mica polytypes are deduced by making use of the characteristics of the mica unit layer and stacking mode. The algebraic properties of the vector-stacking symbol of Ross et al. (1966) are examined, and a simple algorithm for deducing the space group from this symbol is presented. A method considered for enumerating all possible stacking sequences of mica polytypes makes use of a computer.
 
Electron microprobe analysis of vanadium in presence of titanium by compositional measurement of chromite in chondritic meteorites
 
Ultra-high vacuum dynamic scanning force microscopy (dynamic SFM) has been performed on in situ cleaved and as-grown (001) surfaces of low-T, m = 18 and m = 20, antigorite from the Kovdor Mine, Russia. The internal microstructure of the same crystal before cleavage has been checked by conventional TEM on FIB-cut cross-sections. The structural wave is imaged by dynamic SFM with a similar to 0.25 nm topographic amplitude (outcropping tetrahedral sheet) on cleaved and as-grown surfaces, and with a similar to 0.5 nm topographic amplitude (outcropping tetrahedral + octahedral sheets) mostly found on cleaved surfaces. Atomic resolution imaging was successfully applied on the cleavage surface through imaging individual atomic features on the outer hexagonal net of the emerging (Mg, O, OH) octahedra of the half-wave. The antigorite cleavage crack undulates through a single octahedral sheet, thereby avoiding rupture of strong Si-O bonds. The two tetrahedral reversals, which form the edges of the modulation repeat, are found to be strongly non-equivalent in structure: across < 010 >, one reversal is sharp as expected from the standard models of the antigorite structure, whereas the other reversal is unexpectedly "extended." The latter suggests some scheme of anti-polar positioning of silicate tetrahedra along < 010 > at the 6-membered ring reversal. High-resolution transmission electron microscopy (HRTEM) structure imaging of antigorite viewed down to < 010 > confirms spread out electron densities at this reversal. Numerous step height measurements on (001) surfaces show incremental results as integral multiples of 0.25 nm, the spacing between O,OH surfaces along c*. Many of them differ in height from integral multiples of the unit cell repeat along c* and could be explained from carving the bulk wave structure. For all surfaces and steps, local stoichiometry and global electro-neutrality of the surface are satisfied.
 
Backscattered electron image of titanomagnetite showing the fine oxidation-driven exsolution lamellae of ilmenite in MIL 03346 ,106. 
The Miller Range (MIL) 03346 nakhlite contains ~20% mesostasis, which contains skeletal titanomagnetite. The titanomagnetite contains trellis-type {111} lamellae of ilmenite similar to those found in terrestrial titanomagnetites that have experienced subsolidus redox reactions during cooling of their host rocks. We have characterized the MIL 03346 titanomagnetite-ilmenite intergrowths by a combination of focused ion beam (FIB), energy-dispersive spectroscopy (EDX), and high-resolution transmission electron microscopy (TEM). The resulting structural and chemical analyses have been combined with temperature and fO2 data from previous studies of MIL 03346, as well as previous work on two-oxide thermobarometry of nakhlites. Our calculations show that as MIL 03346 and other nakhlites cooled below 800 °C, they recorded increasingly reducing conditions, such that the lowest temperatures calculated correspond to fO2 conditions as low as 4 log fO2 units below the FMQ buffer. However, the MIL 03346 lamellae must have formed by oxidation and thus record a very late stage low-temperature (<350 °C) oxidation event. When considered together with previous studies of MIL 03346 and nakhlites in general, the overall cooling history could be explained by early oxidation followed by intermediate stage reduction caused by S2 loss by degassing, followed by late loss of Cl by degassing.
 
The incommensurately modulated structure of the mineral natrite has been refined for the first time. Two single-crystal grains, Lv and Kh, from two different occurrences [Mt. Karnasurt, Lovozero massif (Lv), Kola peninsula, Russia, and the pegmatite of Mt. Koashva, Khibiny massif (Kh)], have been investigated at 293 and 120 K using synchrotron X-ray data. The average structures of both minerals are identical and the basic features of the structural modulations are similar to the synthetic γ-Na2CO3 phase previously published. The γ (incommensurate) → δ (lock-in) phase transition reported at low temperature for the synthetic compound was not observed down to 120 K in natural natrite. Crystal-chemical aspects, especially about the second coordination sphere for the carbon atoms, are examined to explain the different structural behaviors observed at low temperature. The possible role played by the minor isomorphous substitutions in natural natrite specimens is also discussed.
 
The solubility of H2O-CO2 fluids in a synthetic analogue of a phono-tephritic lava composition from Alban Hills (Central Italy) was experimentally determined from 50 to 500 MPa and 1200 and 1250°C. H2O and CO2 contents in experimental glasses were determined by bulk analytical methods and FTIR spectroscopy. For the quantification of volatile concentrations by IR spectroscopy we have calibrated the absorption coefficients of water-related and carbon-related bands for phono-tephritic compositions. The determined absorption coefficients are 0.62 ± 0.06 L mol-1cm-1 for the band at ~4500 cm-1 (OH groups) and 1.02 ± 0.03 L mol-1cm-1 for the band at ~5200 cm-1 (H2O molecules). The coefficient for the fundamental OH stretching vibration at 3550 cm-1 is 63.9 ± 5.4 L mol-1cm-1. CO2 is bound in the phono-tephritic glass as CO32- exclusively; its concentration was quantified by the peak height of the doublet near the 1500 cm-1 band with the calibrated absorption coefficient of 308 ± 110 L mol-1cm-1. Quench crystals were observed in glasses with water contents exceeding 6 wt% even when using a rapid quench device, limiting the application of IR spectroscopy for water-rich glasses. H2O solubility in the ultrapotassic melts (7.52 wt% K2O) as a function of pressure is similar to the solubility in basaltic melts up to 400 MPa (~8 wt%) but is higher at 500 MPa (up to 10.71 wt%). At 500 MPa and 1200°C, the CO2 capacity of the phono-tephritic melt is about 0.82 wt%. The high CO2 capacity is probably related to the high K2O content of the melt. At both 200 and 500 MPa, the H2O solubility shows a non linear dependence on XfH2O in the whole XfH2O range. The variation of CO2 solubility with XfCO2 displays a pronounced convex shape in particular at 500 MPa, implying that dissolved H2O promotes the solubility of CO2. Our experimental data on CO2 solubility indicate that the interaction between phono-tephritic magma and carbonate rocks occurring in the Alban Hills magmatic system may result in partial dissolution of CO2 from limestone into the magma. However, although the CO2 solubility in phono-tephritic melts is relatively high compared to that in silicic to basaltic melts, the capacity for assimilation of limestone without degassing is nevertheless limited to < 1 wt% at the P-T conditions of the magma chamber below Alban Hills. In press 2.3. TTC - Laboratori di chimica e fisica delle rocce JCR Journal
 
Top-cited authors
Frank C Hawthorne
  • University of Manitoba
George R Rossman
  • California Institute of Technology
Donna L. Whitney
  • University of Minnesota Twin Cities
Robert M. Hazen
  • Carnegie Institution for Science
John C. Schumacher
  • Portland State University