Kaersutite, West Head, Victoria

Mineral Matters #450

This week’s Facebook Mineral of the Week Group theme is minerals in the Amphibole Supergroup.

This provides both challenges and opportunities.

Challenge as in providing the correct identification.

Opportunity due the sheer size (number of species) of the group.

Earlier today, I was busy photographing highly exciting black kaersutite specimens.

Kaersutite is an interesting mineral in that it often forms as phenocrysts in alkaline igneous intrusive rocks. Phenocrysts are large, noticeable crystals that form in igneous rocks. During the cooling of molten rock (magma), some minerals start to crystallize earlier and grow larger than others. These larger crystals, or phenocrysts, are then surrounded by a finer-grained matrix when the rest of the magma cools and solidifies.

Most of the kaersutite specimens that I have are loose crystals that have weathered out of the host matrix. This one shows the kaersutite in situ. It was originally in the late Ruth Coulsell mineral collection.

Ruth Coulsell (1912-2000) was a prominent mineral collector from Melbourne, Victoria. She was a foundation member and honorary life member of the Mineralogical Society of Victoria. Ruth was known for her enthusiasm, generosity, and dedication to mineralogy. In recognition of her contributions, the mineral fluornatrocoulsellite (originally coulsellite) was named in her honour.

Below: Kaersutite, West Head, Mornington Peninsula Shire, Victoria, Australia. 33mm wide.