Showing papers on "Tridymite published in 1974"

Oxides and Hydroxides

Abstract: Oxidic minerals can only be recognized differential thermal analytically by the appearance of structural or magnetic changes. The high-low inversion of quartz (~573° C) has been for many years a means of calibrating the temperature, as well as for calorimetric measurements (following a proposal of Faust), but neither the inversion temperature (= ti) nor the heat of reaction (3.1 cal/g after Faust) can be used for calibration, since the ti can vary by a great temperature interval (Keith and Tuttle; Smykatz-Kloss, 1970, see III-8), and the heat of reaction is dependent on the degree of disorder of the crystals. Grim and Rowland have measured 565° for the ti of authigenically formed quartz crystals of soils. The ti-variation of the SiO2 minerals cristobalite and tridymite is still much greater than the ti-variation of quartz crystals (see III-7). Magnetites oxidize in two steps to Fe2O3, reflecting two very strong exothermic effects with peaks at 380 and 580° C. Shape and temperatures of these peaks are dependent on the grain size (Egger), and on some other factors (Kirsch). A weak endothermic effect at 680° C points out the Curie point of Fe2O3 (Egger); the temperature of this peak varies with different contents of titanium and manganese (compare with III-3). In DTA curves of hematite Peters obtained a broad endothermic deflection between 400 and nearly 700° C with a small endothermic peak at 675° C, which probably points out the Curie point of the Fe2O3.

A new interpretation of the structure of disordered α-cristobalite

Abstract: Disordered α-cristobalite which occurs extensively in bentonites, opals and particularly deep-sea cherts, has been previously interpreted in terms of a unidimensionally disordered structure in which cristobalite is interstratified with two-layer tridymite-like sheets. An alternative interpretation is that the structure is essentially tridymitic but that the sheets are stacked with random transverse displacement normal to the c axis, an arrangement similar to the translational, turbostratic stacking postulated for smectites. This interpretation was arrived at after a comparitive study of a silica phase in an Italian bentonite and a deep-sea chert, material which yielded an X-ray powder pattern almost identical with that of disordered α-cristobalite, but electron diffraction patterns and infrared spectra more consistent with tridymite. It is suggested that this type of silica, which has been described almost universally as cristobalite, is more appropriately referred to as disordered α-tridymite.

A New Interpretation of the Structure of Disordered ~-Cristobalite

Abstract: Disordered c~-cristobalite which occurs extensively in bentonites, opals and particular- ly deep-sea cherts, has been previously interpreted in terms of a nnidimensionally disordered structure in which cristobalite is interstratificd with two-layer tridymite-like sheets. An alternative interpretation is that the structure is essentially tridymitic but that the sheets are stacked with random transverse displacement normal to the c axis, an arrangement similar to the translational, turbostratic stacking postulated for smectites. This interpretation was arrived at after a comparitive study of a silica phase in an Italian bentonite and a deep-sea chert, material which yielded an X-ray powder pattern almost identical with that of disordered ~-cristobMite, but electron diffraction patterns and infrared spectra more consistent with tridymite. It is suggested that this type of silica, which has been described almost universally as cristobalite, is more appropriately referred to as disordered ~-tridymite.

Electron microprobe analysis of some minor- and accessory components in mesosiderites

Abstract: Analyses of chromite, ilmenite, whitlockite, farringtonite, schreibersite, and tridymite from four mesosiderite specimen were made using electron microprobe techniques. These minerals are chemically homogeneous with the exception of chromite, which exhibits slight zoning and grain to grain compositional variation in the same specimen. It can be assumed that schreibersite, troilite, and phosphates formed by solid state nucleation from α-NiFe or solid state reactions between mixed metal and silicate fragments at temperatures of approximately 600–700°C during a stage of thermal metamorphism. Chromite was associated with the silicate fraction before metal and silicate fragments have mixed. It is shown that the results of this work are consistent with the theory of mesosiderite genesis evolved byPowell (1969, 1971).