Talcose rocks usually form from hydrothermal or regional metamorphism of dolomitic marbles or ultramafic & mafic rocks, resulting in talc schists, soapstones, and steatites.
The Allamoore Formation (mid-Mesoproterozoic, 1.25 billion years) of western Texas contains commercial quantities of talc (magnesium hydroxysilicate - Mg3Si4O10(OH)2). Published research indicates that Allamoore talcose rocks have an unusual origin. They formed by hydrothermal metamorphic replacement of an evaporitic lacustrine magnesite succession by hydrothermal connate or meteoric waters.
Locality: Bridges Talc Mine (a.k.a. Texola Talc Mine), just north of I-10, northwest of town of Allamoore, Allamoore Talc District, ~10-12 miles west of Van Horn, southeastern Hudspeth County, Trans-Pecos Mountains, far-western Texas, USA (about 31¡ 05Õ 43Ó North, 105¡ 01Õ 33Ó West).
Allamoore talc (8.3 cm across) from the Precambrian of Allamoore, Texas, USA.