Talc
Mg₃Si₄O₁₀(OH)₂ · Monoclinic · also called soapstone, steatite
Talc is the bottom of the Mohs scale and the substance of talcum powder. Massive talc rock, called soapstone, has been carved for millennia because it is soft enough for hand tools and tough enough to survive.
Properties
- Mohs hardness
- 1
- Streak
- white
- Lustre
- pearly, greasy, dull
- Colour
- White, pale green, or grey, usually with a slightly translucent, soapy appearance.
- Cleavage
- Perfect basal cleavage, though massive soapstone rarely shows it.
- Fracture
- Uneven
- Specific gravity
- 2.58–2.83 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm talc
- Softest mineral there is — a fingernail carves it easily, and it defines hardness 1.
- Unmistakably greasy, soapy feel.
- Massive soapstone can be carved with a knife like hard wood.
What talc gets confused with
Each of these is settled by one test. Run that test first.
Gypsum is harder (2) and does not feel soapy; talc is scratched by anything.
Serpentine is harder (3–5) and resists a fingernail.
Where it is found
Altered ultramafic rocks and magnesium-rich metamorphic belts.
Is it worth anything?
No specimen value; soapstone is sold cheaply as carving stock.