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

The new mineral roymillerite Pb 24 Mg 9 (Si 9 AlO 28 )(SiO 4 )(BO 3 )(CO 3 ) 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, α = 1.86(1), β ≈ γ = 1.94(1), 2 V (meas.) = 5(5)°. The IR spectrum shows the presence of britvinite-type tetrahedral sheets, CO 3 2 - , BO 3 3 - , and OH − groups. The chemical composition is (wt%; electron microprobe, H 2 O and CO 2 determined by gas chromatography, the content of B 2 O 3 derived from structural data): MgO 4.93, MnO 1.24, FeO 0.95, PbO 75.38, B 2 O 3 0.50, Al 2 O 3 0.74, CO 2 5.83, SiO 2 7.90, H 2 O 1.8, total 99.27. The empirical formula based on 83 O atoms pfu (i.e. Z = 1) is Pb 24.12 Mg 8.74 Mn 1.25 Fe 0.94 B 1.03 Al 1.04 C 9.46 Si 9.39 H 14.27 O 83 . The crystal structure was determined using single-crystal X-ray diffraction data. The new mineral is triclinic, space group P 1 ¯ , with a = 9.315(1), b = 9.316(1), c = 26.463(4) Å, α = 83.295(3)°, β = 83.308(3)°, γ = 60.023(2)°, V = 1971.2(6) Å 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 (CO 3 ) 10 (BO 3 ,SiO 4 ). The strongest lines of the powder X-ray diffraction pattern [ d , Å (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.