Quartz SiO2 is a common mineral in Earth’s crust. In places where meteor impacts
ID: 285681 • Letter: Q
Question
Quartz SiO2 is a common mineral in Earth’s crust. In places where meteor impacts have occurred, such as Meteor Crater, other distinctly different SiO2 minerals occur, such as stishovite and coesite.
a) Are quartz, coesite and stishovite isomorphs or polymorphs?
b) We find coesite and stishovite in shocked Coconino Sandstone at Meteor Crater. We infer that coesite and stishovite formed from the transformation of quartz at high pressure and temperature. What type of phase transformations are these? Explain your answer.
Explanation / Answer
a) Answer: Quartz, coesite, stishovite are polymorphs. In mineralogy, it means that a single chemical composition (SiO2) can exist with two or more different crystal structures. The crystal structure will then change to that of the more stable structure, and a different mineral will be in existence with the changing pressure-temperature environment.
b) Answer:
Transformation of quartz to coesite and stishovite is a densification process with the increasing pressure at low temperature. The mineral stishovite can be formed on earth only due to the high pressures generated by a meteorite impact. First, anions form a close-packed sub-lattice, governed by the strong repulsion between them. Next, cations redistribute onto the interstices.
With increasing pressure and low temperature, quartz undergoes a displace transformation to coesite (Monoclinic), and coesite undergoes a reconstructive transformation to stishovite (tetragonal) at even higher pressures. Thus, coesite and stishovite have metastable polymorphs.
Displacive Transformations involve only small adjustments to the crystal structure without breaking of bonds. Little rearrangement in displacive transformations refers to no change in energy at the transformation temperature or pressure, and the transformations are instantaneous and reversible cause no occurrence of unstable polymorphs.
Reconstructive Transformations involves extensive rearrangement of the crystal structure with breaking of chemical bonds and reassembling the atoms into a different crystal structure. This usually involves a large energy change at the transformation temperature or pressure. The extensive rearrangement slows down the transformation. The Very slow rate of the transformation can cause the occurrence of unstable polymorphs (metastable).