Original paper
Rediscovery of manandonite in the Sahatany Valley, Madagascar
Ranorosoa, Nadine; Fontan, Francois; Fransolet, André-Mathieu
European Journal of Mineralogy Volume 1 Number 5 (1989), p. 633 - 638
22 references
published: Nov 13, 1989
manuscript accepted: May 30, 1989
manuscript received: Feb 15, 1989
DOI: 10.1127/ejm/1/5/0633
Abstract
Abstract Manandonite occurs as white flakes with a pearly lustre on the micaceous (001) cleavage, associated with elbaite and quartz lining vugs of saccharoidal albite, collected in the type locality, i.e. the Antandrokomby pegmatite. Optically pseudo-uniaxial to biaxial positive (2V about 10-15°). α - ß = 1.604(5). The crystals are often twinned on (001) as 6-fold sectors. Manandonite is a 1:1 layer silicate (7 Å serpentine-kaolinite group) 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 Å, space group C2221). The strongest X-ray lines are 6.92 (100)(002), 4.362 (25)(110), 3.447 (80)(004), 2.489 (15)(131,201), 2.376 Å (35)( 132,202). A wet-chemical analysis gave SiO2 23.02, B2O3 7.71, Al2O3 47.85, Fe2O3 0.36, MnO 0.02, MgO 0.03, CaO 0.07, Na2O 0.13, Li2O 5.38, H2O 15.40, sum 99.97 wt.%, which yields the ideal formula, Li2Al4[(Si2AlB)O10](OH)8, with Z = 2. Dmeas. = 2.78(2); Dcalc. - 2.79. The thermal behaviour and the infrared spectrum constitute additional features to differentiate manandonite from cookeite.
Keywords
manandonite • lithium • boron • serpentine-kaolinite group • Madagascar