The Mineral chamosite

Chamosite is a member of the Chlorite, and it forms a series with the mineral Clinochlore. Chamosite is the iron rich end member, and Clinochlore is the magnesium-rich end member. Their properties are very similar, and they are difficult to distinguish from each other, though Chamosite is harder, heavier, and usually less transparent than Clinochlore.

Chemical Formula

(Fe²⁺,Mg)₅Al₂Si₃O₁₀(OH)₈

Color

Greenish-gray, gray, greenish-black, brown

Crystal System

Monoclinic

Properties

Streak White

Hardness 2.5 - 3

Transparency Translucent

Specific Gravity 3 - 3.4

Luster Vitreous to pearly

Cleavage 1,1

Fracture Uneven

Tenacity Thin flakes are flexible but not elastic.

Other ID Marks Larger crystals and masses may have a slightly greasy feel.

Crystal Habits

Crystals, which are usually very small, are tabular and pseudohexagonal in shape, but also may be prismatic. Also in foliated, flaky, scaly, and rosette forms. May also be massive, radiating, and globular masses of dense flakes.

3D Crystal Atlas

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Additional Information

Composition Basic magnesium iron aluminum silicate.

In Group Silicates; Phyllosilicates; Chlorite Group

Striking Features Color and lack of elasticity

Environment As an alteration mineral of metamorphic iron deposits, but also as a secondary mineral in igneous traprock environments and granite pegmatites.

Rock Type Igneous, Metamorphic

Other Names

Thuringite

Noteworthy Localities

Chamosite is more common then perceived, but it usually is just in small uninteresting flakes labelled as Chlorite without being specifically identified. Specific occurences include the Yaogangxian Mine, Hunan Province, China; Klenovec, Slovakia; Ashland, Middlesex Co., Massachusetts; Washington Pass, Okanogan Co., Washington; Atlantic City, Fremont Co., Wyoming; and Moat Mountain, Carroll County, New Hampshire. The New Jersey traprock quarries of Paterson and Prospect Park, Passaic Co., and the Milington Quarry, Somerset  Co. contain Chamosite coatings and dustings upon other minerals.

Common Mineral Associations

Calcite, Albite, Orthoclase, Quartz, Magnetite, Almandine

Distingushing Similar Minerals

Muscovite and other micas - Are more elastic then Chlorites.
Talc - Softer (1).
Other Chlorite minerals - Indistinguishable by practical methods.

chamosite Photos

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