Whilst shopping at my favourite shop, Kremer Pigmente for some aerinite, I came across a mineral pigment I did not know, and obviously just had to buy it. It is called pentagonite, and Kremer indicate that it is sourced in Goa, India.
Well, the bottle of pale, turquoise blue pigment has arrived. I’ve made a grain dispersion and have just had a look under the microscope … and disappointingly, it is colourless in plane polarised light with low birefringence under crossed-polars. However that is not a surprise, because pentagonite is that unusual thing, a silicate mineral that can be used as a pigment. Many silicates, however colourful in hand specimen do not retain their colour when finely ground. Anyone who has done Minerals 101 will remember the streak test. A mineral that gives a good ‘streak’ when chalked onto fired, unglazed tile is also likely to be a good pigment. Most silicates give a white streak. Pentagonite is calcium vanadium silicate (Ca[VO]Si4O10.4H2O).
As I bought it on a whim, I never bothered to check out what it was until now. I was slightly cheered here as I realised that I am aware of a polymorph called cavansite. There is much to be said on the naming of minerals, but CaVanSite is boringly named after its constituent elements. I feel the mineral namers missed a trick here as it is my wish to discover a new mineral and call it caravansite. For a split second, when I spotted the cavansite at a mineral dealers, I thought someone had beaten me to it. So I bought that on a whim too. Pentagonite and cavansite were first discovered in Oregon, USA, found infilling cavities and fissures in flood basalts. The minerals have been found in the same environment in the Deccan Traps of Southern India.
I would be really grateful to know if any artists have used this material and if so what they thought of it as a pigment?
©Ruth Siddall 2015