High-pressure polymorph modifications of some minerals in impactites: Geological observations and experimental data

The paper presents a review of literature data on the polymorph modifications of SiO 2 (coesite and stishovite), C (diamond and lonsdaleite), Mg 2 SiO 4 (ring woodite), and MgSiO 3 (majorite) found in naturally occurring impact structures (astroblemes) and synthesized in experiments. Much attention is devoted to the description of ringwoodite and a high-density phase of pyroxene composition, which were obtained by the authors in experiments on the shock-wave loading of rocks under pressures of 30–70 GPa. The high-density polymorph modifications listed above are demonstrated to be formed by the following three mechanism transforming the original material: (i) crystallization from an impact melt, (ii) martensite phase transition, and (iii) migration phase transition (the latter two mechanisms occur in the solid phase). Both in nature and in laboratory experiments, high-density polymorph modifications are formed under the effect of shock waves under dynamic pressures exceeding the static pressure values by factors from 1.5–2 to 10. We were the first to experimentally obtain ringwoodite and the high-pressure phase of pyroxene composition via the diaplectic transformations of biotite and garnet. Our experiments confirm calculation data on the disproportionation of the shock-wave energy between minerals composing a rock according to their volumetric contents. The experimental data also confirm our earlier conclusions that the pressures under which the shock-thermal decomposition of minerals starts are controlled by the types of the crystalline structures of these minerals.