Henritermierite
Ca3(Mn3+,Al)2(SiO4)2(OH)4
A rare manganese-bearing hydrogarnet notable for being tetragonal rather than cubic, found in metamorphosed manganese deposits in Morocco.
- Mohs hardness
- 6.5-7
- Color
- brown-red to orange-brown
- Type
- mineral
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Overview
Henritermierite is an unusual garnet-supergroup mineral because it is tetragonal rather than the cubic symmetry typical of garnets. It is a hydrogarnet containing trivalent manganese, which both colors it and, with ordering of hydroxyl groups, lowers its symmetry.
Its color ranges from brown-red to orange-brown, and it usually occurs as small crystals or grains. The mineral is rare and prized by collectors of garnet-supergroup species.
Henritermierite was named after the French geologist Henri Termier and is best known from manganese deposits in Morocco.
Formation & geology
Henritermierite forms in metamorphosed and hydrothermally altered manganese deposits, where manganese, calcium, and water are available at relatively low temperatures. The hydrogarnet substitution (hydroxyl replacing silica) and the ordering of manganese drive its tetragonal structure.
It occurs with other manganese minerals such as braunite, manganese oxides, and calcite in manganese ore bodies.
The type and best-known locality is the Tachgagalt (N'Chwaning-style) manganese deposit region of Morocco, with additional occurrences in manganese deposits elsewhere.
How to identify it
Look for small brown-red to orange-brown garnet-like crystals in manganese-rich rock. Hardness is about 6.5-7 with a pale streak.
Its defining feature, tetragonal symmetry, is not visible to the naked eye; suspected henritermierite is confirmed by optical or X-ray methods showing it is birefringent rather than isotropic like cubic garnets. The manganese-deposit setting and association with braunite are useful field clues distinguishing it from ordinary spessartine or andradite.
Uses & significance
Henritermierite has no commercial or gem use; it is a rare collector and research mineral. Scientifically it is important as one of the few non-cubic garnet-supergroup minerals, illustrating how cation ordering and hydroxyl content can lower garnet symmetry.
Fine specimens from Moroccan manganese deposits are sought by collectors of rare species. It has no established metaphysical tradition.
Frequently asked questions
Why is henritermierite unusual?
It is tetragonal rather than cubic, a rarity among garnets, caused by manganese and hydroxyl ordering in its structure.
What gives henritermierite its color?
Trivalent manganese produces its brown-red to orange-brown hue.
Where is henritermierite found?
In metamorphosed manganese deposits, most famously in Morocco.
How is its tetragonal nature detected?
Through optical or X-ray methods showing birefringence, since unlike cubic garnets it is not optically isotropic.
Henritermierite guides
In-depth guides for identifying, valuing, and understanding Henritermierite.
