Chlorite: The mineral Chlorite information and pictures

About

Chlorite is the group name for about 10 related minerals. However, the term Chlorite can be used both to describe the group in general, or as a specific term to describe any green member of the Chlorite group whose exact identity is not practical to be determined. Although Chlorite is not recognized as an individual mineral species by the IMA, mineral collectors still refer to it by its traditional name, except when specifically referring to the individual member minerals such as Clinochlore, Chamosite, and Cookeite.

Chlorite is very common, and is often an uninteresting green mineral coating the surface of more important minerals. However, there are some crystal forms and varieties that are attractive on their own right. Chlorite also forms as inclusions within other minerals, especially Quartz, where it makes the host mineral green and may even cause phantom growths.

Identification

Chemical Formula: Below is a simple formula encompassing the common members of the Chlorite Group:
(Mg,Fe,Li)₆AlSi₃O₁₀(OH)₈

The formula for the three most common individual group members are:
Cookeite: LiAl₅Si₃O₁₀(OH)₈Clinochlore: (Mg,Fe²⁺)₅Al₂Si₃O₁₀(OH)₈
Chamosite: (Fe²⁺,Mg)₅Al₂Si₃O₁₀(OH)₈
Composition: Basic iron magnesium aluminum silicate
Color: Light to dark green, grayish-green, black. Some rarer varieties are white, yellow, brown, pink and purple.
Streak: White
Hardness: 2–2.5
Crystal System: Monoclinic
Luster: Vitreous, pearly, or dull
Cleavage: 1,1
Fracture: Uneven
Tenacity: Thin flakes are flexible but not elastic.
Transparency: Transparent to translucent
Specific Gravity: 2.6–3.3
Group: Silicates; Phyllosilicates; Chlorite Group
Rock Type: Igneous, Metamorphic

Rankings

Popularity (1–4): 2
Prevalence (1–3): 1
Demand (1–3): 3

Crystal Forms & Aggregates

Crystals are usually tabular and pseudohexagonal in shape, but also may be prismatic. Also as thick flakes, micaceous masses and groupings, and in foliated, flaky, and scaly forms. May also be massive and in globular masses of dense flakes.

Chlorite commonly form pseudomorphs and dense coatings of other minerals, assuming the original minerals crystal shape.

Other ID Marks

Has a slightly greasy feel.

Striking Features

Color and lack of elasticity

Environment

Most often in metamorphic environments, especially in Serpentine deposits; also as a secondary mineral in volcanic basalt and in hydrothermal replacement deposits.

Noteworthy Localities

Although Chlorite is a fairly common mineral and found worldwide, localities producing material worthy of collections are surprisingly limited. Maderanertal, in Uri, Switzerland, is famous for its shiny green Chlorite coatings on Quartz and Adularia. Many interesting Quartz phantoms caused by Chlorite have been found there.

In the U.S., Excellent pseudomorphs of Chlorite after Garnet come from Michigamme, Marquette Co., Michigan. Chlorite as a matrix to nice Magnetite and Pyrite crystals comes from Chester, Windsor Co., Vermont. Good crystals come from the Green Mountain Mine, Day Book, Yancey Co., North Carolina. Strange pseudomorphs of Chlorite after all types of minerals came from the Tilly Foster Mine, Brewster, Putnam Co., New York.

Other localities include Trumbull, Fairfield Co., Connecticut; the Warren Brothers Quarry, Acushnet, Bristol County, Massachusetts; and the New Idria District in the Diablo Range, San Benito Co., California.