Chloritoid
| Chloritoid | |
|---|---|
|
Chloritoid crystal group on matrix from Nuristan Province, Afghanistan (size:6.3 x 3.5 x 3.0 cm) | |
| General | |
| Category | Nesosilicates |
| Formula (repeating unit) | (Fe,Mg,Mn)2Al4Si2O10(OH)4 |
| Strunz classification | 09.AF.85 |
| Dana classification | 52.03.03.01 |
| Crystal symmetry | Monoclinic prismatic 2/m or triclinic pinacoidal 1 |
| Unit cell |
a = 9.50 Å, b = 5.50 Å, c = 18.22 Å; β = 101.9°; Z = 4 or a = 9.46 Å, b = 5.50 Å, c = 9.15 Å; α = 97.05° β = 101.56° γ = 90.10° |
| Identification | |
| Color | Dark gray, greenish gray, greenish black |
| Crystal habit |
Tabular pseudohexagonal crystals; rosettes, commonly coarsely foliated with foliae typically curved or bent; also massive |
| Crystal system | Monoclinic or triclinic |
| Twinning | Common on {001}, polysynthetic may be lamellar |
| Cleavage | Perfect on {001}, distinct on {110}; parting on {010} |
| Tenacity | Brittle |
| Mohs scale hardness | 6.5 |
| Luster | pearly on cleavage surfaces |
| Streak | White, grayish, or very slightly greenish |
| Diaphaneity | Translucent |
| Specific gravity | 3.46 – 3.80 |
| Optical properties | Biaxial (+) or (–) |
| Refractive index | 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 angle | Measured: 36° to 89° |
| Dispersion | r > v; strong |
| References | [1][2][3] |
Chloritoid is a silicate mineral of metamorphic origin. It is an iron magnesium manganese alumino-silicate hydroxide with formula: (Fe,Mg,Mn)2Al4Si2O10(OH)4. 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.[1][2]
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.[2][3]
References
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