The amazing minerals of the Larsemann Hills

by Edward S. Grew and Christopher J. Carson
Monday, December 29, 2014

Four minerals were discovered on Stornes Peninsula in the Larsemann Hills of East Antarctica based on fieldwork there from 2003 to 2004. In part because of these minerals and other rare boron and phosphate minerals found in this pristine region, Stornes Peninsula is now protected as an Antarctic Specially Protected Area — the highest level of environmental protection in Antarctica. Below are some details about these special minerals.

Boron Minerals

Prismatine: a borosilicate mineral composed largely of magnesium, iron and aluminum, that together with its boron-poor analog kornerupine, has been found in 70 to 80 localities worldwide, mostly in ancient rocks crystallized at depth like those found in the Larsemann Hills.

Grandidierite: a green-blue borosilicate also containing magnesium, iron and aluminum found in about 40 localities worldwide in a variety of rocks. Forms lath-like prisms.

Tourmaline: widespread in a variety of sedimentary, metamorphic and igneous rocks worldwide. Worm-like intergrowths of black tourmaline with quartz are characteristic of the Larsemann Hills, where tourmaline also occurs in fine-grained, sugary textured aggregates or in layers of “tourmalinite,” a massive black rock composed almost exclusively of tourmaline and quartz.

Boralsilite: the first new mineral found in the Larsemann Hills. This borosilicate was first described in 1998 in a specimen collected near Prismatine Peak some 10 years earlier by Australian photographer Douglas Thost, who has done extensive geological and glaciological fieldwork in Antarctica. During the 2003–2004 season, we found boralsilite at nine localities on Stornes Peninsula. There are only two other localities worldwide where this mineral has been found to date: Almgjotheii, Rogaland, Norway, and Horní Bory, Bory Granulite Massif, Czech Republic.

Dumortierite: a widespread borosilicate mineral of aluminum that contains minor amounts of arsenic and other metals. In the Larsemann Hills it is found in bright-blue fibrous mats and gray centimeter-sized prisms.

Werdingite: a borosilicate of magnesium, aluminum and iron related to boralsilite in its crystal structure known only from a single thin section in which it is present as a microscopic constituent.

Phosphate Minerals

Wagnerite: a new polytype of magnesium fluorphosphate that forms bright orange masses up to 3 centimeters across. (Polytypes of a mineral differ from one another in crystal structure, but not enough to be considered distinct mineral species.)

Stornesite-(Y): discovered in the Larsemann Hills and only found there. It is a sodium-calcium-magnesium-rich phosphate, named for the Stornes Peninsula and for the element yttrium, the most abundant of the rare-earth elements (hence the ‘Y' designation to distinguish it from a stornesite containing another rare-earth element). Its closest relative is the yttrium-free meteoritic mineral chladniite.

Tassieite: discovered in the Larsemann Hills and only found there. It is named for Tassie Tarn on Stornes Peninsula, which has an outline resembling that of Tasmania. The mineral is a blue-green sodium-calcium-iron-magnesium phosphate containing water molecules in its crystal structure.

Chopinite: discovered in the Larsemann Hills and named for French mineralogist Christian Chopin of the École Normale Supérieure, Paris, for his major contributions to phosphate mineralogy. It is the magnesium-dominant analog of the iron-phosphate mineral sarcopside. Four grains less than 1 millimeter across in a single thin section from Brattnevet, which is located between Stornes and Broknes peninsulas, constitute all the known terrestrial examples of chopinite; the only other occurrence is meteoritic.

Mélonjosephite: a microscopic calcium-iron phosphate enclosed in apatite known from just four localities in the world other than the Larsemann Hills.

Isokite: a fluorphosphate of calcium and magnesium found in microscopic veinlets cutting through wagnerite; it is known from no more than 10 localities in the world.

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