If it’s very soft and feels a bit greasy and/or soapy, there’s a good chance it’s talc.

Physical Properties
Chemical formula Mg3Si4O10(OH)2
Class Phyllosilicate
Sheets of linked tetrahedra
Crystal system Monoclinic
Habit Usually massive
Usually very fine-grained
Rare tabular pseudohexagonal
Color Gray
Hardness 1
Specific gravity 2.8
Cleavage Perfect basal (001)
Fracture Flexible
Luster Resinous
Transparency Translucent
Streak White
Optical Properties
PPL Colorless
XPL 1st order grays and yellows
May appear smeared or
poorly defined
δ 0.05
Special properties
Soft, greasy feel
after Perkins, 320

Talc in Hand Sample

A different talc
Talc in soapstone

Talc in Thin Section

Thin Section GigaPans

Talc, plane polars
Talc, crossed polars
Talc tremolite schist, plane polars
Talc tremolite schist, crossed polars

Talc, PPL

Talc, XPL

Talc tremolite schist, PPL

Talc tremolite schist, crossed polars

Further Reading

Talc at webmineral.com
Talc at mindat.org