Smith College - Geology 222b - Petrology

Petrographic Data File



Mg-rich variety known as Sismondine
Mn-rich variety known as Ottrelite
Forms series with Carboirite

Crystal System Monoclinic or Triclinic 2/m
Crystal Habit

Foliated plates or massive

Pseudohexagonal tabulate crystals are rare


{001} perfect
{110} good
{010} parting

Not visible in thin section.

Twinning common on {001}.


(x) colorless, olive green, gray
(y) blue-gray, indigo, blue-green
(z) colorless, yellow-green, greenish brown

Optic Sign Biaxial (+) , (-) Most are optically positive but negative minerals have been found
2V 45° to 68° with some outliers


Optic Orientation

X or Y = b (monoclinic);

Z-c angle= 2°-30°; Y = b (triclinic)

Two polymorphs, one monoclinic, the other, triclinic.

Refractive Indices
alpha =
beta = 
gamma =


Values decrease in relationship with the replacement of Fe by Mg.
Max Birefringence 0.006-0.022 High Positive Relief
Elongation yes  
Extinction inclined  
Dispersion r > v  
Distinguishing Features Named for its superficial resemblance to chlorite, chloritoid is characterized by its dark green to gray-green color. It has a hardness of 6.5 and exhibits many optical qualities such as pleochroism, high relief and frequent lamellar twinning.
Occurrence Results from low to medium grade regional metamorphism as well as in hydrothermal environments. Common associative minerals are muscovite, chlorite, garnet, staurolite and kyanite.
Editors Sue Scheufele ('05), Louisa Hall ('15) Data Sources: Deer, W. A., R. A. Howie, and Jack Zussman. Rock-forming Minerals: Volume 1. Ortho-and Ring Silicates. Longmans, 1962.


A cluster of chloritoid crystals in thin section (Sample CH-8b).

This rollover shows chloritoid, the elongate mineral, in both plane polarized light and cross polarized light. Note the lamellar twinning present in xpl. The width of the field of view is 7.75 mm (Sample DR-2b).

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