LONDONITE, A NEW MINERAL SPECIES: THE Cs-DOMINANT ANALOGUE OF RHODIZITE FROM THE ANTANDROKOMBY GRANITIC PEGMATITE, MADAGASCAR

Abstract

Londonite, (Cs,K,Rb) Al 4 Be 4 (B,Be) 12 O 28 , is the Cs-dominant analogue of rhodizite. It occurs with rhodizite at Antandrokomby, a dike of granitic pegmatite in the Manandona valley, Antsirabe region, and at Ampanivana and Antsongombato, in the Betafo region south of Mahaiza, all in Madagascar. Londonite occurs in the inner zones and in miarolitic cavities of highly evolved pegmatites rich in red tourmaline. Associated minerals include microcline, quartz, albite, tourmaline (elbaite – liddicoatite – schorl series), Cs-rich beryl, spodumene, danburite, Mn-rich fluorapatite, hambergite, microlite, manganocolumbite, manganotantalite, behierite, and hafnian zircon, among others. Londonite occurs as equidimensional crystals a few millimeters across, and exceptionally up to 7 cm. The dominant morphology is dodecahedral {110} and rare deltoid dodecahedral {221}, modified by tristetrahedral {211}, tetrahedral {111} and very rare cube {100} faces. Crystals are colorless to white to yellow, with a white streak. The luster is vitreous. Crystals are translucent to transparent. Optically, the mineral is isotropic, with an index of refraction of 1.693. Mohs hardness is 8. It has no cleavage or parting, and is brittle with a conchoidal fracture. Its density is 3.34 g/cm 3 (meas.) and 3.42 g/cm 3 (calc.). Londonite is isometric with space group P 43 m. The cell parameters are: a 7.321 (1) A, V 392.30 A 3 , and Z = 1. The three strongest X-ray powder-diffraction lines measured for londonite [ d in A( I )( hkl )] are 2.9898(100)(211), 2.1132(70)(222), and 2.4410(50)(300,221). Electron-microprobe analyses reveal a wide range of K and Cs contents and an overall low abundance of Rb. The empirical formula based on 28 atoms of oxygen and calculated from an average of five analyses from Antandrokomby is: (Cs 0.48 K 0.38 Rb 0.09 Na 0.03 Ca 0.02 Mn 0.01 ) ∑1.01 (Al 3.98 Li 0.02 Fe 0.01 ) ∑4.01 Be 4 [(B 10.99 Si 0.01 ) ∑11.00 Be 1 ] ∑12.00 O 28 , with the proportion of B and Be fixed by stoichiometry. The mineral is named after David London, Professor of Geology and Geophysics, University of Oklahoma, in recognition of his experimental studies of evolved granitic melts and his contributions to our understanding of the origin of granitic pegmatites.