Grossite
Chemical formula: CaAl<sub>4</sub>O<sub>7</sub>
Grossite is a very rare calcium aluminate, found mainly in meteorites and in specific, high-temperature metamorphic rocks.
Description
## Characteristics Grossite is an oxide mineral, chemically a calcium aluminate. It most commonly occurs as very small, anhedral (irregular) grains or poorly formed, short-prismatic crystals, rarely exceeding a millimeter in size. It is colorless, pale greenish, or bluish. It exhibits a vitreous luster. Due to its composition and genesis, it is of great interest in mineralogy and planetology. ## Physical Properties This mineral is relatively hard, with a Mohs hardness of 6.5 to 7. It is transparent to translucent. Its density ranges from 2.73-2.78 g/cm³, which is typical for aluminosilicate and aluminate minerals. It does not exhibit magnetic properties or fluorescence under ultraviolet light. ## Colors and Varieties Grossite is usually colorless but can take on pale shades of blue or green. The coloration is likely due to trace amounts of other elements. No named varieties of this mineral are recognized. ## History and Name Grossite was officially recognized as a new mineral by the IMA in 1994. Its name honors Shulamit Gross (1923-2012), an Israeli geologist who dedicated a large part of her career to studying the geologically unique Hatrurim Formation in Israel, where this mineral was first found in terrestrial rocks. ## Applications Grossite has no commercial or industrial applications. Its significance is purely scientific, as an indicator of extreme rock-forming conditions and as a component of primordial Solar System matter preserved in meteorites.
Diagnostic features
## Identification Identifying grossite based on visual characteristics is practically impossible and requires advanced laboratory techniques such as X-ray diffraction (XRD) or electron microprobe analysis (EMPA). A key diagnostic clue is its occurrence environment – calcium- and aluminum-rich inclusions (CAIs) within carbonaceous chondrites or specific mineral associations in the Hatrurim Formation. ## Distinguishing from Similar Minerals In its typical environment (e.g., in meteorites), it can be confused with other colorless minerals forming CAIs, such as gehlenite, anorthite, spinel, or hibonite. Differentiation requires analysis of chemical composition and crystal structure. Compared to calcite, it is much harder. From quartz, with which it has similar hardness, it differs completely in chemical composition and formation environment. ## Crystal Forms Grossite forms small, poorly developed crystals with an equant or short-prismatic habit. Most often, however, it occurs as irregular, anhedral grains within the rock mass or within inclusions in meteorites.
Geological environment
## Genesis Grossite forms in two distinct, yet equally extreme environments. The first is the early Solar System, where it crystallized as one of the first solid phases from the hot protosolar nebula. These primordial crystals have been preserved as calcium- and aluminum-rich inclusions (CAIs) in carbonaceous chondrites. The second environment consists of terrestrial contact metamorphic zones with very high temperature and low pressure, formed by the combustion of underground bitumen deposits, as in the case of the Hatrurim Formation. ## Mineral Associations In terrestrial rocks (Hatrurim Formation), grossite co-occurs with minerals such as brownmillerite, mayenite, gehlenite, larnite, wollastonite, and spinel. In meteorites, its typical associated minerals in CAIs are spinel, perovskite, hibonite, and minerals from the melilite group. ## Localities The most important terrestrial locality, which is the type locality, is the Hatrurim Formation in Israel. The mineral has also been found in similar formations in Maqarin, Jordan. Grossite is also a significant component of many meteorites, with classic examples including the carbonaceous chondrite Allende (Mexico) and Vigarano (Italy).
Rarity
Very rare
Collector aspects
## Quality Criteria For collectors, grossite is primarily a systematic and scientific mineral. Terrestrial specimens are extremely rare, and any material in which the mineral has been unequivocally identified is valuable. Samples with visible, even microscopic, crystals are most highly prized. In the case of meteorites, the value of a specimen is related to the size and richness of grossite-bearing CAIs, as well as the rarity of the meteorite itself. ## Popular Localities For collectors specializing in rare minerals, the most desired material is from the type locality – the Hatrurim Formation in Israel. Among meteorite collectors, the Allende meteorite inclusions are the best-known source of grossite.
Care and storage
## Cleaning Grossite specimens, due to their extreme rarity and typically microscopic size, require special care. Cleaning should be kept to a minimum. Gentle dust removal with a soft brush or careful use of compressed air is permissible. Washing in water is not recommended unless absolutely necessary – in which case, distilled water should be used. ## What to Avoid Ultrasonic cleaners and all chemical agents, including acids and bases, should be strictly avoided, as they could damage not only the mineral itself but also co-occurring rare mineral phases. Although it is a high-temperature mineral, sudden temperature changes should be avoided. ## Storage Grossite specimens should be stored under stable conditions, in specialized "micromount" boxes that protect them from mechanical damage and dust. Exposure to moisture and contamination should be avoided.