License: Creative Commons - Attribution Author: James St. John
“Madocite” (field of view ~2.7 cm across) from the Precambrian of Ontario.
Here’s a rock from Ontario having a mineral combination you don’t see very often. This sample is from a metamorphosed mafic dike that intrudes tremolite-talc schists hosted in dolomitic marble. The metamorphism has converted the original mafic igneous minerals into a mixture dominated by tourmaline (black) and pyrite (brassy gold). Some of the small bright white patches are talc. Tourmaline-pyrite metamorphic rocks lack a formal name, so the term “madocite” has been used by some geologists. Published mineral analysis indicates that “madocite” contains tourmaline, pyrite, tremolite, and some phlogopite mica.
Age of metamorphism: probably ~1.24 billion years, during the Mesoproterozoic. Subsequent metamorphism occurred at ~1.1 billion years, during assembly of the Rodinia supercontinent.
Initial metamorphic agent: probably emplacement of the nearby Deloro Granite.
Locality: Canada Talc Limited mine at Madoc, south-central Hastings County, southeastern Ontario, southeastern Canada.
Most info. from:
Dunn (1998) - Formation of dolomite-hosted massive talc at Madoc, Ontario. Geological Society of America Abstracts with Programs 30(7): 229-230.