Henritermierite

Chemical formula: Ca₃Mn₃+₂(SiO₄)₂(OH)₄

Henritermierite is a rare, reddish-orange mineral from the garnet group, distinguished by the presence of manganese and characteristic, well-formed crystals.

## Characteristics Henritermierite is a mineral belonging to the garnet group, specifically to the hydrogarnet subgroup. Its chemical formula indicates the presence of calcium and trivalent manganese. It forms well-developed, isometric crystals, most often in the form of rhombic dodecahedra or deltoidal icositetrahedra, reaching sizes up to 2 cm. It usually occurs as single crystals or small aggregates embedded in the host rock. Its intense, reddish-orange to red color is one of its most characteristic features. ## Physical Properties Henritermierite crystals exhibit a hardness of about 6.5 on the Mohs scale. They have a vitreous luster, which becomes more resinous on fracture surfaces. The mineral is typically translucent, and in thin fragments, it can be transparent. Its density is approximately 3.4 g/cm³. ## Colors and Varieties The dominant and only known color of henritermierite is intense reddish-orange, transitioning to red or reddish-brown. No color or commercial varieties are distinguished. ## History and Name The mineral was first described in 1969 by C. Gaudefroy, M. Orliac, F. Permingeat, and A. Parfenoff. Its name honors Henri Termier (1895–1971), a French geologist and paleontologist, professor at the Sorbonne in Paris, for his contributions to the geology of Morocco, where the mineral was discovered. ## Uses Due to its rarity and small crystal sizes, henritermierite has no industrial applications. It is solely an object of interest for mineral collectors and scientific institutions.

Properties

Mohs hardness: 6.5
Luster: Vitreous, Resinous
Streak: Red-brown
Density: 3.406
Cleavage: None
Fracture: Uneven, Conchoidal
Transparency: Transparent, Translucent
Crystal system: Cubic

Diagnostic features

## Identification Key diagnostic features of henritermierite include its intense reddish-orange color, regular (isometric) crystal habit, and high hardness. Its occurrence in a specific geological environment – in manganese deposits – is also characteristic. ## Distinguishing from Similar Minerals Henritermierite can be confused with other red garnets, such as spessartine or almandine. Differentiation often requires advanced research methods (chemical analysis, density measurements). Spessartine is often more orange and forms crystals in other rocks (e.g., pegmatites). In its occurrence, it can be mistaken for other manganese minerals, but its regular crystal form is usually a distinguishing feature. ## Crystal Forms Henritermierite crystallizes in the isometric system, forming classic garnet shapes. The most common are well-formed rhombic dodecahedra and deltoidal icositetrahedra, as well as forms that are a combination of these.

Geological environment

## Genesis Henritermierite is a mineral of hydrothermal or metamorphic origin. It forms as a result of the alteration of manganese- and calcium-rich rocks under relatively low-temperature conditions. Its typical environment is manganese deposits, where it forms in fissures and small rock cavities. ## Mineral Associations This mineral co-occurs with other manganese minerals. In its type locality (Tachgagalt, Morocco), it was found in association with marokite, gaudefroyite, hausmannite, krednerite, braunite, and calcite. ## Localities The most important and historically first occurrence of henritermierite, from which the best specimens originate, is the Tachgagalt manganese deposit in Ouarzazate Province, Morocco. This is its type locality. Beyond Morocco, its presence has also been noted in the Wessels Mine in Kuruman, in the Kalahari Manganese Fields region of South Africa, which is also famous for exceptional manganese minerals.

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, sharp-edged crystals of intense, reddish-orange color and good luster. Crystals embedded in a contrasting rock matrix are highly valued, especially when accompanied by other rare minerals like gaudefroyite. Crystal size is a key factor – specimens with crystals exceeding 1 cm are considered exceptional. ## Popular Localities By far the most sought-after specimens come from the type locality – the Tachgagalt deposit in Morocco. They are characterized by classic development and color. Specimens from South Africa, though rarer on the market, are also valued by specialists collecting minerals from the Kalahari region.

Care and storage

## Cleaning Henritermierite specimens can be safely cleaned with a soft, dry brush to remove dust. For heavier soiling, distilled water may be used. Ultrasonic cleaners should be avoided, as they can damage crystals, especially those embedded in the rock matrix. ## What to Avoid Contact with acids and other aggressive chemicals, which can damage its surface, should be avoided. Although relatively hard, it should be protected from impacts and scratching by harder minerals (e.g., quartz, corundum). It is not sensitive to light, but extreme temperature changes should be avoided. ## Storage Henritermierite specimens are best stored in individual, padded collector boxes to prevent abrasion and mechanical damage. It displays well in showcases, protected from dust and direct vibrations.

Sources

IMA Mineral List (CNMNC)