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

Abstract

The new mineral roymillerite Pb24Mg9(Si9AlO28)(SiO4)(BO3)(CO3)10(OH)14O4, 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/cm3. Roymillerite is optically biaxial, negative, α = 1.86(1), β ≈ γ = 1.94(1), 2V (meas.) = 5(5)°. The IR spectrum shows the presence of britvinite-type tetrahedral sheets, $${\text{CO}}_{3}^{2 - }$$ , $${\text{BO}}_{3}^{3 - }$$ , 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 $$\bar{1}$$ , with a = 9.315(1), b = 9.316(1), c = 26.463(4) A, α = 83.295(3)°, β = 83.308(3)°, γ = 60.023(2)°, V = 1971.2(6) A3. The crystal structure of roymillerite is based built by alternating pyrophyllite-type TOT-modules Mg9(OH)8[(Si,Al)10O28] and I-blocks Pb24(OH)6O4(CO3)10(BO3,SiO4). The strongest lines of the powder X-ray diffraction pattern [d, A (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.