A treasure trove of minerals discovered in the Larsemann Hills
The Larsemann Hills was first sighted by Norwegians in 1935, but it was not until the late 1980s that detailed geological investigations commenced.
These early investigations reported large, abundant prisms of a near-black mineral, which the geologists described as ‘tourmaline’. Several years later, Chinese and Australian geologists showed that the prisms, as large as piano keys, were actually ‘prismatine’ – a much rarer mineral containing boron and silicon (Figure 1) – and another rare borosilicate, ‘grandidierite’. The rocks making up a prominent peak on central Stornes contain prismatine in such abundance that it has been named Prismatine Peak (originally called Tourmaline Peak).
In 2003-04, we found the mineral bands to be associated with nodules of the common calcium phosphate mineral, ‘apatite’. Both the boron-rich rocks and the apatite contained a variety of minerals, four of which are new to science.
The Larsemann Hills contains sedimentary and volcanic rocks that were laid down between 900 and 550 million years ago. Some of these rocks were unusually rich in boron. The rocks were subsequently buried, ‘metamorphosed’ (changed from one type of rock into another) at temperatures of 800-860°C, and squeezed by tectonic forces during the collision of East Gondwana and West Gondwana some 515-530 million years ago. The resultant amalgamation formed the super-continent Gondwana. Under these conditions the metamorphic rocks melted, resulting in two other rock types, ‘granite’ and ‘pegmatite’, both of which are common in the Larsemann Hills.
Six borosilicate minerals have been found so far in the Larsemann Hills – prismatine, grandidierite, boralsilite, werdingite, dumortierite and tourmaline. The most unusual of these is boralsilite, which was discovered in the Larsemann Hills in 1998 and named for its composition, boron, aluminium and silicon.
We found boralsilite at nine localities on Stornes during the 2003-04 field season. The colourless prismatic mineral can be seen with the naked eye and recognised in the field, and is distinguished under the microscope by its striped appearance (Figure 2). Outside the Larsemann Hills, boralsilite is known only from Rogaland, Norway.
The distinctive borosilicate mineral assemblage in the Larsemann Hills is attributed to a variety of factors, including the high boron content of the source rocks and their relatively low water content, which caused an unusual enrichment of boron, but not of other elements normally concentrated in pegmatites, such as lithium and beryllium.
Large brown nodules of apatite – which can be up to 10 cm across – are associated with boron-rich rocks and other rock types (Figure 3). Eight phosphate minerals have been found in apatite nodules, three of which are new and, to date, have only been found in the Larsemann Hills. The three new phosphate minerals – stornesite-(Y), chopinite and tassieite – are microscopic and very rare.
Stornesite-(Y) is a sodium-calcium-magnesium-rich phosphate, named for Stornes and for the element yttrium-(Y). Its closest relative is the yttrium-free phosphate ‘chladniite’ reported in two meteorites.
Chopinite is dominantly magnesium phosphate with iron. It was named in honour of French mineralogist, Christian Chopin. Four sub-millimetre-sized grains were found in a single thin section of apatite.
Tassieite (Figure 3 insert) is a sodium-calcium-iron-magnesium phosphate, containing water lodged in its structure. It was named for Tassie Tarn south of Johnston Fjord, which has an outline resembling that of Tasmania.
The Larsemann Hills is unique in its mineral diversity. Four new mineral species have been found, while three other minerals are known from only a handful of other locations worldwide. Several rare species are also found in unusual abundance in the Larsemann Hills. This unique mineralogy, as well as the region’s other scientific, environmental, aesthetic and logistical values, has led to its recent designation as an Antarctic Specially Managed Area.
PROFESSOR EDWARD GREW, Department of Earth Sciences, University of Maine, USA
DR CHRIS CARSON, Geoscience Australia