Showing papers in "American Mineralogist in 1967"

The oxygen isotope and cation exchange chemistry of feldspars

Abstract: Cation exchange experiments between alkali and alkaline-earth feldspars and corresponding 2-3 molal aqueous chloride solutions were performed at a fluid pressure of one kilobar over the temperature range 350°-800°C. Oxygen isotope analyses of the exchanged feldspar indicate that essentially complete oxygen isotope equilibration between solution and feldspars accompanies the cation exchange. Oxygen isoptope fractionations obtained this way were proved to be equilibrium fractionations by their agreement with those obtained by true isotope exchange reactions between synthetic feldspars and pure water. The oxygen isotope fractionation factor (alpha) between alkali feldspar and water in the temperature range studied is given by the expression 10 3 ln alpha=2.91 (10 6 T -2 )-3.41. No isotope fractionation was discernible between albite and potassium feldspar. However, the alkali feldspars were found to concentrate O 18 relative to the alkaline-earth feldspars, indicating a relationship between the Al/Si ratio in feldspar and tendency to concentrate O 18 . The plagioclase-water fractionation follows the equation: 10 3 ln alpha=(2.91-0. 76beta)(10 6 T -2 )-3.41-0.41beta where beta is the An content of plagioclase. Observations made during the course of this work suggest that the mechanism of oxygen and cation exchange in these experiments involves fine-scale solution and redeposition in a fluid film at the interface between exchanged and unexchanged feldspar. A mechanism involving simple solid-state diffusion cannot explain the observed communication between the oxygen at the interfacial boundary and the solution.

The Gypsum-Anhydrite Equilibrium at One Atmosphere Pressure

Abstract: The equilibrium temperature for the reaction CaSO*l4*s.2H*l2*sO=CaSO* l4*s+2H*l2*sO*l(liq. soln)*s has been determined as a function of activity of H* l2*sO (a*lH*l2*sO*s) of the solution. Synthetic gypsum and anhydrite or 1:1 mixtures were stirred in solutions of known a*lH*l2*sO*s (calculated from vapor pressure data for the Na*l2*sSO*l4*s and H*l2*sSO*l4*s solutions), at constant temperature for as much as 12 months. The reversible equilibrium was approached from both sides and is defined by: a*lH*l2*sO*s=0.960 at 55°, 0.845 at 39°, 0.770 at 23°C. Provided the solids do not change in composition, the equilibrium at constant P and T is a function of a*lH*l2*sO*s only and is independent of the constituents in solution. Extrapolation to the bounding system CaSO*l4-*sH*l2*sO(a*lH*l2*sO*s=1.000) yields 58°±2°C. This is within thermodynamic calculations (46°±22°C) but higher than solubility measurements (38° to 42°C). The new data indicate that in sea-water saturated with halite and gypsum should dehydrate above 18°C. The scarcity of anhydrite in modern evaporite deposits is predicted by the present results. The available data on the temperature- salinity conditions under which anhydrite and gypsum exist in the Recent supratidal flat sediments of the Trucial Coast, Persian Gulf, are compatible with the present experimental data.

The Mineralogy of Thingmuli, a Tertiary Volcano in Eastern Iceland

Abstract: A series of olivine-tholeiite, basaltic-andesite, icelandite and pitchstone lavas and dykes which make up a Tertiary central volcano in eastern Iceland have been examined mineralogically with the electron-probe. The composition of the coexisting one-phase magnetites and ilmenites has been used to determine the temperatures and oxygen fugacities of their equilibration. These data, for the rocks which contain oxide microphenocrysts, indicate that the tholeiites have liquidus temperatures near 1080oC, faliing to 925oC for a porphyritic pitchstone; the oxygen fugacities of the series approximate to those defined by the synthetic fayalite-magnetite-quartz bufier. The groundmass pyroxenes in the basalts and basaltic-andesites are augites and pigeonites which vary in composition in a similar way to those of the Skaergaard intrusion; olivine apparently shows an analogous recurrence of crystallization to the Skaergaard, reappearing in the icelandites as an iron-rich phenocrystic phase and being absent in the tholeiites and basaltic-andesites. The groundmass plagioclase becomes progressively more sodic in the basalts and basaltic-andesites, and its range of zoning becomes increasingiy restricted with increase in the amount of the glassy residuum. Of the secondary minerals, only chlorophaeite is sufficiently abundant for analysis and its analyses shorv some response to the composition of the rocks in which it is found.

Properties of a Synthetic Magnesium-Aluminum Carbonate Hydroxide and Its Relationship to Magnesium-Aluminum Double Hydroxide, Manasseite and Hydrotalcite

Abstract: A synthetic mineral prepared by titrating a mixed solution of MgClz and AlCl: with NaOH in a COz-free system and then diaiyzing the suspension for 30 days at 60'C was a hydrated Mg-AI carbonate hydroxide with the formula MgoAlzCOa(OH)re.4HzO and properties of manasseite and hydrotalcite. The synthetic mineral is hexagonal with a:6.14t, c:15.6lz A and Z:1. Dehydration and corresponcling contraction of basal spacings were reversible up to 300\"C and irreversible above 450'C. Above 300'C both OH water and COz were evolved, and at 450'C the structure was destroyed. A one-dimensional structure analysis indicates a structure consisting of brucitic layers alternating with hydrous aluminum layers.

The Microcline-orthoclase Transition Within a Contact Aureole

Abstract: Microcline perthites from Precambrian pegmatites in the l'ront Range, Colorado, have been converted to orthoclase in a zone around a Tertiary intrusive stock. This stock, near the town of Eldor, is 2 miles across and sholvs fairly well exposed vertical contacts The microcline-orthoclase transition, the only major manifestation of contact metamorphism, was investigated optically and by X-ray powder diffraction along five traverses. Outside the contact a.ureole and microcline commonly consists of clear crystals with typical crosshatched twinning and shows a. 2V of 80o + 5o and an obliquity ranging from 0.84 to 0.93. The orthoclase from near the contact is always turbid, untwinned, with axial angles I (010) from 50o to 66o. The perthitization and composition (OrzzAbx) of the feldspars remain roughly constant across the contact aureole, except within the last few feet from the contact, where an increased albite content or the disappearance of the perthite lamellae may occasionally be observed. Both the nature of the microcline-orthoclase transition and its distance from the contact depend strongly on the configuration of the contact in that particular area. In accordance with the heat flow calculations, the transition is relatively sharp and close (1200 feet or less) to pretruding corners of the intrusive, whereas, near re-entrant sections of the intrusive, the transition is gradational and relatively distant (over 2400 feet) from the contact. Heat flow calculations further indicate that the microcline-orthoclase transition may occur at temperatures below 400oC, i.e. more than 50oC Iower than was hitherto known from an experimental approach. The order of the transition is not specified by this investigation although the occurrence of some intermediate microcline in the transition zone may be evidence for a continuous type of transformation.

Experimental Formation of Kaolinite from Montmorillonite at Low Temperatures

Abstract: Naand Al-montmorillonites under hydrothermal conditions at 175\" and 220oc for scopic data. The hydrated state of the kaolin when first formed is noted'