Mineralpedia Details for Olmsteadite
Named after Frank Olmstead and first found at the Hesnard mine and later the Big Chief mine, and White Cap mines. All three are zoned pegmatites near Keystone. Olmsteadite was the first secondary transition-metal phosphate containg niobium indicating a paragenetic relationship between Columbite-Tantalite and a phosphate, probably Triphylite. Occurs as thick prismatic, very lustrous, sub-adamantine, deep red-brown, well-developed crystals to 1mm. (Moore, PB et al AM v.61, (1976). South Dakota School of Mines display specimen contain crystals to about 10mm. Can be confused with deep red Sphalerite or dark smoky Quartz, both of which are found in the same matrix of advanced altered phosphate rock with cores of Siderite and Hydroxylapatite.
Ref. Handbook of Mineralogy, Anthony et al (1995) and MSA at http://www.handbookofmineralogy.org/pdfs/olmsteadite.pdf
Olmsteadite from White Cap mine, Pennington Co., South Dakota, United States
Red bladed to tabular crystals, less than 1cm.
Olmsteadite from Big Chief mine, Pennington Co., South Dakota, United States
Deep red tabular crystals to 1cm in Siderite.