Reidite

Chemical formula: Zr⁴⁺SiO₄

Reidite is a rare polymorph of zircon, formed under conditions of extremely high pressure, most commonly in impact craters.

## Characteristics Reidite is a high-pressure polymorphic form of zircon (ZrSiO₄), meaning it has the same chemical composition but a different, denser crystal structure. It occurs as microscopic grains or aggregates, often within zircon grains that have undergone partial transformation due to a shock wave. Due to its size, its visual characteristics are difficult to assess without specialized equipment. In microscopic images (e.g., in polarized light), it exhibits characteristic features distinguishing it from zircon, such as a higher refractive index and stronger birefringence. ## Physical Properties As a mineral occurring as microscopic inclusions, most of its physical properties are determined theoretically or using advanced laboratory techniques. It has a significantly higher density than zircon (approx. 5.2 g/cm³ compared to 4.7 g/cm³), which is a result of its compact crystal structure. Hardness is estimated to be similar to zircon, around 7.5 on the Mohs scale. The luster, observed on synthetic crystals, is adamantine. ## Colors and Varieties Reidite is typically colorless. No colored or commercial varieties are distinguished. ## History and Name The mineral is named in honor of Alan F. Reid (1933–1991), an Australian crystal chemist who first synthesized this high-pressure ZrSiO₄ phase in the laboratory in 1969. As a natural mineral, it was identified and described only in 2001 by C.A. Glass and colleagues in material from the Chesapeake Bay impact crater in Virginia, USA. ## Uses Reidite has no commercial or industrial applications. Its significance is purely scientific – it is a key indicator of shock metamorphism, allowing for the identification and study of impact structures (impact craters) on Earth and other celestial bodies.

Properties

Mohs hardness: 7.5
Color: Colourless
Luster: Adamantine
Streak: white
Density: 0
Cleavage: Good on {101}
Fracture: Irregular/Uneven
Transparency: Transparent
Crystal system: Tetragonal

Diagnostic features

## Identification Identification of reidite is impossible without advanced laboratory techniques. It is unrecognizable in the field. In the laboratory, it is identified using polarized light microscopy (characterized by higher interference colors than co-occurring zircon), and final confirmation is obtained through Raman spectroscopy or electron backscatter diffraction (EBSD), which allow its structure to be distinguished from that of zircon. ## Distinguishing from Similar Minerals Reidite is most often confused with zircon, from which it forms. Differentiation involves identifying differences in crystal structure. In microscopic images, reidite often forms characteristic, lamellar intergrowths within zircon crystals, which is diagnostic evidence of shock pressure. ## Crystal Forms Reidite does not form macroscopic, well-developed crystals. It occurs as microscopic, anhedral grains or as planar lamellae within zircon crystals, oriented according to its crystallography.

Geological environment

## Genesis Reidite is a mineral formed under conditions of shock (impact) metamorphism. It forms from zircon under extremely high pressure (above 30 GPa) and relatively moderate temperature, generated during the impact of a large meteorite. It is considered one of the most important indicators of such events. ## Mineral Associations This mineral occurs almost exclusively in association with zircon, from which it forms. It is accompanied by other shock minerals, such as coesite, stishovite (high-pressure polymorphs of quartz), maskelynite (glass formed from plagioclase), and minerals of the rocks impacted by the meteorite (quartz, feldspars, micas). ## Localities Reidite has been identified in several confirmed impact craters worldwide. Key localities include: - Chesapeake Bay Crater, Virginia, USA (discovery site). - Ries Crater, Bavaria, Germany. - Xiuyan Crater, Liaoning, China. - Woodleigh Crater, Western Australia. Its presence has also been confirmed in ejecta (material ejected from the crater) found in various parts of the world.

Rarity

Very rare

For collectors

## Quality Criteria Reidite is not a collector's mineral in the traditional sense, as it does not form crystals visible to the naked eye. Its value is purely scientific. For research institutions and specialized collectors of impact structures, the most valuable samples are those (typically thin rock sections) in which the presence of reidite has been analytically confirmed and is clearly visible under a microscope, e.g., in the form of distinct lamellae within a zircon crystal. Geological context and origin from a well-documented impact crater are crucial.

Care and storage

## Cleaning Specimens containing reidite (typically thin rock sections or samples from craters) do not require special cleaning. If necessary, they can be wiped with a dry, soft cloth or brush to remove dust. Due to the microscopic nature of reidite, any chemical or mechanical cleaning is inadvisable and may damage the surrounding rock matrix. ## What to Avoid Reidite is metastable under Earth's surface conditions. Heating above 300°C can cause its retrograde transformation back to zircon. High temperatures must be strictly avoided. Samples should be protected from chemicals and ultrasound. ## Storage Reidite samples, often in the form of thin sections for microscopic examination or small rock fragments, are best stored in specialized specimen boxes or containers, protecting them from dust, moisture, and sudden temperature changes. They do not require special display conditions beyond standard protection from external factors.