Micas, feldspars and columbite-tantalite minerals from the zoned granitic lepidolite-subtype pegmatite at Namivo, Alto Ligonha, Mozambique
Neiva, Ana M. R.
European Journal of Mineralogy Volume 25 Number 6 (2014), p. 967 - 985
published: Feb 28, 2014
ArtNo. ESP147052506006, Price: 29.00 €
The Namivo granitic lepidolite-subtype pegmatite is concentrically zoned. Textural and chemical studies show at the crystal scale and within each zone that muscovite evolved to lithian muscovite and the latter to ?mixed form? in the outer and inner intermediate zones and core. ?Mixed form? also evolved to lepidolite in the inner intermediate zone and core. Each of these micas represents an evolution from the pegmatite outer intermediate zone to the inner intermediate zone and core. Some crystals are progressively zoned while others are reversely zoned, but overgrowths and replacements also occur. Albite and K-feldspar evolved from the wall zone to the core and feldspar thermometry records a decrease in temperature. Columbite-(Mn) shows a compositional trend from the outer intermediate zone to the core typical of this mineral from a lepidolite-subtype granitic pegmatite. The chemical zonation of this pegmatite is derived from crystallization of an undercooled granitic melt in which a probably local constitutional zone refining of fluxing and incompatible elements contributes to the textural and chemical changes from the wall zone to the core. The unexpected occurrence of lepidolite and Fe/Mg-containing polylithionite, the richest micas in Si, Licalc. and F, in the outer intermediate zone is attributed to disequilibrium crystallization from an undercooled melt. They are associated with tantalite, and Ta precipitation may have been caused by the local decrease of Li and F in the melt. These minerals are uncommon in this zone of granitic pegmatites.