New Mineral Names

Abstract

A l m e i d a i t e (I M A 2 0 1 3-0 2 0) , P b (M n , Y) Z n 2 (Ti,Fe 3+) 18 O 36 (O,OH) 2 , is a new mineral from Novo Horizonte, Bahia, Brazil. The mineral was found in altered dacite, 2 m away from a hydrothermal quartz vein that intercepts the dacite. Associated minerals include rutile, hematite, quartz with rutile inclusions, and xenotime-(Y), although these did not belong to the original dacite but to the hydrothermal quartz veins. Almeidaite likely formed as a product of the reaction of hydrothermal fluids with the dacite host rock, whereby the dacites are part of a package of metavolcanic acid rocks resulting from a peraluminous and alkaline magmatism with an age estimated as 1.75 B.y. The formation was deeply fractured and heated during the Braziliano orogenic event (600-450 Ma), generating a swarm of hydrothermal quartz veins. Almeidaite forms platy crystals up to 30 × 30 × 6 mm, dominated by the basal pinacoid {001} with typically scalloped and imperfect faces and bounded by mostly steep rhombohedra and the hexagonal prism {110}. Most crystals are multiply twinned with non-planar contact surfaces that are approximately parallel to the c axis. Almeidaite is black with a brown streak, is opaque with submetallic luster, is brittle, shows no cleavage or parting and has conchoidal fracture. Mohs hardness = 6; D meas = 4.68(5), D calc = 4.616 g/cm 3. Almeidaite is non-fluorescent in long-wave or shortwave UV light. It is optically uniaxial (+) with high birefringence. Reflectance values were measured between 400 and 700 nm in 20 nm intervals. The values for COM wavelengths [R min , R max % (nm)] in air are: