“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. — Photo: James St. John · CC BY 2.0
Madocite
A complex lead-antimony-arsenic sulfosalt found in the Grenville marbles at Madoc, Ontario.
Intro adapted from Wikipedia (CC BY-SA).
Madocite is an IMA-recognized mineral species in the sulfosalt class, with the chemical formula Pb₁₉(Sb,As)₁₆S₄₃.
It crystallizes in the orthorhombic crystal system. It is a member of the Madocite structural group.
Key properties: specific gravity 5.98; Gray-black, shining streak; Metallic luster.
IMA symbol Mdc, IMA status: approved, named in 1966.
Found in the marbles of the Precambrian Grenville Limestone, at the margin of an intrusive (Madoc, Canada); in a hydrothermal deposit in dolomitic limestones with other As–Tl minerals (Jas Roux, France).
It is found associated with Jamesonite, boulangerite, arsnopyrite (Madoc, Canada); chabournéite, pierrotite, parapierrotite, stibnite, pyrite, sphalerite, twinnite, zinkenite, andorite.
The name is derived from the locality at Madoc, Canada.