Rediscovery of manandonite in the Sahatany Valley, Madagascar

Manandonite occurs as white flakes with a pearly lustre on the (001) cleavage, associated with elbaite and quartz lining vugs of saccharoidal albite, collected in the type locality. The mineral is optically pseudo-uniaxial to biaxial positive, the crystal often twinned on (001) as 6-fold sectors. It is a 1:1 layer silicate and can be considered as a two-layer polytype with a C- centred orthorhombic-shaped cell (a 5.057, b 8.765, c 13.769 Angstrom). A wet-chemical analysis yielded the idealized formula Li (sub 2) Al (sub 4) [(Si (sub 2) AlB)O (sub 10) ](OH) (sub 8) , with Z = 2. The thermal behaviour and the IR spectrum constitute additional features to differentiate manandonite from cookeite.