Formula: (Fe2+,Mg,Al,Fe3+)6(Si,Al)4O10(OH,O)8
Phyllosilicate (sheet silicate), chlorite group
Forms a series with clinochlore
Specific gravity: 2.95 to 3.30
Hardness: 3
Colour: Green, grey-green, black
Solubility: Decomposed by hot hydrochloric acid (Dana)

Plutonic igneous environments
Sedimentary environments
Metamorphic environments
Hydrothermal environments
Basaltic cavities

Chamosite occurs in many different environments:
- In granitic bodies formed by the hydrothermal alteration of micas such as biotite.
- In granite pegmatites.
- In sedimentary ironstones. Abundant in soils and marine clays. Found in coal deposits.
- Authigenically formed under reducing conditions in the presence of decomposed organic material.
- Formed in low-grade regional and contact metamorphosed rocks.
- As a hydrothermal alteration of wall rock around sulphide deposits.
- In amygdules in volcanic rocks.

Chamosite has been found as a replacement of a wide variety of silicates, including garnet.

Associated minerals include siderite, kaolinite, quartz, magnetite, pyroxene, plagioclase, olivine, calcite, feldspar, garnet, muscovite, biotite, amphibole, pyroxene, dolomite, chromite, talc, serpentine, chloritoid, ilmenite, titanite, zircon, zeolites, tourmaline, hematite and rutile.

At Girardville, Quebec, Canada, chamosite is found at the contact with the enclosing gneiss and in altered zones of the pegmatite. Hematite is generally found on its surface.

At Saint-Pierre-de-Broughton, Quebec, Canada, chamosite occurs in the albite-stilpnomelane schist and carbonate rock contacts. It is common in the anatase- and brookite-bearing fissures, often forming coatings on and inclusions in albite, anatase, brookite, and quartz.

At the Yaogangxian mine, Hunan, China, chamosite occurs on quartz.

At the Braen quarry, Passaic county, New Jersey, USA, chamosite has been found as pseudomorphs after analcime and natrolite.

Common impurities: Mn,Ca,Na,K