Tridymite

About: Tridymite is a research topic. Over the lifetime, 840 publications have been published within this topic receiving 14831 citations.

Carbon Dioxide at High Pressure and Temperature

Abstract: We present a review of recent studies of carbon dioxide (CO 2 ) at high pressures and temperatures and describe the experimental evidence for a dipole interacting bent CO 2 phase IV and for a non-molecular extended phase V. Phase IV is stabilized above 12 GPa and 1000 K; the Raman activation of the internal bending mode (v 2 ) indicates a loss of molecular inversion symmetry in this new phase. Above 35 GPa, and 1800 K. the CO 2 molecules associate to form a covalent solid (V) with a structure based on singly bonded CO 4 tetrahedra, similar to that of SiO 2 tridymite, We discuss the stability and properties of this polymeric form of CO 2 .

The mineralogy of the Tokomaru silt loam and the occurrence of cristobalite and tridymite in selected North Island soils

Abstract: The origin of cristoba lite has been invest igated in a se lect ion of soils f rom North Island . These include Tokoma ru silt loam in the Manawatu , and Hamilton clay loam, Naike clay and Te Kowhai s i lt loam in the Hamilton basin . The occurrence of cristobal ite , in con j unct ion with a s s ociated sand and silt mineralogy, has been used to interpret both the provenance of soil parent materials and aspects of soil gene s i s . Cristoba lite isolated from the soils is a-cristobalite ( opal-C) and is inva riably accompanied by t ridymite ( t ridymite-M st ructure ) . SEM data suggest that rather than discrete phases , cri stoba l ite and t ridymite usually occur intergrown with each other . Togethe r they const itute up to 15% of the soils examined and usually occur in higher concent rat ions in upper soil hori zons . Oxygen isotope abundances for cristoba lite and tridymite range from 7 . 5 -8 . 4 °/00 and are independent of grain s i ze and relat ive proportions of c ristobalite and t r idymite. These data , together with the highly ordered c ryst al st ructure and subhedral morphologies of grains , indicate that c r istobalite and tridymite formed at high temperature . Both are shown to occur in a number of the rhyolitic tephras f rom the Taupo Volcanic Zone ( e . g . Aokautere and Okareka ashes) and leucocratic lavas from the Egmont cent re . Cristobalite and t ridymite in the soils the refore probably o riginated from volcanic sources . Various forms of amorphous silica were also identified in the soils of the Hamilton basin . In upper soil horizons, microfossils and phyt o l iths dominate but at depth the Naike c lay and Hamilton clay loam cont a in two inorganic forms of amorphous silica . These comprise

Irradiation-induced amorphization of AlPO4

Abstract: AlPO 4 , in the mineral form berlinite , is isostructural with α-quartz. We have investigated the irradiation-induced amorphization of hydrothermally-grown berlinite and found that — like quartz and other silicas but unlike most other phosphates — it undergoes solid-state radiolyis, with an efficiency fifty times that of quartz at room temperature, and amorphizes at an absorbed ionization dose of about 1 GGy. High-resolution TEM revealed that — unlike quartz in which small amorphous inclusions nucleate — electron-irradiated AlPO 4 proceeds uniformly to an aperiodic state, much as do cristobalite and tridymite, and 20 times faster. It was found also to amorphize under 1.5 MeV Kr + ion irradiation at a collisional energy density (10 eV/atom) similar to that for quartz and in keeping with the degree of structural freedom afforded by its tetrahedral network structure. The critical ion fluence for amorphization was found to increase by a factor of 5 between 300 and 600 K. Radial distribution functions derived from energy-filtered electron diffraction patterns from regions amorphized by electrons resemble those of electron-amorphized quartz with some additional features.

The opal-CT nanostructure

Abstract: Opal-CT, a silica polymorph in marine sedimentary siliceous rocks, exhibits commonly a form of spheres about ten micrometers in diameter (“lepispheres”). Quantitative infrared analyses were performed with a special attention to the band related to the Si O Si angle and to the [SiO4] tetrahedral framework. Opal-CT includes about 40% non-crystalline structures mixed with crystalline ones related to tridymite. Under TEM a single opal-CT lepisphere section exhibits a nanostructure constituted of a stacking of domains 1.5 nm thick, alternatively crystalline and non-crystalline. The crystalline domains match with a monoclinic form of tridymite which pass to non-crystalline domains with a distortion of the Si O Si angle. The regular stacking of crystallites 1.5 nm thick composed with about three tridymite modules is assigned to a tridymite superstructure. The paradoxical crystallization at sea water temperature of a high temperature mineral is here explained by the polymerization of silica dimers from sea water enriched with dissolved silica. Opal-CT hydrothermal in origin differs from the sedimentary one by its infrared spectrum and its optical properties that matches with the silica polymorph lussatite. It is inferred that the hydrothermal opal-CT exhibits a structure significantly different from the sedimentary one (pseudo-orthorhombic, even orthorhombic).