Quartz grain boundaries

Fig. 5. SEM-BSE image showing Au, eiectrum, native Bi and BiTe aiioys precipitated in an anneaied amphiboiite-facies metabasait iayer. The inset CL image reveais aiioys and suifides in contact with new generations of quartz (red) and piagiociase (dark) that records dissoiution-precipitation reactions isoiated aiong the grain boundaries of a previous anneaied matrix.

Alicudi rocks, like other Aeolian volcanics, contain a variety of meta-morphic and magmatic xenoliths. Metamorphic xenoliths are represented by predominant quartz-rich rocks displaying evidence of partial melting along grain boundaries, and by a few biotite gneiss and granulite... 

The structural picture of the BIF is emphasized not only by differences in composition and thickness of the layers, but also by textural characteristics. In metamorphosed BIF granoblastic or hornfels textures are very common in the barren quartz and semi-ore siderite-quartz bands equidimensional quartz grains with undulating boundaries constitute their matrix. Alternation of higher-order bands, consisting of grains of different size but of the same texture, within layers is a usual phenomenon in slightly metamorphosed rocks. 

Some speculations on the nature of high-angle grain boundaries in quartz rocks. In Mineral and Rock Deformation Laboratory Studies, The Paterson... 

Microstresses arise inside the material as a result of the unequal thermal expansion coefficients of the glass and of the crystals. This phenomenon is characteristic of polyphase ceramics. In porcelains, the stresses arise in particular at the boundary between large quartz crystals and glass. Their formation is also contributed to by the modification inversion of quartz. The stresses sometimes bring about formation of microscopically visible cracks inside the quartz grains or around them. The structure of porcelain thus contains weak points which enhance the development of fracture on loading. 

The facts mentioned above indicate that one of the ways to increase the mechanical strength of porcelain is to change the glassy phase composition in order to increase its thermal expansion coefficient, and thus reduce the microstress at the boundary with quartz. Practical experience shows that the same effect is obtained just by reducing the quartz grain size the small quartz grain size suppresses the adverse effects of microstresses at the boundaries. 

Kodiyalam S, Kalia RK, Kikuchi H, Nakano A, Shimojo F, Vashishta P (2001) Grain boundaries in gallium arsenide nanocrystals underpressure A parallel molecular-dynamics study. Phys Rev Letters 86 55-58 Leoni S, Nesper R (2000) Elucidation of simple pathways for reconstractive phase transitions using periodic equi-surface (PES) descriptors. The silica phase system. 1. Quartz-tridymite. Acta Crystallogr A 56 383-393... 

Chemical sensors for gas molecules may, in principle, monitor physisorp-tion, chemisorption, surface defects, grain boundaries or bulk defect reactions [40]. Several chemical sensors are available mass-sensitive sensors, conducting polymers and semiconductors. Mass-sensitive sensors include quartz resonators, piezoelectric sensors or surface acoustic wave sensors [41-43]. The basis is a quartz resonator coated with a sensing membrane which works as a chemical sensor. 

Post-crystallization exchange can take many forms that will complicate thermometry. Figure 3 shows four cartoons of the isotope distribution predicted for various processes of hydrothermal alteration of quartz by a low fluid. If diffusion is radial and inward from the grain boundary then a concentric zonation of is predicted with a smooth error function shaped gradient (Fig. 3 A and Fig. 4). If the rim of the crystal... 

Figure 10. Contour plots of apparent temperatures and fractionations (%o in italics) as a function of modal percentages in an idealized three-mineral rock composed of quartz, feldspar, and hornblende, predicted by the Fast Grain Boundary diffusion model to be recorded by (a) quartz and hornblende, (b) feldspar and hornblende, and (c) quartz and feldspar. These figures show the relation of discordance and the mode of each mineral in a rock (from Eiler et al. 1993).

Fluid inclusions in quartz cement are situated both in close proximity to the boundary between detrital quartz grains and cement as well as within the overgrowths themselves. The inclusions chosen for microthermometry are interpreted as primary inclusions that were trapped during growth of quartz cement. Inclusions located in healed fractures were not selected for analysis. 

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