Showing papers in "American Mineralogist in 1972"

Determination of Liquidus Relations in Synthetic Silicate Systems With Electron Probe Analysis: The System Forsterite-diopside-silica at 1 Atmosphere

Abstract: A new method of determining the equilibrium liquidus diagram with the use of electron probe analysis has been applied to the petrologically important system forsterite (Ms,SioJ-diopside (CaMgSLOu)-silica at I atm and has been found to be very efrcient. The analysis shows that the diopside solid solutions in this system have a small amount (less than 5 weight percent) of forsterite component near liquidus temperatures. The analysis has confirmed the incongnrent melting of pure diopside that has been used for a secondary standard in the temperature scale, the existence of iron-free pigeonite on the join diopside-enstatite (MgSiOs) between diopside 6 enstatite 94 and diopside 23 enstatite 77 (weight percent) at the solidus, and the nonbina.ry nature of the joins forsterite-diopside and diopsidesilica.

Laboratory Dissolution of Plagioclase Feldspars in Water and Organic Acids at Room Temperature

Abstract: Freshly fractured albite, oligoclase, labradorite, bytownite, anorthite, and a high-K variety of plagioclase in particle sizes between 4r[ pm and 150 arm were disolved at roorn temperature in deionized water, 0.01 M acetic and aspartic acids (weakly complexing), and salicylic and citric acids (strongly complexing). Solutes of centrifuged solution were analyzed for pI[, and for Si, Al, Fe, Mg, Ca, Na, and K. Ca-rich plagioclases dissolved in organic a,cids more readily than did Na-rich plagioclases, whereas Na-rich plagioclases were the more soluble in deionized water. AI was preferentially dissolved over Si in particular from Ca-rich plagioclase in complexing acids. Ca was relatively more soluble than was Na in both organic acids and water. The organic solvents, in order of increasing effect of dissolution, were acetic, aspartic, salicylic, and citric acids, which is the order of complexing capacities. Citric acid was more effective t.han other acids in extracting Al and Ca, particularly from Ca-rich plagioclases, presumably because of the formation of Aland Cacomplexes.

Atomic Arrangement of Merwinite, Ca3Mg[SiO4]2, an Unusual Dense-packed Structure of Geophysical Interest

Abstract: Merwinite, Ca"MglSionlr, is an important and persistent phase in petrological systems crucial to mantle and crustal chemistry and to the cement and blast furnance industries. Despite numerous studies on its crystal chemistry, all reports recorded heretofore in the literature are erroneous in various details. The crystal cell data are: space group P\/a, a 73.254(21), b 5.253(il, c 9.328(17)A, p 91.90(15)', Z 4. The unit formula, CaoMgtSi0l", constitutes the asymmetric unit of the crystal structure with all atoms in general positions. Solution of the structure proceeded from Patterson and p-general syntheses, and led to ,B(hftl) 0.061 for 23O0 independent reflections.

Cation determinative curves for mg-fe-mn olivines from vibrational spectra

Abstract: Infrared spectra in the region 4000-200 cm-1 have been measured for several a.nnlysed olivine minerals. Ten or more bands arc observed below 1000 em-•, the frequencies of wl1ich are composition dependent and move to lower energies witl1 increasing Fe and Mn in the olivine. Best-fit equations relating peak ma....Urna to olivine composition were computed for each of the three binary series and a triruJgular determinative grid constructed for Mg-Fe-Mn olivines. Deviations from linearity in certain determinative curves of Mg-Mn and Fe-Mn olivines correlate with Mn•• ordering in M(2) positions. The accuracy of an olivine composition determinative curve is inversely propor tional to the frequency shift. be!;ween end-member compositions. In the forsterite-fayali te eries, theo com­ positions estimated from bands 4 and 5 in the infrn.red spectra. are ru close to the true compositions as estimates based on established X-ray d spacings and cell parameter data.

Free Energies of Formation and Aqueous Solubilities of Aluminum Hydroxides and Oxide Hydroxides at 25°C

Abstract: comparison of the standard free energies of formation at 25'c of corundum, diaspore, boehmite, gibbsite, and bayerite estimated from carefully selected uq,ruor6 solubility data with those of calorimetric origin .permits selection of a set which is more consistent with stabilities observed in field and laboratory than the calorimetric set alone. The selection process involves acceptance of -378'2 and -1160 kcal per mole for AG'1 of aALo\" and Al\"* (aq) respectively and assignment of -311'0 kcal per mole for the AG'1 of AI(OE); (aq)' Selected values for the solids are:

Diffuse reflectance spectra of several clay minerals

Abstract: The diffuse reflectance spectra in the 0.3 to 2.1 micrometer wavelength region have been measured for several sa,mples of clay minerals sometimes found in the atmospheric aerosol. Exa.mples of spectra are shown for different hydration states of several montmorillonite and kaolin group minerals. It was found that these clays show absorption features in the wavelength regions where liquid water absorbs. These absorption bands are removed by dehydration, as would be expected. It is concluded that the major features of the near infrared absorption spectra of montmorillonite and kaolin clays are determined primarily by the water present in the clay stmctures. In addition, the spectra show evidence of strong absorption in the near ultraviolet and short wavelength visible region. This latter absorption is a general feature of all the clay samples investigated.

Diaspore-corundum Equilibrium Determined by Epitaxis of Diaspore on Corundum

Abstract: The pressure-temperature equilibrium curve between diaspore and comndum and vapor was experimentally determined above Bg0\" c and l.b kbar in two ways, by observing presence or absence o.f epitaxial growth of diaspore crystallites on corundum grains and by recording weight changes on single crystals of corundum. Equilibrium points were used to derive thermodynamic data for diaspore, which allowed calculation of the equilibrium curve below 890\" c. This curve passes through 1 bar at 220. C, 120 bar at 860\", 24@ ba,r at 40b\", and 7500bar at460\".

Stability of Acmite-Jadeite Pyroxenes at Low Pressure

Abstract: pJnoxenes based on a pressure of breakdown of pure albite to jadeite f quartz of 16.7 kb at 600\"c and of 14.5 kbar at s00'c. rf there is solubility of iron-albite in albite at 600\"C, it is probably less than 4 to S mol percent. Acmitic pyroxenes occurring in low-pressure, high-temperature, silica-saturated environments sucli as alkali granites and related pegmatites will be lowest in jadeite component. Acmitic pyroxenes in silica-undersaturated igneous rocks should show appreciably more jadeite component if they equilibrated with albite and nepheline. For the silica-saturated case, acmitic pyroxene can show high jadeite component only in high-pressure, low-temperature environments. Acmitic pyroxenes co-existing with albite and quartz might be used as a pressure indicator if an independent estimate of temperature can be made.