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Formula: NaAl3Al6(Si4B2O18)(BO3)3(OH)3O
Cyclosilicate (ring silicate), borosilicate,
tourmaline group
Crystal system: Trigonal
Specific gravity: 3.128 calculated
Hardness: 7 to 8
Streak: White
Colour: Pink, yellowish brown to grey or near colourless
Luminescence: Weak dark red fluorescence under short wave UV
Environments
Pegmatites
Hydrothermal environments
Ertlite is a very boron-rich mineral of the
tourmaline group that forms under low-temperature hydrothermal
conditions as a late-stage phase in pegmatites
(AM 110.12.1865–1876).
Localities
At the Stoffhütte pegmatite, Deutschlandsberg, Deutschlandsberg District, Styria, Austria, ertlite has been found
as pale green crystals in smoky quartz
(Mindat photo).
Ertlite from the Stoffhütte pegmatite -
Image
There are two co-type localities, the Sahatany Valley, Ibity, Antsirabe II District, Vakinankaratra, Madagascar, and
Sakangyi, Kyauk-Pyat-That, Mogok Township, Pyin-Oo-Lwin District, Mandalay Region, Myanmar.
The Sahatany Valley, Ibity, Antsirabe II District, Vakinankaratra, Madagascar, is the source of the holotype specimen.
The holotype occurs as pink to brownish grey or near colourless euhedral crystals and aggregates, up to 10 mm in diameter,
with vitreous lustre, conchoidal fracture and white streak. Holotype ertlite has a Mohs hardness of 7 to 8, and a
calculated density of 3.128. It formed during a late stage of open pocket crystallisation in a highly fractionated
granitic
pegmatite dike of
lithium - cesium -
tantalum -enriched type (LCT-type).
Sakangyi, Kyauk-Pyat-That, Mogok Township, Pyin-Oo-Lwin District, Mandalay Region, Myanmar, is the source of the co-type
specimen. Cotype ertlite has a Mohs hardness of 7 to 8 and a calculated specific gravity of 3.135. Cotype
ertlite crystallised in a very similar pegmatite
environment to that of the holotype. The co-type sample has the lowest unit-cell volume ever recorded for a natural
tourmaline.
At both localities, ertlite occurs as a late-stage hydrothermal phase in open pockets within highly fractionated,
boron-rich granitic
pegmatites
(AM 110.12.1865–1876).
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