Showing papers on "Tridymite published in 1980"

Silica solid solution and zoning in natural plagioclase

Abstract: Microprobe analyses of plagioclase from some lunar basalts have anomalous stoichiometry as compared to terrestrial feldspars. The anomaly indicates the presence of the "excess silica" substitution, [ ]Si_4O_8. This coupled substitution leaves a vacancy in the feldspar lattice and is effectively SiO_2, dissolved in the feldspar. Detailed study indicates that core-rim [ ]Si_4O_8, zoning is a common phenomenon in plagioclase from lunar basalts. Anorthite-albite zoning is also typically present, so plots of [ ]Si_4O_8 as a function of anorthite content provide a convenient measure of the [ ]Si_4O_8 variations present. Most lunar basaltic plagioclase shows a monotonic increase in [ ]Si_4O_8 with albite content, typically ranging from 0 mole percent [ ]Si_4O_8 in the cores to 5-7 percent in the rims. This zoning is produced by two cooperating processes: a falling temperature of formation, and growth from a progressively siliceous liquid. In addition, rapid growth and delayed nucleation of plagioclase and silica phase appear to be important to the disequilibrium incorporation of [ ]Si_4O_8. By contrast, plagioclase from a wide variety of terrestrial occurrences lacks [ ]Si_4O_8. The Absence of [ ]Si_4O_8 in many terrestrial igneous plagioclases is primarily related to the presence of H_2O, which decreases the liquidus temperatures relative to those in lunar rocks. Terrestrial basalts which have comparable crystallization temperatures to their lunar counterparts lack [ ]Si_4O_8 in part because of an absence of strong in situ differentiation. In addition, plagioclase typically crystallizes prior to pyroxene (the reverse of the mare basalts), so kinetic effects have a reduced role. If a melt is in equilibrium with cristobalite (or tridymite or quartz), the silica activity is buffered, and the [ ]Si_4O_8, content of the plagioclase is directly related to the temperature. [ ]Si_4O_8, incorporation, however, is typically not an equilibrium process, so there is little hope of using it directly as a geothermometer.

On the split-atom model for hexagonal tridymite

Abstract: Probable oxygen positions in the structure of hexagonal tridymite are estimated by assuming Jess distortion of the silica tetrahedra, the centers of which are kept at the silicon positions in the hexagonal structure. This geometrical consideration leads to a tridymite framework characterized by puckered six-membered rings which have ditrigonal shapes in the projection along c. There are six ways to form the tridymite framework. This appears to support the split-oxygen model proposed by the X-ray study. It is suggested that the hexagonal structure of tridymite may be an average of micro domains, each having the framework structure just noted, distributed statistically among the six possible orientations. In the recent X-ray study of hexagonal tridymite at about 460° C (Kihara, 1978), the refinement, carried out on the Gibbs' structure (Gibbs, 1927), resulted in the oxygen atoms having highly anisotropic and large thermal motions (model I). The shortest axis of each oxygen ellipsoid was on a Si—Si line (root-mean-squared amplitude ~ 0.2 Λ) while the remaining two axes were nearly equal and large in magnitude (~0.4Ä). An alternative model (model II) has oxygen atoms which are located on six positions on a circumference normal to each Si—Si line with equal occupancy of the silicon atoms are located on the corresponding single positions as in the model-I structure. This was successfully refined and gave compatible atomic distances and angles with those in other silica structures. This split-oxygen model thus seems to be reasonable for describing the structure of hexagonal tridymite at about 460° C. The difference Fourier map, however, did not succeed in giving a clear resolution of these oxygen positions. These were separated by about 0.4 Ä in the model-II structure determined by the procedure of full-matrix, least-squares refinement. On the other hand, no refinements for split-silicon models were attempted because silicon was 96 Κ. Kihara: Split-atom model for hexagonal tridymite Table 1. Si —Ο and Ο — Ο distances determined by X-ray studies of ζ quartz, χ cristobalite, and monoclinic and triclinic tridymites. The averaged values are given in parentheses

Tridymite: Effect of hydrostatic pressure to 6 kbar on temperatures of two rapidly reversible transitions

Abstract: Trajectories of two reversible phase transitions in a low-Na synthetic tridymite have been determined to 6 kbar by differential thermal analysis (DTA) in hydrostatic apparatus using Ar or CO2. The temperature of the lower transition increases from ≈111 ° C at 1 bar linearly with pressure with slope ∼15 deg kbar−1. Pressure raises the temperature of the upper transition from 157 ±2 ° or 159 ° C (independently determined) at 1 bar witħ a slope of ≈53 deg kbar−1, up to ∼0.7 kbar; for the data above that pressure, the initial slope is ≈64 deg kbar−1. Above 2−1/2 kbar, the variation is linear with slope ≈70 deg kbar−1. No evidence for other transitions was found at any of the apparent changes of slope. Hystereses for both transitions decreased at high pressures compared to 1-bar. Preferred values for the transition enthalpies, together with these slopes and the Clausius-Clapeyron equation, yield estimates for the volume changes at the transitions of ≈0.01 (lower) and 0.15 to 0.25 (upper) cm3 gfw−1. These calculated volume changes are not consonant with many of the high temperature volumetric data on tridymites of varying origins.

Quartz glass optical fibre with high tensile strength - obtd. by coating rod blank with boric oxide, or boric oxide and silica, prior to drawing of fibre

Abstract: The fibre is made by drawing a quartz glass blank; and the bank is first coated with a layer (I) preventing the crystallisation of silica to tridymite and/or cristobalite on its surface Boric oxide, or boric oxide and silica, are pref used as layer (I) When a conventional blank is heated in a drawing furnace tridymite and cristobalite small crystals form on its surface and do not completely melt; on cooling, these crystals peel off, leaving microfissures which reduce the tensile strength of waveguide fibres The invention minimises the formation of such crystals

Incorporation of impurities in tridymites from a used silica brick

Abstract: The incorporation of impurities in tridymites taken from a used silica brick have been studied by means of a high-resolution microprobe, X-ray fluorescence, X-ray diffractometry, and optical microscopy. The bulk impurity content of the brick changes strongly from the hot zone (4.7 wt%) to the colder part of the brick (7.6 wt%), indicating material transport along the temperature gradient. The transport medium for migration processes is probably a melt occurring in narrow veins between large tridymite crystals at high temperatures. The average impurity contents of homogeneous tridymite single crystals are 0.49 wt% in the hot zone of the brick and 0.81 wt% in the cold zone. Al2O3, TiO2, and Na2O are main impurity constituents; the tridymites do not contain significant amounts of Fe2O3 or CaO. The a parameters of tridymites decrease by about 0.26% from 4.9837 A to 4.9709 A from the hot zone to the cold zone of the brick and the c dimension shows a smaller decrease (0.11 %) from 8.2023 A to 8.1933 A.

Change of X-ray diffraction pattern of tridymite by heating and cooling

Abstract: 単斜型6C tridymite(珪石レンガ,湯ケ原)と斜方型10C tridymite(石神山)の加熱変化を高温下でX線単結晶回折法と高温顕微鏡観察により検討した。珪石レンガ単斜型では室温から450℃ 間に5変態が存在し,貫井等の結果を確認した。また斜方10C型の加熱変化も貫井等の斜方5C型のそれと類似する。斜方10C型では,120～150℃ 間でO(6+I,1,1)相が室温相と共存しながら出現する。 また単斜型と斜方型の関係を調べるため,各tridymiteの加熱冷却実験を行なった。その結果,湯河原産単斜型は150℃ 以上の加熱冷却で斜方10C型に変化し,珪石レンガ単斜型は加熱急冷の場合のみ2C型に変化する。斜方型を加熱徐冷したが,単斜型への変化は観察されなかった。

Pressure-induced phase transitions in tridymite

Abstract: Phase transitions in tridymite were examined at pressures up to about 30 kbar by X-ray single-crystal and optical methods, using a hydrothermally synthesized monoclinic crystal of MC type tridymite and a natural crystal (from Kumamoto, Japan) of orthorhombic symmetry tentatively named PO-5 type. In MC type tridymite, a reversible pressure-induced phase transition has been observed at about 5 kbar without destroying the crystal. The unit-cell dimensions of the high-pressure phase were a:17.07, D: 9.81 and c: 81.26A in an orthorhombic lattice. The SiOo tetrahedron displacement on this phase transition is discussed. PO5 type tridymite did not change to any other phase.