Stilpnomelane Locality: Sterling mine, Antwerp , Jefferson County, New York , USA ( Locality at mindat.org ) Size: 5.4 x 4.4 x 1.7 cm. This classic and historic New York piece is from the Sterling Iron Mine at Antwerp. Foliated, two-toned gray plates of bladed stilpnomelane are set in matrix. Stilpnomelane is an uncommon layered silicate, and this is a very highly representative, old-time specimen. This old mine closed in 1910. Accompanied by an expertly handwritten, faded label from an older collection. The collection this came out of was a museum stash dating to prior to World War I. — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Stilpnomelane
A black, lustrous potassium-iron phyllosilicate of banded iron formations and low-grade schists.
Intro adapted from Wikipedia (CC BY-SA).
Stilpnomelane is an IMA-recognized mineral species in the silicate class, with the chemical formula (K,Ca,Na)(Fe,Mg,Al)₈(Si,Al)₁₂(O,OH)₃₆·nH₂O.
It crystallizes in the triclinic crystal system (space group P-1). It is a member of the Stilpnomelane, Clay structural group.
Key properties: Mohs hardness 3–4; specific gravity 2.59–2.96; Pearly to vitreous on cleavage surface, may be submetallic luster; refractive index 1.639; optically biaxial.
IMA symbol Stp, IMA status: approved, named in 1827.
Found in banded iron formations and enclosing slates and schists. In metamorphic rocks of the blueschist and greenschist facies, and in some metamorphosed massive sulfide deposits.
It is found associated with Quartz, siderite, apatite, hematite, minnesotaite, greenalite, chamosite, grunerite (iron formations); magnetite, chlorite, quartz, pyrrhotite.
Its name comes from the Greek for shining and black, in allusion to its luster and color.