Alumino-oxy-rossmanite

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Formula: ☐Al₃Al₆(Si₅AlO₁₈)(BO₃)₃(OH)₃O
Cyclosilicate, borosilicate, tourmaline group
Crystal System: Trigonal
Specific gravity: 3.07 measured, 3.092 calculated for the empirical formula
Hardness: 7
Streak: White
Colour: Pink
Luminescence: Not fluorescent under UV
Environments

Pegmatites

Alumino-oxy-rossmanite is a new mineral, approved in 2020 and to date (June 2023) reported only from the type locality

Localities

At the type locality, the Hengl quarry, Eibenstein an der Thaya, Raabs an der Thaya, Waidhofen an der Thaya District, Lower Austria, Austria, pink alumino-oxy-rossmanite is an early magmatic aluminium-rich oxy-tourmaline from a small pegmatitic body embedded in amphibolite and biotite-rich paragneiss. The most common associated minerals are quartz, albite, microcline and apatite. Beryl and, in places schorl, are also found as primary pegmatitic phases. Because of the low mode of associated mica (muscovite), it is assumed that the silica melt which formed this pegmatite crystallised under relatively dry conditions, in agreement with the observation that alumino-oxy-rossmanite contains a lower amount of OH than most other tourmalines. Alumino-oxy-rossmanite also exhibits the most aluminium-rich end-member composition of the tourmaline supergroup, with theoretical content ~54 wt% Al₂O₃. The significant content of tetrahedrally coordinated aluminium could reflect the relatively high-temperature conditions (~700°C) inferred for crystallisation of the pegmatite (AM 107.157-166), which is poor in lithium and fluorine (HOM).

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