From Dwarf Fortress Wiki
Talc is an unremarkable white stone found in large quantities within dolomite. Like dolomite, talc is also magma-safe. Despite its softness in real life, talc can somehow be used in Dwarf Fortress for masonry and even mechanics.
[INORGANIC:TALC] [USE_MATERIAL_TEMPLATE:STONE_TEMPLATE] [STATE_NAME_ADJ:ALL_SOLID:talc][DISPLAY_COLOR:7:7:1][TILE:'|'] [ENVIRONMENT_SPEC:DOLOMITE:CLUSTER:100] [SOLID_DENSITY:2691] [IS_STONE] [MELTING_POINT:12700]