Editorial Feature

Chloritoid - Occurrence, Properties, and Distribution

May 14 2014

Chlorotoid is a silicate mineral that was first described in 1837, from the Ural Mountains region of Russia. It was named for its similarities of the chlorite-group minerals.

Properties of Chloritoid

The following are the key properties of Chloritoid:

Cell Data
- Space Group: C2/c
- a = 9.50(3)
- b = 5.50(2)
- c = 18.22(5)
- β = 101°57(5)'
- Z = 4
  Or
- Space Group: C1
- a = 9.46(1)
- b = 5.50(1)
- c = 9.15(1)
- α = 97:05(2)°
- β = 101:56(2)°
- γ = 90:10(2)°
- Z = 4

Crystal Data
- Monoclinic or triclinic, both pseudohexagonal
- Point Group: 2/m or 1
- Rarely as rosettes of pseudohexagonal tabular crystals; commonly coarsely foliated with foliae
- typically curved or bent; massive
- Twinning: Common on {001}, may be lamellar; twin axes [100], [110], and [130] observed
- X-ray Powder Pattern: 4.498 (100), 4.449 (100), 2.963 (90), 1.5813 (80), 2.367 (70), 2.306 (70), 2.639 (50)

Chemical Composition

Elements	Content
Al₂O₃	40.12
FeO	27.06
SiO₂	23.91
H₂O⁺	7.03
Fe₂O₃	1.23
MgO	0.51
TiO₂	0.20
MnO	0.16
CaO	0.04
F	0.01
H₂O^-	0.01
K₂O	trace
Total	100.28

Optical Properties
- Optical Class: Biaxial (+) or (-)
- Pleochroism: X = olive-green to yellow; Y = grayish blue to blue; Z = colorless to pale greenish-yellow
- Orientation: X or Y = b (monoclinic)
- Z ^ c = 2°-30°
- Y ˜ b (triclinic)
- Dispersion: r > v; strong
- α = 1.705-1.730
- β = 1.708-1.734
- γ = 1.712-1.740
- 2V(meas.) = ∼45°-70°

Estimated Properties

Electron density	Bulk density (electron density) = 3.43 g/cm³ note: Specific gravity of Chloritoid = 3.47 g/cm³
Photoelectric	PE_Chloritoid = 5.82 barns/electron U = PE_Chloritoid x ρElectron density = 19.95 barns/cm³
Fermion index	Fermion index = 0.01 Boson index = 0.99
Radioactivity	Chloritoid is not radioactive

How to Identify Chloritoid

Chloritoid is non-fluorescent and occurs in a range of colors, from dark gray to dark green, grayish-black, green-black, and green-gray. It has a transparent to translucent appearance, with pearly luster and green-gray streak. Its fractures are brittle, like that of non-metallic minerals and glasses. It exhibits two-dimensional platy forms, or massive crystal forms.

The hardness of the mineral is 6.5, and its average density is 3.54 g/cm³.

Global Distribution

Chloritoid is distributed in the following places:

Kosoi Brod, Mramorskii Zavod, south of Yekaterinburg, Russia
Salmchateau, near Ottre, Ardennes Mountains, Belgium
Nadels and Zermatt, Valais, Switzerland
PrÄagraten, Tirol, Austria
Along the coast between Stonehaven and Aberdeen, Scotland
Tintagel, Cornwall, England
Natick, Kent Co. Rhode Island, USA
Chester, Hampden Co. Massachusetts
Clove Valley, Duchess Co. New York
Bull Mountain, Patrick Co. Virginia
Chibougamau and St. Giles, Quebec, Canada
Kalgoorlie, Western Australia.

Occurrence of Chloritoid and Useful Mineral Association

Chloritoid occurs in regionally metamorphosed pelitic sediments, lower grade staurolite zones, garnet, schists in the biotite, quartz-carbonate veins, and other hydrothermal environments.