Tridymite

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

Coexistence of tridymite polymorphs and cristobalite in tridymite crystals

Abstract: flexions are also visible on X-ray photographs using longer exposure times. Together with these chanqes there appears a stroNq increase of the dl ffuse background in the pow.der diagrams showing a characteristic modulation, . which can be connected with overdamped soft modes at the·l'-point and in (~\ 0) -directions observed by inelastic neutron scattering (Boysen,Eckold, Symp. on Neutron Scattering, Berlin, 1984). En conclusion the transition is characterized by a rotational rearrangement of (nearly) rigid (Si,Al)04-tetrahedra combined

Microstructural Behaviour of Dissolution of Quartz Grains into Feldspathic Fusions

Abstract: In his previous paper, the writer has reported a quantitative investigation on the behaviours of dissolution of coarse quartz gains into feldspathic fusions, one of the most important reactions occurring in porcelain bodies during firing (this Journal 63 [712] 432 (1955)). The present one gives the results of a study on the same subject from a microstructural point of view.By firing the small tablets consisting of 97% fine ground potash-, or soda-feldspar and 3% coarse quartz grains (Table 1), one fired at 1150°-1450°C, with heating rates of 60°, 120°C, and 180°C per hr., and the other with 0.5-4 hrs. soaking at 1300°, 1350°, or 1400°C. They were studied with aid of polarization microscope, and occasionally by means of X-ray.The results obtained were summarized as follows:(1) The reaction between quartz and feldspar started from a little lower temperatures than 1150°C, with the formation of very thin reaction layers between them.(2) Refractive index distributions of glasses formed in the fired bodies were measured. From the results obtained, it is inferred that the uniformity of the melting phases formed in the bodies of quartz-potash-feldspar system were raised with the increase of firing temperatures and holding times (Fig. 4), and that in the case of the bodies of quartz-soda-feldspar system, unhomogeneity of the phases was, on the contrary, rather increased with their SiO2 content being also gradually increased with temperature and time (Fig. 5). These differences were mostly due to the difference of diffusion velocity of SiO2 which entered into the melting phases accompanied with dissolution of quartz grains.(3) In soda-feldspar fusions, the diffusion layers formed around the quartz grains temporarily showed double construction layers (Fig. 1-9), and, with soaking time, these layers gradually changed into continuous ones with the increase of their thickness. The temporary formation of these double diffusion layers was due to the fusion of soda-feldspar itself, whose fusion behaviour showed a marked change at a certain temperature range.(4) Quartz grains kept their phases metastablly until they dissolved at 1150°-1450°C., and converted to neither cristobalite nor tridymite, though a weak pseudo biaxial interferrence figures which were perhaps owing to the stresses formed with rapid cooling, were observed (Table 2). In the diffusin layers and the glassy phases, cristobalite or tridymite were also not found.

X-ray diffractometric determination of tridymite in silica refractories

Abstract: L'article est une etude critique des possibilites d'estimer la teneur en tridymite dans les refractaires de silice par diffraction des rayons X (XRD). A l'occasion de l'examen du polymorphisme de SiO 2 dans des briques de silice ayant servi pendant environ 25 ans dans les parois d'un four a coke, on a montre que des briques de silice ayant les memes teneurs en polymorphes de SiO 2 (a savoir : quartz, cristobalite et verre de silice) conduisent a des taux de comptage differents pour la tridymite. L'observation microscopique des briques montre que l'intensite diffractee est liee, plus ou moins directement, a la taille des cristaux de tridymite, une taille qui depend des conditions de temperature lors de la formation de tridymite. Cela signifie que des echantillons de reference (des standards) pour la tridymite doivent etre prepares selon les memes conditions que celles subies par les echantillons etudies, faute de quoi l'estimation par XRD de la teneur en tridymite peut etre erronee.

Very-dense silica minerals in the Shergotty SNC meteorite: Evidence for extreme shock pressures

Abstract: Electron and X-ray diffraction confirm that SiO2 in Shergotty consists of two very dense SiO 2 polymorphs in addition to stishovite. All three high-density structures represent quench phases from post-stishovite polymorphs that are stable from 47 to > 80 GPa. Introduction. Shock metamorphism of meteorites and terrestrial samples is characterized by deformation microstructures and high-pressure minerals and glasses[1, 2]. Quartz is a important indicator of shock metamorphism because it forms planar deformation features (PDFs) [1-6] and transforms into the high-density polymorphs coesite and stishovite [7-8, 1-2, 6]. Experimental [9-14] and theoretical [14-18] studies of SiO2 at high pressure have shown that there are several “post-stishovite” structures that are more dense than stishovite. Natural examples of dense SiO2 phases were discovered in Shergotty and interpreted as “post-stishovite” structures that were produced during the shock metamorphism of Shergotty [19-21]. The dense SiO2 was initially thought to have the αPbO2 structure [20], but subsequent diffraction experiments have shown more complex structures [19, 21]. Here we combine field-emission scanning electron microscopy (FESEM), powder X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) to characterize two new SiO2 structures and discuss these polymorphs in terms of the peak shock pressure experienced by Shergotty. Petrography. Silica in Shergotty mostly occur as large (>150 μm) wedge-shaped grains typical of β-tridymite. They are embedded in clinopyroxene or between clinopyroxene, mesostacis, and "maskelynite". Each grain is surrounded by radiating cracks that initiate at the surfaces of the silica grains and penetrate deep (up to 600 μm) in the Shergotty matrix (Fig. 1). The radiating cracks are similar to those reported from ultra-high pressure metamorphic rocks around coesite grains [22] and are indicative of a large volume increase after decompression. The individual silica grains consist of mosaics of many of domains (10-60 μm), many displaying an orthogonal intergrowth of two or more sets of lamellae with different brightness in back-scattered electron images (Fig. 1b). Electron microprobe analyses show that the lamellae and lamellae-free areas are almost pure SiO2 with minor amounts of Na2O (0.40 wt. %) and Al2O3 (1.14 wt. %). A 120 μm disc containing a large silica (> 60 μm) grain was cored out with a high-precision diamond micro-drill for successive X-ray and TEM investigation. Electron diffraction. Our initial electron diffraction data fit a post-stishovite structure similar to an α-PbO2 –like structure[20]. Since the initial investigation we have obtained diffraction patterns from three distinct zone axes, providing seven diffraction vectors to constrain the structure of this SiO2 polymorph (Table 1). The corresponding d-spacings cannot be indexed using any structures of low-pressure SiO2 polymorphs, including tridymite. Similarly, the diffraction data are inconsistent Figure 1. BSE images of SiO2 showing radiating cracks in surrounding maskelynite and cpx, (a). FESEM BSE images (b), show a fine lamellae microstructure in the SiO2 grains.