A member of the serpentine group with an ideal composition of approximately (Fe2+, Mn2+, Mg)3-x(Fe3+, Al)x(Si2-xAlx)O5(OH)4. Berthierine is commonly found in unmetamorphosed sedimentary iron formations. Berthierine occurs more commonly as either an apparent trigonal (possibly 1T) or less commonly as an apparent monoclinic (possibly 1M) polytype, and both polytypes are generally intergrown. Berthierine is often confused in the older literature with chamosite, a member of the chlorite group, but not to be confused with berthierite, a sulfide mineral.
Cf., brindleyite, chamosite