BY STEVE VOYNICK

Talc is well-known to mineral collectors for its hardness or, more precisely, its lack of hardness. Ranking 1.0 on the Mohs hardness scale, talc is the softest of the more than 5,400 recognized minerals.

Talc, or basic magnesium silicate, chemical formula Mg3Si4O10(OH)2, crystallizes in the monoclinic system; usually as foliated or compact masses and only occasionally as tabular crystals. Translucent to opaque and exhibiting perfect one-directional cleavage, talc has a pearly luster, a greasy feel, and colors ranging from white to pale shades of green, yellow, brown, and gray. With a specifi c gravity of 2.75, it is slightly denser than quartz.

Mineral hardness is a function of its atomic structure. Talc is a phyllosilicate or sheet silicate, specifi cally a three-layered sheet silicate with an octahedral layer of magnesium and hydroxyl ions “sandwiched” between two tetrahedral silica layers. This arrangement creates stacks of tightly bonded, threelayered sheets with a neutral charge.