Grossular
Chemical formula: Ca<sub>3</sub>Al<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>
Grossular is a calcium-aluminum silicate from the garnet group, valued for its wide range of colors, from colorless, through green and yellow, to orange-brown.
Description
## Characteristics Grossular, a calcium-aluminum silicate, belongs to the broad and diverse garnet group. It forms characteristic, well-developed crystals in the form of a rhombic dodecahedron or a deltoidal icositetrahedron, often with a high luster. It also occurs as granular aggregates or massive forms. Its name is not related to the color green, as is often thought, but to the wide palette of colors in which it occurs. ## Physical Properties Grossular is characterized by a hardness of 6.5-7.5 on the Mohs scale, which makes it a relatively scratch-resistant mineral. It has a vitreous luster, sometimes transitioning to resinous, especially in the hessonite variety. It is a transparent to translucent mineral. Its density ranges from 3.57-3.61 g/cm³. ## Colors and Varieties Grossular is one of the most color-diverse garnets. The most important varieties are: - **Hessonite**: A variety with colors ranging from orange and honey-yellow to reddish-brown, known as the "cinnamon stone." It is characterized by an internal texture resembling turbid syrup or wavy, hot air. - **Tsavorite**: An intensely green, emerald-like variety, whose color is caused by the presence of vanadium or chromium. It is the most valuable variety of grossular. - **Leucogarnet**: A rare, colorless form of grossular. - **Hydrogrossular**: Usually a greenish, opaque, and massive variety containing hydroxyl (OH) groups in its structure. It is sometimes mistaken for jadeite. ## History and Name The name "grossular" comes from the Latin name for gooseberry, *ribes grossularia*, in reference to the light green, spherical crystals first found in the Wiluy River region of Siberia. The mineral was described by Abraham Gottlob Werner in 1808. ## Uses Due to its color, luster, and hardness, grossular is a valued gemstone and collector's item. Tsavorite and hessonite are of greatest importance in jewelry. Massive, compact varieties are sometimes used as carving material.
Diagnostic features
## Identification The key diagnostic feature of grossular, like other garnets, is its crystallographic form – most often a rhombic dodecahedron or a deltoidal icositetrahedron. Other important features include hardness (6.5-7.5), vitreous or resinous luster, and lack of cleavage. In the hessonite variety, a characteristic "oily" or "syrupy" internal texture is present. ## Distinguishing from Similar Minerals - **Green grossular (tsavorite)** can be confused with emerald, which is, however, softer (7.5-8) and crystallizes in a different system (hexagonal). - **Hessonite** is sometimes mistaken for citrine (a variety of quartz), which is much lighter and has lower hardness, and for spessartine, from which it is distinguished by optical properties and density. - **Massive hydrogrossular** is similar to jadeite and nephrite (jade), from which it can be distinguished by measuring density and refractive index. ## Crystal Forms Grossular most often forms isometric crystals in the shape of rhombic dodecahedra and deltoidal icositetrahedra, often with very well-developed and lustrous faces. It also occurs as granular aggregates, crusts, and massive forms.
Geological environment
## Genesis Grossular is a typical mineral of metamorphic rocks. It forms mainly as a result of contact or regional metamorphism of carbonate rocks (limestones, marls) contaminated with silica and clay. It is characteristic of skarns, which are rocks formed at the contact of magmatic intrusions with carbonate rocks. It also occurs in serpentinites and rodingites. ## Mineral Associations Minerals often associated with grossular include diopside, vesuvianite, wollastonite, epidote, calcite, scapolite, and scheelite. ## Localities Significant deposits and occurrences of grossular are found in many places around the world: - **Tsavorite**: Tsavo National Park in Kenya and neighboring Tanzania are the main sources of this valuable variety. - **Hessonite**: The most beautiful specimens come from Sri Lanka, Brazil, and India. It also occurs in Canada (Asbestos mine, Quebec) and the USA (California). - **Other varieties**: The famous Jeffrey mine in Quebec (Canada) supplied world-class grossular crystals in various colors, including green, colorless, and pink. Valuable specimens also come from Mexico, Pakistan, Italy (Val d'Ala), and Siberia in Russia.
Rarity
Not very common
Collector aspects
## Quality Criteria The collector's appeal of grossular depends on its variety. For tsavorite and hessonite, the intensity and purity of color and transparency are most important. For crystalline specimens, key factors are: the sharpness and luster of the crystal faces, its size, lack of damage, and an attractive association with accompanying minerals on the host rock (matrix). ## Popular Localities The most sought-after by collectors are specimens from historical, now inactive localities, such as the Jeffrey mine in Canada, which was famous for its perfect crystals. Tsavorite specimens from Kenya and Tanzania are valued for their gemological qualities, and classic, green crystals from Val d'Ala in Italy adorn many mineralogical collections.
Care and storage
## Cleaning Grossular specimens are relatively durable and can be safely cleaned in warm soapy water with a soft brush. Ultrasonic cleaners are generally safe for clean, inclusion-free stones, but caution should be exercised with specimens containing internal fractures. ## What to Avoid Avoid contact with strong acids and sudden temperature changes. Despite its significant hardness, grossular is brittle and can be damaged by a strong impact. Steam cleaning is not recommended. ## Storage To avoid scratches, grossular specimens should be stored separately, away from minerals of higher hardness (such as diamond or corundum) and from softer specimens that it could damage. It is best to keep them in separate pouches or in compartments of a display case lined with soft material.