Showing papers in "American Mineralogist in 1986"

Measurement of water in rhyolitic glasses; calibration of an infrared spectroscopic technique

Abstract: A series of natural rhyolitic obsidians were analyzed for their total water contents by a vacuum extraction technique. The grain size of the crushed samples can significantly affect these analyses. Coarse powders must be used in order to avoid surface-correlated water. These analyses were used to calibrate infrared spectroscopic measurements of water in glass using several infrared and near-infrared absorption bands. We demonstrate that infrared spectroscopy can yield precise determinations of not only total dissolved water contents, but also the concentrations of individual H-bearing species in natural and synthetic rhyolitic glasses on spots as small as a few tens of micrometers in diameter.

Hydroxyl sites in SiO2 glass: a note on infrared and Raman spectra

Abstract: There is some interest in the speciation of water in silicate glasses and melts, and the effect of bulk composition on this speciation. We have obtained infrared and Raman spectra for Si02-H20 glasses with low (1200 ppm) and high (6.3 wt%) water contents. The lowwater-content glass shows an asymmetric band in the fundamental O-H stretching region, consistent with an SiOH species involved in a range of hydrogen-bonding environments. The high-water-content sample shows features due to both hydroxyl groups, as observed for the low-water-content glass, and molecular H20 dissolved in the glass. This sample also shows a Raman band at 3598 cm-i, which probably corresponds to a new type of SiOH species. We have also obtained the Raman spectrum for a sodium silicate glass Na20' 4Si02 containing 1 wt% H20. Addition of water to this composition causes considerable changes in the silicate band region below 1200 em-i. The O-H stretching bands are very different to those observed for the Si02-H20 glasses, and a weak band near 2350 cm-1 may correspond to SiOH groups involved in intratetrahedral hydrogen bonding, across an edge of the Si04 tetrahedron.

Raman spectra of some tectosilicates and of glasses along the orthoclase-anorthite and nepheline-anorthite joins

Abstract: . Raman spectra are reported for powdered crystalline anorthite, sanidine, leucite, a-nepheIme, an~ ~-carnegie~te ~nd glasses .ofCaA12Si20s (An), KAlSi30s (Or), and NaAlSiO. (Ne) composItIOns. Quahtatlve compansons between features in the spectra of the isochemical c~stal~ a~d glasses indicate that the glasses maintain short-range (coordination) relationShIPSsImIlar to those of the crystalline polymorphs. Intermediate-range order (TO. ring structures; T = Si,Al) of the glasses is inferred from the frequency of the dominant symmetric stretch v,(T -0- T) band in their Raman spectra. The ring structure of An glass is similar to that of the crystalline feldspars, consisting predominantly off our-membered TO. rings. Its Raman spectrum indicates that the melt-quenched glass of Or composition contains domains ~fleucite-like structure, consisting of alternating four- and six-membered TO. rings, and ~egIons ~fpredominantly six-membered ring structures. The ring structure ofNe glass consIsts of sIx-membered TO. rings, similar to its isochemical crystalline polymorphs, although the rings are highly puckered. High-frequency antisymmetric-stretching features in the spectra of the tectosilicate glasses are explained in terms of site symmetries of TO. tetrahedra due to the distribution of SiO. and AIO. units in the glass. Variations in the ring sizes ~n glasses along the Or-An and Ne-An joins are reflected by shifts of the v,(T-0T) bands m the glass spectra. Antisymmetric-stretching features in the spectra of glasses along the Or-An join vary systematically as Si:Al changes from 3: 1 in Or to 1:1 in An but vary only slightly in the spectra of glasses along the Ne-An join because of the common Si:Al of all glasses along that join.

High-chlorine amphiboles from oceanic rocks; product of highly-saline hydrothermal fluids?

Abstract: High-precision analyses of chlorine and fluorine in amphiboles were performed by electron microprobe for five selected samples of metagabbro, amphibolite and metadiabase from the Mathematician Ridge failed spreading center, East Pacific Ocean. Fluorine is consistently very low, less than or equal to 0.2 wt.o/o in all amphiboles. Chlorine concentration is variable and forms a trimodal distribution that correlates with amphibole type in the following way: greenschist-grade actinolites generally contain less than or equal to 0.05 wt.o/o Cl; amphibolite-grade hornblendes contain 0.1 to 0.6 wt.o/o Cl; small irregular zones within actinolite bear up to 4.0 wt.o/o Cl. The high-chlorine amphiboles (hastingsite) probably formed as a result of locally elevated chlorine activity in hydrothermal fluids at upper greenschist-grade temperatures. The amphibolite-grade hornblendes with a few tenths of a percent chlorine may have formed in the presence of a saline hydrothermal fluid or a fluid of near seawater salinity. The amount and distribution in the ocean crust of saline hydrothermal fluids and their associated chlorine-bearing alteration products may be an important factor in understanding ocean hydrothermal systems, particularly in terms of the amount and nature of variabilitv in these svstems.

Formation of tourmaline-rich gem pockets in miarolitic pegmatites

Abstract: AssrRAcr Analysis of fluid inclusions in spodumene, beryl, tourmaline, and quartz from miarolitic pegmatites ofAfghanistan, coupled with lithium aluminosilicate stability relations and with previous studies from gem pegmatites of San Diego County, California, indicate that pocket development in tourmaline-rich, miarolitic rare-element pegmatites occurs between approximately 475 and, 425C and between 2800 and 2400 bars. This range of P and ?r is comparable to the conditions of late-stage crystallization in geochemically similar massive (nonmiarolitic) pegmatites. Whether gem pockets form may be dependent largely on the timing of tourmaline crystallization. Formation of tourmaline removes an alkali borate component from residual pegmatitic melt, with the consequent deposition of other alkali aluminosilicate and oxide-forming minerals and exsolution of copius amounts of HrO. If tourmaline crystallization is inhibited until the late stages of pegmatite consolidation, the large quantities of HrO that are liberated may form pegmatitic pockets. IxrnonucrroN

Ionic modeling of mineral structures and energies in the electron gas approximation; TiO 2 polymorphs, quartz, forsterite, diopside

Abstract: The modified electron gas (MEG) theory has been used to calculate non-empirical shortrange repulsive parameters, which were then employed to calculate minimum-energy structural configurations for quartz, forsterite, diopside and the TiO, polymorphs using the ionic model. The structure parameters of the TiO, polymorph models match the observed ones to within a few percent, but do not model the slight distortions of TiOu octahedra precisely. Calculated structure energies show rutile more stable than anatase by less than 4 kJ/mole and more stable than brookite by about 20 kJ/mole. The model forsterite structure matches the observed one rather well; the sense of SiOo tetrahedral distortions are correct in the model, although the absolute values are not precise. Distortions of model Ml and M2 polyhedra are qualitatively correct but exaggerated in magnitude. Modeling of quartz is less successful; the model structure resembles high quartz closer than low quartz, but the SiO-Si angle is 163'rather than the observed 145'. Diopside is modeled extremely poorll some Ml-O and M2-O distances are long by up to l30o/0. Resulting changes in Ml and M2 coordination lead to charge balanced oxygen atoms in the model structure, in contrast to local electrostatic charge imbalances that exist on all oxygens in the observed structure.

Phase relationships of gabbro-tonalite-granite-water at 15 kbar with applications to differentiation and anatexis

Abstract: Phase relationships have been determined at 15 kbar with variable H_2O contents for three rocks-a gabbro, a tonalite, and a muscovite granite-representing the compositional trend of the calc-alkaline rock series. Experiments were conducted in 0.5-in. piston-cylinder apparatus using Ag-Pd or Pt capsules. The results obtained are acceptable representations of the phase relationships for elucidation of petrological processes. Selected runs with 5% H_2O were completed using Au capsules to overcome the problem of iron loss, up to the temperature limit of Au, for reliable electron microprobe analyses of glass, garnet, clinopyroxene, amphibole, and plagioclase. No glasses could be analyzed in the gabbro. The data provide K_D values as a function of temperature for pairs of coexisting minerals and liquid. The chemical variations are used to evaluate (1) the fractionation trends in hydrous magmas at 55 km in thickened continental crust or uppermost mantle and (2) the products of anatexis of the thickened crust. At this depth, fractionation of hydrous basalt, or anatexis of gabbro and tonalite or their metamorphosed equivalents, appears to produce liquids diverging from the calc-alkaline trend. Anatexis of granite produces a liquid less siliceous than itself, with some chemical characteristics of syenite.

Relationships between radiation damage and trace water in zircon, quartz, and topaz

Abstract: The presence of trace amounts of water or hyroxide ion can dramatically affect the response of a mineral to radiation from both internal and external sources. In zircon, where radioactive decay of U causes structural damage (metamictization), water can enter the structure after a threshold of damage is reached and stabilize the metamict state by annealing local charge imbalance. Quartz crystals with both amethyst and citrine zones have molecular water in the citrine zones and dominantly hydroxide ion in the amethyst zones. This may be due to the reduction of the amethyst Fe(IV) color center by atomic H formed by radiolysis of the water. In topaz, special hydroxide sites correlate with the formation of a brown color center. The crystallographically identified hydroxyl on the (OH,D site does not appear to be correlated with radiation damage. The mobility of H and the ease with which H-O species may be formed in silicates appear to explain much of the role of hydrous species in radiation-damage processes in minerals. These species occur in several charge states and may anneal local charge imbalance. The occurrence of trace amounts of water in even nominally anhydrous minerals makes this interaction common.

Mineralogy and radiation effects of microlite from the Harding Pegmatite, Taos County, New Mexico

Abstract: Microlite, rangrng from crystalline to metamict, is a principal accessory in several lithologic units of the Harding pegmatite, Taos County, New Mexico. From the sequence of lithologic units within the pegmatite, crystallization of microlite from the pegmatite magma is inferred to have begun relatively late, after the formation of the beryl and quartz zones, and continued throughout the formation of the core zones and subsolidus replacement units. Over 200 chemical analyses of microlite determined by electron microprobe are reported, and they are consistent with the accepted structural formula, A2-^B2X6Y.-,' pHrO, where principally A : Ca, Na, IJ, Mn; B : Ta, Nb, Ti; X : O; and Y : F, OH, O. General features of microlite crystal chemistry identified include (l) a positive correlation between A-site vacancies and the maximum Y-site vacancies, (2) a positive correlation between Na and F, and (3) a negative correlation between U and F. The latter is consistent with the interpretation that U at the A site in the pyrochlore structure is analogous to the uranyl group , UO3* , requiring O in place of F at the Y site. Microlite compositions from four of five lithologic units examined are chemically distinct in terms of linear combinations of the variables IJ, Fe, Ti, Bi, Ca, Ce, Pb, F, Mn, Ba, Sb, Th, Ta, and Na. The cleavelanditeunit microlites, the exception, not surprisingly have chemistries like those of the quartzlath spodumene zone, which the cleavelandite unit in part has replaced. The earliest-formed microlites from the quartz-lath spodumene zone have the highest Ta, Na, and F and are low in U and Mn. In contrast, microlites from the later microcline-spodumene zone and the replacement units are generally higher in Ti, Mn, and U and lower in F and Na. Chemical changes ascribed to primary hydrothermal alteration from residual pegmatitic fluids include increases in Ca, Mn, and Ti and an overall loss of A-site cations. Alteration effects and chemical zoning within crystals, analyzed in terms of simple end members, identifies the following principal substitution schemes: BNbBTa,BTa BNb, ACaYO + ANaYF and AEYE aNaYF for core-to-rim zoning in crystals and ACaYO ANaYF and ACaYO AEY! for primary alteration. Secondary (weathering) alteration results in decreases in Na, Ca, and F and increases in HrO. The effects of alpha-recoil damage due to the decay of constituent U have been examined. Because of the wide variation in U content (0.1-10 wto/o UOr), the microlites exhibit the full range of structural periodicity from completely crystalline ( 20 dpa). Based on X-ray and electron ditrraction analysis, the progressive structural modification of microlite udth increasing alpha dose involves (1) formation of isolated defect aggregates (i.e., individual alpha-recoil tracks) up through doses of l0'4 alphas/mg with no detectable effect on the materials's ability to diffract X-rays or electrons, (2) continued damage and overlap of these defect aggregates yielding coexisting regions of amorphous and crystalline domains at l0rs to 1016 alphas/mg, and (3) complete amorphization at doses greater than 10'7 alphas/mg.