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My understanding is that soapstone is talc (hydrated magnesium silicate) which has been metamorphically transformed. However, it is not clear to me how this transformation affects the talc.

What is the difference between talc and soapstone?

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Soapstone is a form of talc. It is basically talc plus other minerals, such as micas, chlorite and amphiboles, depending on the initial rocks that were metamorphosed. Soapstone is also called a talc-schist.

Soapstone is formed by the metamorphism of ultramafic protoliths, such as dunite or serpentinite and the metasomatism of siliceous dolomites. The metamorphism occurs in the regions where tectonic plates are present, where the rocks are subjected to heat and pressure, with flowing liquids, but without melting. Soapstone is derived from hydrothemal alteration of peridotite and related ultramafic rocks.

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My understanding is that soapstone is talc (hydrated magnesium silicate) which has been metamorphically transformed

Not quite. Soapstone is talc-rich rock. The way you put, you had talc to begin with, then it was metamorphosed, and something happened to the talc to transform it into soapstone. This is not accurate.

You had something else to begin with (see Fred's answer), and it was transformed into talc-containing soapstone during metamorphism.

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