Roymillerite, Pb24Mg9(Si9AlO28)(SiO4)(BO3)(CO3)10(OH)14O4, a new mineral : mineralogical characterization and crystal chemistry

• The new mineral roymillerite Pb24Mg9(Si9AlO28)(SiO4)(BO3)(CO3)(10)(OH)(14)O-4, related to britvinite and molybdophyllite, was discovered in a Pb-rich assemblage from the Kombat Mine, Grootfontein district, Otjozondjupa region, Namibia, which includes also jacobsite, cerussite, hausmannite, sahlinite, rhodochrosite, barite, grootfonteinite, Mn-Fe oxides, and melanotekite. Roymillerite forms platy single-crystal grains up to 1.5 mm across and up to 0.3 mm thick. The new mineral is transparent, colorless to light pink, with a strong vitreous lustre. Cleavage is perfect on (001). Density calculated using the empirical formula is equal to 5.973 g/cm(3). Roymillerite is optically biaxial, negative, alpha = 1.86(1), beta ae gamma = 1.94(1), 2V (meas.) = 5(5)A degrees. The IR spectrum shows the presence of britvinite-type tetrahedral sheets, , , and OH- groups. The chemical composition is (wt%; electron microprobe, H2O and CO2 determined by gas chromatography, the content of B2O3 derived from structural data): MgO 4.93, MnO 1.24, FeO 0.95, PbO 75.38, B2O3 0.50, Al2O3 0.74, CO2 5.83, SiO2 7.90, H2O 1.8, total 99.27. The empirical formula based on 83 O atoms pfu (i.e. Z = 1) is Pb24.12Mg8.74Mn1.25Fe0.94B1.03Al1.04C9.46Si9.39H14.27O83. The crystal structure was determined using single-crystal X-ray diffraction data. The new mineral is triclinic, space group P , with a = 9.315(1), b = 9.316(1), c = 26.463(4) , alpha = 83.295(3)A degrees, beta = 83.308(3)A degrees, gamma = 60.023(2)A degrees, V = 1971.2(6) (3). The crystal structure of roymillerite is based built by alternating pyrophyllite-type TOT-modules Mg-9(OH)(8)[(Si,Al)(10)O-28] and I-blocks Pb-24(OH)(6)O-4(CO3)(10)(BO3,SiO4). The strongest lines of the powder X-ray diffraction pattern [d, (I, %) (hkl)] are: 25.9 (100) (001), 13.1 (11) (002), 3.480 (12) (017, 107, -115, 1-15), 3.378 (14) (126, 216), 3.282 (16) (-2-15, -1-25), 3.185 (12) (-116, 1-16), 2.684 (16) (031, 301, 030, 300, 332, -109, 0-19, 1-18), 2.382 (11) (0.0.-11). Roymillerite is named to honor Dr. Roy McG. Miller for his important contributions to the knowledge of the geology of Namibia.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geologi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geology (hsv//eng)

Nyckelord

New mineral

Roymillerite

Britvinite

Crystal structure

IR spectroscopy

Kombat Mine

Namibia

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