Search

Cristobalite and tridymite in andesite and obsidian exhibit bright blue emissions in color cathodoluminescence (CL) images. CL spectra of cristobalite and tridymite show emission bands at 400 and 430 nm, respectively. Their CL intensity decreases with an increase in the electron irradiation time, similar to the short-lived blue luminescence reported in hydrothermal quartz. The reduction rate of the CL intensity of hydrothermal quartz is small as compared to cristobalite and tridymite. The initial CL intensity of both minerals immediately after the electron irradiation is positively correlated with the Al2O3 content. This suggests that the blue emissions of cristobalite and tridymite can be attributed to the [AlO4/M+]0 defect center (M: H+, Li+, Na+, and K+). The electron irradiation of cristobalite and tridymite diffuses the monovalent cations of H+, Li+, and Na+ and destroys their crystal structure, resulting in short-lived blue luminescence. The CL intensity after 600 s of electron irradiation depends on the K2O impurity content. In contrast to other monovalent cations, the K+ cation is not diffused because of its large ionic radius. The CL emissions of cristobalite and tridymite after 600 s of electron irradiation can be attributed to the [AlO4/M+]0 defect (M: K+). CL images of cristobalite in obsidian from Utah, USA, show a heterogeneous distribution of CL intensity, with oscillatory CL zoning on the rim and radial lath-shaped textures with bright CL superimposed on weak luminescence background. This implies that lath-shaped textures with bright emissions might have crystallized from rhyolitic magma at temperatures higher than the recrystallization temperature minerals in the other areas.

Unit-cell parameters, Raman scattering profiles and DTA curves were measured on monoclinic tridymite from fired silica brick. Ten peaks appear in the Raman measurements from 100 to 1250 cm at 25°C, and their positions and widths show no significant change up to 110°C. The monoclinic unit-cell parameters, a, b, c and , slightly increase or remain nearly constant with increasing temperatures up to 110°C. Both the Raman spectra and the unit-cell parameters undergo significant changes in the range from 110 to 115°C. Only two endothermic peaks, at 117 and 166°C, appear in the DTA curve measured on a powdered sample of nearly pure monoclinic phase in a heating run from 25 to 300°C. In X-ray single-crystal refinements, the structural parameters including anisotropic temperature factors of atoms were fully determined for the monoclinic phase at 25, 80, 90 and 100°C, and the orthorhombic phase at 140°C. The difference between the temperature factors on both sides of the transition point is the most remarkable: 0.0286 A in average for all O atoms at 100°C in the monoclinic phase and 0.0713 A at 140°C, both for Beq. The averaged Si-O distances calculated for the atomic mean positions decrease with increasing temperatures: 1.599 A at 25°C, 1.595 A at 100°C and 1.576 A at 140°C, whereas the values corrected for the thermal displacement parameters remain unchanged at 1.61 A through all the temperatures studied.

The experimental Raman spectra of monoclinic tridymite and its higher temperature orthorhombic modification were successfully simulated with valence force fields applied to the X-ray determined structural parameters. The normal modes contributing to the corresponding Raman peaks in both the phases share essentially the same types of atomic displacements or bond deformations. In the modes with the wave numbers between 100 and 600 cm−1, predominant are the librations of Si-O-Si planes around Si-Si lines, being followed by bending of Si-O-Si or O-Si-O angles. The librational motions of SiO4 units around Si appear in the modes below 100 cm−1. Significant bond stretching and compression appear in the modes with wave numbers higher than 700 cm−1: the deformations are classified into 10 types, 7 for Si-O-Si and 3 for SiO4. Some modes in the highest frequency group between 1150 and 1250 cm−1 show symmetric bond stretching and compression of SiO4 units, while those between 1000 and 1150 cm−1 antisymmetric stretching and compression of Si-O-Si. A low-frequency normal mode is identified to have the polarization vectors of O atoms, which are nearly parallel with the largest principal axes of their thermal ellipsoids. A disorder model is presented to account for the extremely large mean square displacements of O atoms, and a possibility is suggested for the normal mode to boost the hopping of the SiO4 units between the two orientations.

In order to elucidate many geophysical problems induced from the global techtonics, dynamics of the mass transfer has attracted an interest. In association with the seismic researchs, material transformations, such as transition, decomposition, recrystallization, melting, etc. have been also greatly intrested under the conditions of heating, compression and stress field. Pressure-induced amorphizations of many minerals such as quartz, cristobalite, tridymite, anorthite, serpentine, fayalite and several other hydrates have been reported. These amorphizations and shear-instability will play a role to conduct the transformational ductile flow in the subduction zone, which would be the origin of the deep earthquake. Several different types of the pressure-induced amorphization were proved by the X-ray diffraction studies under high pressure and heating condition using multianvil high-pressure appararus and diamond anvil pressure cell. If the amorphization is the quenchable case, the pressure-quenched amorphous substance was examined by electron microscope, Raman spectrosope, EXAFS together with X-ray diffraction study.

The effects of the addition of a small amount of MgCr2O4, (0.3 wt.%) to the system Mg2SiO4 (forsterite)-CaAl2Si2O8(anorthite)-CaMgSi2O6(diopside)-SiO2 at 1 atm have been studied. The system is an important model to the crystallization and generation of tholeiitic basalt. The primary field of spinel is enlarged compared with that in the Cr-free system at the expense of anorthite primary field, and as a result forsterite and anorthite do not coexist at the liquidus temperatures. Two invariant points have been confirmed : one at about 1298°C shows liquid coexisting with forsterite, protoenstatite, diopside, and spinel, and the other at the temperature below 1265°C shows the assemblage of spinel, protoenstatite, diopside, tridymite, and liquid. The first mineral assemblage corresponds to that of spinel lherzolite. In the Cr-free system this assemblage is not stable at 1 atm and an invariant point having plagioclase lherzolite mineral assemblage is present. It is generally said that the transformation from spinel lherzolite to plagioclase lherzolite takes place at about 8 kbar at 1000°C. The results obtained in the present study indicate that the presence of small amount of Cr in magma significantly reduces this transformation pressure. Another important aspect of the result is that the position of the invariant point is possibly moved by the change of bulk Cr content in the system. If the liquid generated by the partial melting of the spinel lherzolite mantle has the composition of this point, Cr content of the mantle would give significant effect to the composition of magma.

Kalsilite shows the domain structure corresponding to the disordered oxygen atoms at two mirror equivalent sites. The domain structure was considered to be due to the high-low inversion, and the structural model of the high-form was constructed. The heating experiment was carried out, which revealed that the X-ray powder pattern changes at 1138 K. It was confirmed that kalsilite undergoes a displacive transformation like quartz and tridymite. The high-form is hexagonal, probably P63mc, with a=5.30(1), c=8.65(1)A. The high-form was also considered to be divided into two groups, high-kalsilite (P63mc) and disordered high-kalsilite (P63⁄mmc). The Zellengleichen subgroup relations are found among these three polymorphs.

Monoclinic tridymite was found in volcanic rocks from three localities; Yugawara, Harius and Mt. Myoho in Japan. The unit cell parameters of the Yugawara tridymite are a=25.89, b=5.00, c=18.59A and β=117.7°, and space group is Aa or A2⁄a. This monoclinic tridymite was transformed to the form with n=10 by rapid cooling from high temperature.

Precious and common opals from the Hosaka mine, Fukushima Prefecture, Japan were studied by means of optical and transmission electron microscopies. The precious opal showing weak play of color of red, green and violet consists of amorphous silica sphere, whose diameter is from 400 to 500 A. Some spheres elongate slightly in one direction, and their packing is not regular. The origin of the play of color in this opal cannot be explained by diffraction of light in regular packing of silica spheres. Commom opal, whose x-ray diffraction pattern corresponds to opal-CT (Jones et al. 1971), consists of short or long acicular crystals which have nearly constant diameter about 200 A but vary in length from 1000 to 100, 000 A in length. The electron diffraction patterns show these crystals to be disordered high tridymite.

Liquidus phase relations on the join forsterite (Mg2SiO4)-anorthite (CaAI2Si2O8)-silica (SiO2) with 0.3 wt% MgCr2O4 were determined in air at 1 atm by ordinary quenching method. Spinel primary field is considerably expanded compared with that in the Cr-free forsterite-anorthite-silica join at the expence of anorthite primary field. As a result, phase assemblages at the points at which 4 phases coexist are completely changed : the assemblages forsterite+spinel+anorth. ite+liquid (piercing point), forsterite+ protoenstatite+ anorthite +liquid (peritectic point), and protoenstatite+anorthite+tridymite+liquid (eutectic point) on the Cr-free join disappear and the assemblages f orsterite +spinel+protoenstatite+liquid and protoenstatite+cristobalite+spinel+ liquid (both are piercing point) are present. However, the phase boundaries between the fields of forsterite and protoenstatite and those of protoenstatite and cristobalite are not affected by the addition of chromium. On the basis of the present data and those of the join forsterite-anorthite-diopside with 0.3% MgCr2O4 (Onuma and Tohara, 1983), phase relations of the system forsterite-anorthite-diopside-silica with 0.3% MgCr2O4 are inferred and its petrologic significance is briefly discussed.

Tridymite from the Steinbach meteorite is monoclinic with cell dimensions: a 18.54, b 5.00, c 23.83 A, β 105.70° and space group C2⁄c or Cc. Its diffraction pattern shows an extremely large number of structural absences, leading to pseudo-orthorhombic superlattice with cell dimensions A 51.86, B 30.00, C 49.20 A. The method of twin analysis (Ito and Sadanaga, 1970) was applied uniquely to derive a crystal lattice from the twin pattern. Pseudohexagonal twinning about the [-201] row of the true monoclinic lattice satisfactorily accounts for these unusual structural extinctions. The symmetry of the twin is represented by the six-color symmetry symbolism. A possible mechanism of twin formation is proposed, based on the recently determined low tridymite structure.

Siliceous hard shales, which were found from the Gamanosawa Formation of Tertiary age, Aomori Prefecture, Japan, are mostly composed of opal-CT. The name of the opal-CT rock is used here instead of the hard shale. The content of SiO2 is in the range of 83 to 94 wt %. Broad diffused X-ray diffraction patterns correspond to the peaks of opal-CT. The spacing of 4 A peak is in the range of 4.10 A to 4.11 A. Bulk density is in the range of 0.90 to 1.25, and apparent porosity in the range of 40 to 60 volume %. Crystallinity of the opal-CT rock is too poor to reveal phase transition. However, when it is heated over 1250°C in air, the crystallinity is improved and reveal phase transition at temperatures ranging from 182°C to 220°C. The spacing of 4 A peak progressively decreased from 4.11 A to 4.05 A by heating it at high temperatures. By hydrothermal treatments at 500°C and 1000 kg/cm2 in a high pressure vessel, the spacing of 4 A peak also decreased from 4.11 Aring; to 4.05 A, and opal-CT gradually transformed into quartz. The opal-CT rocks may be a transitional stage from opal-CT to quartz due to diagenesis in nature.

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。

Usability of rice husk ash was investigated as a siliceous material for calcium silicate lightweight building materials manufactured by hydrothermal reaction. From the results, it was concluded that rice husk ash can be used as a superior siliceous material for manufacture of calcium silicate insulating materials with good thermal durability up to 1, 000°C. Characteristics of rice husk ash, viz, high SiO2 content, reactive silica phase comprising amorphous silica, cristobalite and/or tridymite, and high surface area, are favourable to the formation of well grown xonotlite which forms bodies of insulating materials. In addition, potentiality of rice husk and rice husk ash is briefly introduced as replenishable resources for industry.

The limonite ores from the Horaitakamatsu Mine may be classified into three types according to their external appearance and mineral compositions as follows: (1) Dark brown compact ores, composed of goethite only. (2) Light reddish clayey ores, characterized by the presence of hematite and the intricated mineral contents, namely, montmorillonite, α-cristobalite, quartz, tridymite, goethite and hematite. This complication is ascribed to the fact that the clay altered from volcanic ash mingles in this ores. (3) Dark yellow clayey ores, composed of goethite and quartz. A limonitic substance replacing a kind of moss is found in this mine, and from the results of its investigations, it is ascertained that it is not hydrous ferric oxide but basic ferric sulphate. The results were reported in another paper.

Phase-equilibrium studies of this system, at and just below temperatures where a liquid phase is present, show one ternary compound, the iron analog of cordierite (2FeO. 2Al2O3. 5SiO2), which decomposes at 1210°±l0°C. to mullite, tridymite, and liquid. The fields of stability of corundum, mullite, hercynite, iron cordierite, cristobalite, tridymite, fayalite, and wustite have been delineated. The following invariant points (the first two eutectics and the remainder reaction points) were located: Fayalite+wustite+hercynite+liquid 1148°±5° Fayalite+iron cordierite+tridymite+liquid 1083°±5° Fayalite+iron cordierite+hercynite+liquid 1088°±5° Hercynite+iron cordierite+mullite+liquid 1205°±10° Iron cordierite+mullite+tridymite+liquid 1210°±10° Corundum+mullite+hercynite+liquid 1380°±5° Cristobalite+tridymite+mullite+liquid 1470°±10° Iron cordierite crystallizes with some reluctance, and the metastable invariant points fayalite+tridymite+spinel+liquid at 1073°±5°, and mullite+hercynite+tridymite+liquid at 1205°±10° can be realized. No ferrosilite or almandine garnet could be crystallized from the melts at any temperature even when melts were seeded with these crystalline phases. Natural almandine from Botallack, England (91. 3 per cent almandine), when heated begins to decompose at an appreciable rate as low as 900°C. and yields a mixture of hercynite, iron cordierite, and fayalite. The bearing of these results on petrology and slags is discussed.

At a sudden explosion of Aso Volcano in 1958, about 50.000 tons of old ejecta which had been packed in vent, are erupted. Many kinds of sublimate minerals such as epsomite, ferroan pickeringite, melanterite, kirovite, pyrite, marcasite, chalcopyrite, sulphur, tridymite, chloromagnesite and molysonite are found on the surface or cavities of the ejecta. With the lapse of time after explosion, relative amount of volcanic gases HCl, H2S are decreased, on the contrary SO2, CO2, R and Rn are increased.