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1 Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada
2 Mineral Sciences Division, Canadian Museum of Nature, P.O. Box 3443, Station "D", Ottawa, Ontario K1P 6P4, Canada
3 773 Durshire Way, Sunnyvale, California 94087, U.S.A.
4 Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada
E-mail address: aroberts{at}nrcan.gc.ca
Fencooperite, a new mineral species having the ideal formula Ba6Fe3+3 Si8O23(CO3)2Cl3·H2O, is trigonal, P3m1, with unit-cell parameters refined from powder data: a 10.727(5), c 7.085(3) Å, V 706.1(5) Å3, c/a 0.6605, Z = 1. The strongest seven lines of the X-ray powder-diffraction pattern [d in Å (I)(hkl)] are: 3.892(100)(201), 3.148(40)(211), 2.820(90)(202), 2.685(80)(220), 2.208(40)(401), 2.136(40)(222) and 1.705(35)(421). The mineral occurs as a dominant phase within black aggregates 2 mm across in barium-silicate-rich lenses at Trumbull Peak, Mariposa County, California. It is a primary phase, formed after barite, in an aggregate assemblage that includes alforsite, barite, celsian, gillespite, quartz, pyrrhotite and sanbornite. Additional minerals found within the lenses are anandite, benitoite, bigcreekite, fresnoite, kinoshitalite, krauskopfite, macdonaldite, pellyite, titantaramellite, walstromite, witherite, biotite, diopside, fluorapatite, pentlandite, schorl, vesuvianite and three undefined species. Individual grains of fencooperite are anhedral to somewhat rounded to occasionally platy, are jet black to a dirty grey-brown (on very thin edges), do not exceed 100 µm in size, and have an uneven to subconchoidal fracture. Neither morphological forms nor twinning were observed. The streak is greyish black, tenacity is brittle, luster is vitreous to adamantine, and diaphaneity, opaque to translucent (on thin edges). There is no obvious cleavage; D (calc.) 4.338 (for the ideal formula), 4.212 g/cm3 (for the empirical formula). It is nonfluorescent in ultraviolet light; H (VHN load 10 g) 269 to 367, H (Mohs) 4.5 to 5. Fencooperite is uniaxial negative, O 1.723(4), E 1.711(2), very strongly pleochroic from blue-black (O) to light greenish grey (E); absorption O >> E. Averaged results of electron-microprobe analyses yield BaO 50.51, Fe2O3 12.77, MnO 0.15, SiO2 27.38, Al2O3 1.35, P2O5 0.16, Cl 3.23, CO2 [4.81], H2O [0.98], sum 101.34, less O = Cl –0.73, total [100.61] wt.%. The empirical formula, derived from results of the crystal-structure analysis and electron-microprobe analyses, is Ba5.89(Fe3+2.86Mn2+0.04)
2.90 (Si8.14Al0.47P0.04)8.65 O23.18(CO3)1.95(Cl1.63O1.37)3.00·0.97 H2O, on the basis of O + Cl = 33. The infrared-absorption spectrum shows bands for carbonate and possibly for structural H2O. The mineral name honors Joseph Fenimore Cooper, Jr., Santa Cruz, California, in recognition of his substantive contributions to both mineralogy and mineral collecting in the western United States.
Keywords: fencooperite, new mineral species, hydrated barium–iron silicate carbonate chloride, X-ray data, electron-microprobe data, infrared spectroscopy, Trumbull Peak, Mariposa County, California.
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