Clinohumite
Clinohumite is an uncommon member of the humite group, a magnesium silicate according to the chemical formula (Mg, Fe)9(SiO4)4(F,OH)2. The formula can be thought of as four olivine (Mg2SiO4), plus one brucite (Mg(OH)2). Indeed, the mineral is essentially a hydrated olivine and occurs in altered ultramafic rocks and carbonatites. Most commonly found as tiny indistinct grains, large euhedral clinohumite crystals are sought by collectors and occasionally fashioned into bright, yellow-orange gemstones. Only two sources of gem-quality material are known: the Pamir Mountains of Tajikistan, and the Taymyr region of northern Siberia. It is one of two humite group minerals that have been cut into gems, the other being the much more common chondrodite.
| Clinohumite | |
|---|---|
| General | |
| Category | Nesosilicate |
| Formula (repeating unit) | (Mg,Fe)9(SiO4)4(F,OH)2 |
| IMA symbol | Chu |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (same H-M symbol) |
| Space group | P21/c |
| Unit cell | a = 13.71 Å, b = 4.75 Å, c = 10.29 Å; β = 100.83°; Z = 2 |
| Identification | |
| Color | Brownish to orange, yellow, red |
| Crystal habit | Granular, prismatic, twinned |
| Twinning | Simple, lamellar common on {100} |
| Cleavage | Poor on {100} |
| Fracture | Subconchoidal to uneven |
| Mohs scale hardness | 6 |
| Luster | Vitreous to resinous |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.17–3.35 |
| Optical properties | biaxial (+) |
| Refractive index | nα = 1.623 – 1.702 nβ = 1.636 – 1.709 nγ = 1.651 – 1.728 |
| Birefringence | +0.028 |
| Pleochroism | X = golden yellow, yellow-brown, deep reddish yellow; Y = pale yellow, yellow-orange, light yellow; Z = pale yellow, yellow-orange, colorless |
| 2V angle | Measured: 52° to 90° |
| References | |
| Major varieties | |
| Titanclinohumite | Titanoan; (Mg,Fe2+,Ti)9 [(F,OH,O)2|(SiO4)4] |