Talc
The softest mineral there is, and the one that defines the bottom of the hardness scale — soft enough to leave a mark on paper with a fingernail's pressure.
Talc is the softest mineral on the Mohs scale, and it defines that scale's lowest point: hardness 1 is not a measurement but a comparison with talc. It is a magnesium silicate built in sheets held together by forces weak enough that the sheets slide over one another under a fingertip, which is where both the softness and the characteristic greasy feel come from.
That structure explains almost everything talc is used for. A material that shears easily in one plane is a lubricant; a material that is chemically inert, white and cheap is a filler; and talc is all of those at once.
How to identify it
Talc is identified by feel as much as by sight. It is greasy or soapy to the touch, marks paper, and can be scratched by a fingernail without effort — the standard field test for hardness 1.
It is most often pale green, grey or white, with a pearly lustre on cleavage surfaces. In massive form it is soapstone, soft enough to carve with a knife and hard enough to last centuries once carved.
Uses
The largest uses are industrial rather than cosmetic. Talc is a filler in paper, paint and plastics, where it adds bulk, whiteness and stiffness at low cost, and it is used in ceramics and as a carrier for agricultural chemicals.
Soapstone is carved into vessels, moulds and hearthstones, and used where a material must take heat without cracking. Talc's use in cosmetics is the most familiar and the smallest, and is the one that has attracted concern where deposits carry asbestos minerals alongside the talc.
How it forms
Talc forms where magnesium-rich rocks are altered by hot water — typically in the metamorphism of ultramafic rocks, or where dolomite reacts with silica-bearing fluids. Both routes produce talc as a replacement rather than a primary crystal, which is why it is usually found massive rather than in well-formed crystals.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is a component of
- Serpentinite — rock · where carbon dioxide-bearing fluid carried the alteration a stage further, which is the origin of much of the talc mined worldwide
- Schist — rock · in the schists derived from ultramafic rock rather than from mud, which is where most industrial talc comes from
is used as
- Glass and ceramics — application · in ceramic bodies, and in the cordierite used for kiln furniture and catalytic converter substrates
is an alternative to
- Clay — material · as a mineral filler in paper, paint and plastic, where the choice is between talc's platy particles and kaolin's stacked ones, and both are being bought for opacity per tonne
Sources
- Material WorldOur own writing
- US National Library of Medicine · Mixed — aggregated third-party content, rights retained by depositors
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- COD Advisory Board / Vilnius University · CC0 — contributors place data in the public domain