Madocite

Madocite is a mineral with a chemical formula of Pb₁₇(Sb,As)₁₆S₄₁. Madocite was named for the locality of discovery, Madoc, Ontario, Canada. It is found in the marbles of the Precambrian Grenville Limestone.[6] It is orthorhombic (rectangular prism with a rectangular base) and in the point group mm2. Its crystals are elongated and striated along [001] to a size of 1.5 mm.

Madocite
General
Category	Sulfosalt mineral
Formula (repeating unit)	Pb17(Sb,As)16S41
IMA symbol	Mdc[1]
Strunz classification	2.LB.30
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) (same H-M symbol)
Space group	Pb2a
Unit cell	a = 27.2  Å, b = 34.1 Å, c = 8.12 Å; Z = 4
Identification
Color	Grayish black
Crystal habit	Elongated and striated crystals; massive
Cleavage	{010} Perfect
Fracture	Conchoidal
Mohs scale hardness	3.25
Luster	Metallic
Streak	Grayish black, shining
Diaphaneity	Opaque
Specific gravity	5.98
Pleochroism	Strong, from white to gray
References	[2][3][4][5][6]

Madocite is anisotropic and classified as having high relief. It also displays strong pleochroism.[5]

Madocite is found in small clusters in marble pits (near Madoc, Ontario), and was originally categorized in the 1920s as an unidentified sulfosalt mineral in an assemblage of pyrite, sphalerite, and jamesonite in marble. Later research was done by John L. Jambor in the 1960s who went to the site and collected samples of the assemblages.[7]