Coesite
Chemical formula: SiO<sub>2</sub>
Coesite is a high-pressure, dense polymorphic variety of silicon dioxide (SiO₂), formed under conditions of extreme pressure, e.g., during meteorite impacts.
Properties
- Mohs hardness
- 7.5-8
- Luster
- Vitreous
- Streak
- White
- Density
- 2.92
- Cleavage
- Good on {010}
- Fracture
- Conchoidal
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Identification of coesite is impossible with the naked eye and requires advanced laboratory techniques. In thin rock sections under a polarizing microscope, it is distinguished by high optical relief (much higher than quartz) and characteristic optical properties. Final confirmation is obtained using Raman spectroscopy or X-ray diffraction (XRD), which allow its structure to be unequivocally differentiated from other SiO₂ polymorphs. ## Distinguishing from Similar Minerals At the macroscopic level, rocks containing coesite do not differ from other metamorphic rocks. At the microscopic level, it must be distinguished from other SiO₂ polymorphs, mainly quartz and stishovite. It differs from quartz by a significantly higher refractive index and higher density. Stishovite, even rarer and formed at even higher pressures, has a tetragonal system and even greater density. ## Crystal Forms Natural crystals are extremely rare and microscopic, usually less than 0.5 mm in size. They take the form of hexagonal plates or short, prismatic columns. Most often, it occurs as irregular, anhedral inclusion grains within other minerals, such as garnet, omphacite, or diamond.
Geological environment
## Genesis Coesite is an ultra-high pressure (UHP) indicator mineral. It forms in two main ways: 1. As a result of UHP metamorphism, when continental crustal rocks (e.g., granites, gneisses) are subducted to depths of over 80-100 km in subduction zones. Upon exhumation to the surface, coesite is preserved as a relict within harder, resistant minerals (e.g., garnets) that protect it from retrogression to quartz. 2. As a result of shock metamorphism during large meteorite impacts. The sudden, extreme increase in pressure and temperature at the impact site transforms quartz contained in the target rocks into coesite and other high-pressure minerals. ## Mineral Associations In UHP metamorphic rocks, coesite most commonly coexists with pyrope, omphacite, jadeite, kyanite, talc, phengite, rutile, and diamond. In impact rocks, it is accompanied by shock quartz, lechatelierite (silica glass), stishovite, and minerals typical of the host rock. ## Localities Key localities associated with UHP metamorphism include the Dora Maira massif in the Western Italian Alps (where the largest crystals were found), the Dabie-Sulu region in eastern China, and Kokchetav in Kazakhstan. The classic impact locality is the Meteor Crater (Barringer Crater) in Arizona, USA. Coesite has also been found in the Ries crater in Germany and many other impact structures worldwide.
Rarity
Very rare
For collectors
## Quality Criteria Coesite as such is not an item of collector's trade in the form of isolated crystals. Its collector and scientific value is inextricably linked to the host rock. Most prized are specimens in which the presence of coesite has been analytically confirmed, and its inclusions are visible under the microscope in associated minerals, such as pink pyropes from Dora Maira. The value of a specimen is enhanced by a well-documented locality (e.g., a known UHP massif or impact crater) and a rich mineral association. ## Popular Localities For collectors specializing in metamorphic rocks and indicator minerals, coesite-bearing eclogites and gneisses from the Dora Maira massif in Italy and the Dabie-Sulu region in China are most sought after. Fragments of Coconino Sandstone from Barringer Crater in the USA primarily hold historical and scientific value.
Care and storage
## Cleaning Specimens containing coesite are usually fragments of host rocks and generally do not require cleaning. If necessary, a soft, dry brush can be used to remove dust. Due to the microscopic nature of the grains, any intensive cleaning is not recommended. ## What to Avoid Coesite is a metastable mineral under Earth's surface conditions. Heating it to temperatures above 1100°C will cause its transformation (recrystallization) into quartz. Extreme temperatures and aggressive chemicals, which could damage the host rock containing the coesite grains, should be avoided. ## Storage Specimens should be stored under stable room conditions, away from direct sunlight and heat sources. It is best to keep them in sealed boxes or display cases to protect them from dust and mechanical damage.