Edgrewite

Chemical formula: Ca₉(SiO₄)₄F₂

Edgrewite is a rare calcium silicate from the humite group, occurring as microscopic, colorless to light brown grains in skarns.

## Characteristics Edgrewite is a mineral belonging to the silicates, and within this group, to the humite supergroup. It typically occurs as very small, anhedral (not exhibiting crystal faces) grains, whose size rarely exceeds 200 micrometers. It is observed as inclusions in other minerals, mainly in larnite. It is colorless to light brown and has a vitreous luster. ## Physical Properties Due to the microscopic size of the crystals, many physical properties have not been fully determined. Hardness is estimated at approximately 6 on the Mohs scale, which is typical for minerals of the humite group. The density calculated based on the chemical formula and unit cell parameters is 3.03 g/cm³. The mineral does not exhibit fluorescence under ultraviolet light. ## Colors and Varieties The mineral is usually colorless, sometimes taking on light brown hues. No named varieties have been distinguished. ## History and Name Edgrewite was first described in 2011 by S.N. Britvin and a team of researchers. Its name honors Professor Edward S. Grew, an American mineralogist and petrologist from the University of Maine, for his contributions to the study of boron and beryllium minerals and metamorphic petrology. The mineral was approved by the International Mineralogical Association (IMA) under number 2011-025. The type locality (locus typicus) is the Hatrurim Formation in Israel. ## Uses Edgrewite has no commercial or industrial applications. Its significance is purely scientific and collectible, although due to its microscopic size and rarity, it is an object of interest for specialized micromineral collectors.

Properties

Mohs hardness: 6
Luster: Vitreous
Streak: White
Density: 3.03
Cleavage: None
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of edgrewite is possible only through advanced laboratory methods, such as X-ray microdiffraction (XRD) or Raman spectroscopy, due to its microscopic size and occurrence as inclusions. Visually, it is indistinguishable from many other colorless minerals in the rock. ## Distinguishing from Similar Minerals Under the microscope, it can be confused with other minerals from the humite group (e.g., kumtyite) or other colorless calcium silicates occurring in the same environment (e.g., larnite, brownmillerite). Definitive differentiation requires chemical and structural analysis. ## Crystal Forms Edgrewite forms exclusively anhedral, irregular grains ranging from 5 to 200 micrometers in size. It has not been observed in the form of well-developed, euhedral crystals.

Geological environment

## Genesis Edgrewite forms under conditions of high-temperature, low-pressure contact metamorphism. It occurs in skarns formed at the contact of carbonate rocks with magmatic intrusions. In the type locality (Hatrurim Formation), it is a product of pyrometamorphism of Cretaceous rocks. ## Mineral Associations This mineral co-occurs with other rare minerals of high-temperature skarns. It is most frequently associated with larnite (in which it forms inclusions), brownmillerite, mayenite, ye'elimite, and also minerals from the ettringite group. ## Localities The only confirmed and thoroughly described occurrence of edgrewite in the world is its type locality – the Hatrurim Formation, located in the Negev Desert in Israel. This is a complex of metamorphic rocks known for the occurrence of many rare oxide and silicate minerals.

Rarity

Very rare

For collectors

## Quality Criteria As a micromineral, edgrewite is not evaluated according to typical criteria such as color or clarity. The collectible value of a specimen depends on the abundance of edgrewite grains, their size (the larger, the better), and the confirmation of their identity through scientific analysis. The most valuable specimens come directly from the type locality, with well-documented associated minerals. ## Popular Localities The only source of specimens is the Hatrurim Formation in Israel. Material from this locality is extremely rarely available on the collector's market and is sought after mainly by scientific institutions and highly specialized micromineral collectors.

Care and storage

## Cleaning Specimens containing edgrewite, typically rock fragments, can be gently cleaned with compressed air to remove dust. If liquids are necessary, brief contact with distilled water and subsequent drying is recommended. Due to the microscopic nature of the grains, intensive mechanical cleaning is not advisable. ## What to Avoid Avoid contact with strong acids, which can react with silicates and associated minerals. Do not heat specimens or use ultrasonic cleaners, as these could damage the delicate structure of the host rock and the microcrystals themselves. ## Storage Micromineral specimens, such as edgrewite, are best stored in specialized "micromount" boxes, which protect them from dust and mechanical damage. Avoid humid environments and direct exposure to sunlight.

Sources

IMA Mineral List (CNMNC)