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Formula: NaFe3+3(Al4Fe2+2)(Si6O18)(BO3)3(OH)3O
Cyclosilicate (ring silicate), tourmaline group,
boron-bearing mineral
Crystal system: Trigonal
Specific gravity: 3.216 calculated for the empirical formula
Hardness: 7
Streak: Brown
Colour: Black
Luminescence: No fluorescence under UV
Environments
ferro-bosiite is a new mineral, approved in 2022 and to date (January 2026) reported only from the type
locality.
Localities
At the type locality, the "Marina” granitic pegmatite, Mavuco, Mogovolas District, Nampula Province, Mozambique, the
holotype specimen of ferro-bosiite was found in a giant collapsed cavity in the
pegmatite, which is hosted in
amphibolite. The core zone of the
pegmatite at the surface was characterised by large masses of
white to pale rose quartz associated with
schorl and blue aquamarine. At depth,
however, the pegmatite core changed to become a coarse-grained
rock composed of cleavelandite with minor
quartz and local
amazonitic microcline, with abundant accessory black and multicoloured
tourmaline prisms from a few cm up to over 30 cm, rare large tabular crystals
of pale pink beryl with a pale green core, and granular masses of silvery to
purplish lithium-rich mica. At a depth of
about 25 m, a giant collapsed cavity was encountered. This cavity was mostly filled with collapsed crystals and rocks
partially cemented by a thick crust of whitish late-stage chalcedony. The
collapsed material is mostly composed of cleavelandite in large ‘pillows’ and
large milky quartz crystals in aggregates up to over a ton in weight. In the
collapsed material, several hundreds of kilos of yellow to pink tourmaline
crystals, from a few cm up to about 20 cm, covered by a thick black
tourmaline overgrowth, were discovered. Among the other minerals found in the
collapsed materials, there were several tens of kilograms of native bismuth, and
purple fluorite in octahedral crystals up to 20 cm across.
After the partial
removal of the collapsed materials, a cavity over 12 m long and up to 6 m wide was revealed. The multicolour
tourmaline crystals formed together with the
cleavelandite in the cavity before the collapse, and grew as prisms in the
direction of the antilogous pole (the pole of a crystal that becomes negatively electrified when the crystal is heated),
with a steep pyramidal termination. These crystals were damaged and broke off from the matrix during the pocket rupture;
they were later overgrown by a black tourmaline up to 4 mm thick.
The late-stage overgrowth covered all of the
tourmaline surfaces, but the ferro-bosiite composition is
strictly confined to the
overgrowths on analogous pole (the pole of a crystal that becomes negatively electrified when the crystal is heated)
surfaces, such as the bottom of the crystals broken off from the matrix; the remaining
tourmaline surfaces are covered by black overgrowth of
dravitic composition along with bosiite.
Ferro-bosiite occurs as a black acicular late-stage overgrowth (up to 4mm thick and chemically heterogeneous) at
the analogous pole (the pole of a crystal that becomes negatively electrified when the crystal is heated) of a
multicoloured fluor-elbaite crystal of several centimetres in length. The
black crystals, with a vitreous lustre, have a brown streak and show no fluorescence
(MM 89.886–896).
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