High-temperature Silicate Research Articles

Dissolution of Mafic Minerals and its Implications for the Ascent Velocities of Peridotite-Bearing Basaltic Magmas

Crystal dissolution in high temperature silicate melts is typically controlled by diffusion across a boundary layer in the melt next to the crystal. Experimental studies on basaltic systems show a decreasing rate of dissolution from garnet, clinopyroxene, orthopyroxene, to olivine, with the dissolution rate of garnet about 10 times that of olivine at pressures up to 20 kbars. Preferential dissolution is exhibited by many upper mantle-derived spinel peridotite xenoliths in which embayment of orthopyroxene relative to the juxtaposed olivine grains occurs at the xenolith-host basalt contacts. By combining the data on dissolution rates as a function of pressure, the relationship between the path of adiabatic ascent and the liquidus, and the density and viscosity of basaltic melt, we establish here formulations relating the amount of peridotite dissolution to the ascent times and velocities of alkali basalts in continental and oceanic settings. The results show that alkali basalts may ascend at velocities of u...

Mid- and far-infrared extinction coefficients of hydrous silicate minerals

Extinction coefficients were measured for three kind of hydrous silicate minerals, montmorillonite, chlorite and serpentine, from 7 to 140 μm. The infrared extinction coefficients of these minerals show (1) a few broad bands in the mid-infrared region and (2) a less steep wavelength-dependence in the far-infrared region, in contrast to those of high-temperature magnesium silicates. In the far-infrared region, montmorillonite shows a λ−0.8±0.1 dependence (λ, the wavelength) without any band structure, chlorite has a double maxima structure around 80 μm, and serpentine shows a rather steep dependence with a small peak at 77 μm.

Measurement of the polarized absorption spectra of synthetic transition metal-bearing silicate microcrystals in the spectral range 44,000-4,000 cm?1

A new single beam microtechnique has been developed for measuring the polarized absorption spectra in the region 44,000-4,000 cm−1. Spectra of a natural garnet (Spess70Alm30), measured by the microtechnique and by conventional macrotechniques, are consistent and thus prove the applicability of the microtechnique described. It is possible to obtain well resolved spectra down to about 13,000 cm−1 with crystals as small as about 10 μm. Thus spectra of crystals obtained in routine high-pressure high-temperature silicate syntheses can be measured. The polarized spectra of Mn3+, Fe3+, Fe2+, and Cr3+ in the following synthetic silicate minerals are presented: piemontite (I), acmite (II), orthoferrosilite (III), and kyanite (IV) or uvarovite (V), respectively. O-Cr3+, O-Mn3+, and O-Fe2+ charge transfer band maxima in the UV region are identified at 38,700 cm−1, in V; at 33,200, 35,300, and 39,000 cm−1, in I; and at 32,800, 35,200, and 37,300 cm−1, in III, respectively. Bands in the region ≦25,000 cm−1 are assigned to spin-allowed and spin-forbidden dd transitions as predicted from crystal field theoretical considerations for the foregoing ions in the respective structures.

Fused Glass Penetration into Thermally Grown Silicon Dioxide Films

In the manufacture of fused glass protected planar silicon devices having an underlying barrier layer of silicon dioxide, it is desirable to know the degree of penetration of the glass into the underlying layer as a results of fusing the sedimented glass powder. Penetration depths were determined from etch rate plots using P etch which etches most glasses much more rapidly than silicon dioxide. The penetration of low temperature firing lead borosilicate glasses (<600°C for 5 min) can be limited to less than 100Aå. For high firing temperature silicate glasses (>1100°C for 5 min), a penetration of a few hundred angstroms is more common. Glasses fired at temperatures significantly higher than their softening points (or minimum firing temperatures) show much greater penetrations. The results are explained on a model based on the fact that the softening point of silicate glasses increases sharply with silica composition. As the glass softens, the lower layer of glass fuses with the upper surface to form a thin boundary glass layer with greater silica composition. As the penetration increases, the silica content of this intermediate layer increases sharply and consequently the effective softening point for the region becomes so high that further penetration is severely limited.

Structure of the ore deposits at Santa Barbara, Chihuahua, Mexico

The Santa Barbara mines are grouped in a circle around the village of Santa Barbara located in the Parral mining district, in southern Chihuahua, Mexico. The mines are operated by American Smelting and Refining Company and the bulk of the mineral production comes from eleven vein systems.The pre-mineral rock types consist of a thick calcareous shale formation and andesite flows. The post-mineral rock types consist of dikes and sills of rhyolite and diabase, a thin conglomerate formation, basalt flows, and unconsolidated stream sediments. Pre-mineral faulting took place in two stages, forming four fault systems. Any fault within one system is similar in both strike and dip to another fault within that system. Movement along these faults, vertical in the first-stage faults and horizontal in the second-stage faults, formed openings, breccia zones, and in places horses and wedges of country rock in the faults. The location and explanation of these openings, breccia zones, horses and wedges are the main topic in this paper.Hydrothermal solutions, emanating from depth, were introduced into the faults. The walls and breccia fragments within the faults were silicated and silicified and the high-temperature silicates, garnet, pyroxene, epidote, and idocrase (?) were formed. Accompanying and following the formation of the silicates, the sulfides sphalerite, galena, chalcopyrite, pyrite, and arsenopyrite, with associated gold and an unknown silver mineral were introduced with quartz, calcite, and fluorite. Most of these minerals replaced the silicates and altered shale. The parts of the faults where wide pre-mineral openings, horses or wedges were formed, were filled with quartz and a higher ratio of sulfides than the narrow portions of the faults. Quartz, calcite, fluorite, and barite were among the last minerals deposited. The veins are assigned to the hypothermal class of hydrothermal deposits.