| Chloritoid | |
| Category: | Nesosilicates |
| Imasymbol: | Cld[1] |
| Strunz: | 9.AF.85 |
| Dana: | 52.03.03.01 |
| System: | 1A polytype: triclinic 2M polytype: monoclinic |
| Class: | 1A polytype: pinacoidal 2M polytype: prismatic (2/m) |
| Unit Cell: | 1A polytype: a = 9.46 Å, b = 5.50 Å, c = 9.15 Å; α = 97.05°, β = 101.56°, γ = 90.10° 2M polytype: a = 9.50 Å, b = 5.50 Å, c = 18.22 Å; β = 101.9°; Z = 4 |
| Color: | Dark gray, greenish gray, greenish black |
| Habit: | Tabular pseudohexagonal crystals; rosettes, commonly coarsely foliated with foliaetypically curved or bent; also massive |
| Twinning: | Common on, polysynthetic may be lamellar |
| Cleavage: | Perfect on, distinct on ; parting on |
| Tenacity: | Brittle |
| Mohs: | 6.5 |
| Luster: | pearly on cleavage surfaces |
| Streak: | White, grayish, or very slightly greenish |
| Diaphaneity: | Translucent |
| Gravity: | 3.46 – 3.80 |
| Opticalprop: | Biaxial (+) or (−) |
| Refractive: | nα = 1.713 – 1.730 nβ = 1.719 – 1.734 nγ = 1.723 – 1.740 |
| Birefringence: | δ = 0.010 |
| Pleochroism: | X = olive-green to yellow; Y = grayish blue to blue; Z = colorless to pale greenish yellow |
| 2V: | Measured: 36° to 89° |
| Dispersion: | r > v; strong |
Chloritoid is a silicate mineral of metamorphic origin. It is an iron magnesium manganese alumino-silicate hydroxide with formula . It occurs as greenish grey to black platy micaceous crystals and foliated masses. Its Mohs hardness is 6.5, unusually high for a platy mineral, and it has a specific gravity of 3.52 to 3.57. It typically occurs in phyllites, schists and marbles.
Both monoclinic and triclinic polytypes exist and both are pseudohexagonal.[2] [3]
It was first described in 1837 from localities in the Ural Mountains region of Russia. It was named for its similarity to the chlorite group of minerals.[3] [4]