Showing papers on "Tridymite published in 1973"

Cristobalite; its relationship to chert formation in selected samples from the Deep Sea Drilling Project

Abstract: Mineralogy and relationships of the silica minerals were studied in 96 pelagic siliceous sediments from Legs I through VI of the Deep Sea Drilling Project. Heating cycles at 650°C and 1010°C were used to convert opaline silica to cristobalite, for purposes of estimating opal content. Most of the free silica in these sediments is biogenic (radiolarians and diatoms). Silica deposition was especially abundant during Eocene time. The experimental evidence confirms earlier hypotheses for the diagenetic sequence: solution of biogenic silica within the sediment, precipitation of poorly-ordered cristobalite, gradual inversion to microcrystalline quartz (chert). Cherts formed in clay, silt, and biogenic siliceous ooze all contain cristobalite, as do siliceous mudstones. Cristobalite is absent from the cherts in nannofossil chalk, and from soft biogenic siliceous oozes. New findings include: 1) silica in a calcite matrix may crystallize directly to quartz, without intermediate cristobalite; 2) certain biogenic amorphous siliceous oozes will yield synthetic cristobalite at the surprisingly low temperature of 650°C; 3) cristobalite formed by diagenetic processes is characterized by mixed-layer tridymitic structure, whereas cristobalite formed by heating truly amorphous silica lacks "tridymite" layers.

Effect of silica concentration on the diopsidic pyroxenes in the system diopside-CaTiAl 2 O 6 SiO 2

Abstract: The effect of silica concentration on the solubility of Al and Ti in diopsidic pyroxenes has been investigated at one atmosphere in the system diopside-CaTiAl2O6-SiO2 at temperatures between 1150–1420° C. The composition of pyroxene in the system diopside-CaTiAl2O6-SiO2 is influenced by the total SiO2 content. Near the join diopside-CaTiAl2O6, the pyroxene forms a solid solution with the CaTiAl2O6 molecule, and co-exists with perovskite for compositions greater than 11 weight percent CaTiAl2O6. Anorthite is an accompanying phase. With increasing total SiO2 content a series of mineralogical changes involving Ti-bearing phases occur. Sphene solid solution co-exists with diopside solid solution, anorthite, and perovskite in a small compositional range near the diopside-CaTiAl2O6 join. Additional total SiO2 results in the elimination of perovskite and a decrease of solid solution in the pyroxenes. With further increase in SiO2 content, tridymite appears and the pyroxene is approximately pure diopside. Rutile joins diopside, anorthite, sphene solid solution and tridymite over a broad compositional range in the Ti, Si-rich part of the system. These results demonstrate that increased silica concentration decreases the solubility of Al and Ti in diopsidic pyroxenes and controls the stability of co-existing Ti-bearing phases.

Tridymite-like Crystals in Cristobalitic “Cherts”

Abstract: Weaver and Wise1 have shown that deep sea cristobalitic “cherts”, presumed to be precursors of quartz cherts, are composed in large part of blade-shaped crystals comprising microspheres 3–10 µm in diameter; these crystals were thought to be cristobalite. I have synthesized similar microspheres by hydrothermal crystallization of silica gel; however, the crystal habit of this synthetic material resembles that of tridymite rather than cristobalite. I suggest that both the synthetic microspheres and the deep sea microspheres may be composed of hybrid crystals of interlayered cristobalite and tridymite.

Petrographic Study of the Refractory Performance of High-Fusing Dental Alloy Investments: II. Silica-Bonded Investments

Abstract: Petrographic study of the refractory performance of silica-bonded investments revealed that recrystallized silica bonds consisting of tridymite and cristobalite are formed during burnout The major reaction product created during casting is eskolaite Liquefaction and sintering in the investment at and near its interface with the casting plugs pores and may lower permeability

Method of diffusing ions into quartz glass

Abstract: Quartz glass element, such as a diffusion tube useful in the production of semiconductor elements, capable of forming an outer layer of uniformly fine crystalline silica such as cristobalite or tridymite when heated to a temperature at which such crystalline silica forms containing crystallization promoting nuclei having a rate of diffusion in quartz glass less than that of sodium at elevated temperatures. Such nuclei are present in the outer half of the element wall and the maximum concentration of such nuclei is at the surface of the element and then diminishes inwardly. When the quartz glass element is exposed to elevated temperatures, the nuclei promotes the formation of the outer layer of uniformly fine crystalline silica which imparts thermal dimensional stability for extended periods of use at elevated temperatures.

Problem of the thermal conductivity of coking Dinas

Abstract: Dinas in service in the wall regions of coking furnaces acquire a zoned structure with different thermal conductivity levels in the zones. The thermal shock resistance of the hot zone is about four times less than that of the remaining zones. The thermal conductivity of the hot zone is determined by the extent of the tridymite crystals in the direction of the thermal current and the degree of development of the direct bond of crystal-crystal. In the zone arranged on the coking side the Dinas is carbonized, preventing growth of crystals of tridymite. The porosity in the range 15–25% does not have any decisive influence on the thermal conductivity of the coking Dinas.