Fencooperite, Ba6Fe33+Si8O23(CO3)(2)Cl-3 center dot H2O, a new mineral species from Trumbull Peak, Mariposa County, California

Fencooperite, a new mineral species having the ideal formula Ba6Fe33+Si8O23 (CO3)(2)Cl-3.H2O, is trigonal, P3m 1, with unit-cell parameters refined fro m powder data: a 10.727(5), c 7.085(3) Angstrom, V 706.1(5) Angstrom (3), c /a 0.6605, Z = 1. The strongest seven lines of the X-ray powder-diffraction pattern [d in Angstrom (I)(hkl)] are: 3.892(100)(201), 3.148(40)(211), 2.8 20(90)(202), 2.685(80)(220), 2.208(40)(401), 2.136(40)(222) and 1.705(35)(4 21). The mineral occurs as a dominant phase within black aggregates 2 mm ac ross in barium-silicate-rich lenses at Trumbull Peak, Mariposa County, Cali fornia. It is a primary phase, formed after barite, in an aggregate assembl age that includes alforsite, barite, celsian, gillespite, quartz, pyrrhotit e and sanbornite. Additional minerals found within the lenses are anandite, benitoite, bigcreekite, fresnoite, kinoshitalite, krauskopfite, macdonaldi te, pellyite, titantaramellite, walstromite, witherite, biotite, diopside, fluorapatite, pentlandite, schorl, vesuvianite and three undefined species. Individual grains of fencooperite are anhedral to somewhat rounded to occa sionally platy, are jet black to a dirty grey-brown (on very thin edges), d o not exceed 100 mum in size, and have an uneven to subconchoidal fracture. Neither morphological forms nor twinning were observed. The streak is grey ish black, tenacity is brittle, luster is vitreous to adamantine, and diaph aneity, opaque to translucent (on thin edges). There is no obvious cleavage ; D (calc.) 4.338 (for the ideal formula), 4.212 g/cm(3) (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 analys es yield BaO 50.51, Fe2O3 12.77, MnO 0.15, SiO2 27.38, Al2O3 1.35, P2O5 0.1 6, Cl 3.23, CO2 [4.81], H2O [0.98], sum 101.34, less O = Cl -0.73, total [1 00.61] wt.%. The empirical formula, derived from results of the crystal-str ucture analysis and electron-microprobe analyses, is Ba-5.89(Fe3+ Mn-2.86(2 +) (0.04))(Sigma2.90) (Si8.14Al0.47P0.04)(Sigma8.65) O-23.18(CO3)(1.95)(Cl1 .63O1.37)(Sigma3.00).0.97 H2O, on the basis of O + Cl = 33. The infrared-ab sorption spectrum shows bands for carbonate and possibly for structural H2O . The mineral name honors Joseph Fenimore Cooper, Jr., Santa Cruz, Californ ia, in recognition of his substantive contributions to both mineralogy and mineral collecting in the western United States.