Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 234

Figure 9.4. Shocked and unshocked quartz grains. The unshocked grain is to the
left. Note the striations (shock marks) in the grain on the right. Photograph
courtesy of Professor D. A. Kring, University of Arizona.

shock processes in quartz as a diagnostic tool

Quartz is a relatively stable rock-forming mineral that occurs nearly
ubiquitously in terrestrial crustal rocks. It does not normally display shocked
textures and, where it does, it can be regarded as a reliable indicator of extrater-
restrial impacts (Fig. 9.4). Shocked quartz is found in only two geological settings:
either in direct spatial relationship to impact structures or as a constituent of
distal or global ejecta deposits. Shock effects vary with increasing shock pres-
sure and also to some degree on the pre-shock temperature. Quartz starts to
melt under shock compression of >50 GPa and vaporises at

> 100 GPa. Grieves et

al. (1996)notethat between about 5 and > 50 GPa the following shock effects in

quartz are observed.

(1) Mosaicism which is a highly irregular mottled extinction pattern (as

seen under a polarising microscope).

(2) Planar microstructures (planar fractures and planar deformation fea-

tures). There are two ways in which quartz fails mechanically. These are
by theformation of irregular fractures that are not diagnostic shock
effects and by the formation of regular planar microstructures that are
diagnostic shock effects. There are two types of planar microstructures:
planar fractures (PFs) and planar deformation features (PDFs).

(3) Partial to complete isotropization or amorphisation (diaplectic glass).

Diaplectic glass has the same morphology as the original quartz mineral
and shows no evidence of fluid textures. The refractive index of diaplec-
tic glass decreases with increasing pressure until shock-fused quartz
(lechatelierite) is formed.

(4) Partial transformation to stishovite and coesite. These are high-pressure

polymorphs that occur as fine-grained aggregates formed by partial