Sunday, 2 June 2013

Kilchoanite

Sub Title: AN ELUSIVE MINERAL!

A short bit of science.(skip this bit if you want)

The basic difference between a rock and a mineral is that the former is made up of one or more of the latter. Rocks can be infinity variable as the proportions of the minerals vary. Minerals are much more precisely defined, and have a set chemical formula AND crystal structure. Different crystal structure produces a mineral that may be quite different, in colour, shape or other properties, even if chemically identical. These different forms are known as polymorphs
Different polymorphs are formed when the temperature and/or pressure at the time the crystal was formed differs from the 'normal' range.
When a new mineral is discovered the location is given the accolade of Type Locality and is registered in the official texts along with the Mineral Name. Any new discovery of the mineral, anywhere in the world and even if of better quality, is still referenced back to the original name and type locality.
Enough! back to Kilchoan.

Kilchoan is the type locality of not only Kilchoanite, but of two other minerals, Rustumite and Dellaite. see: Mindat for details if you are interested. Not bad when you consider there are only 30 type localities in the whole of Scotland and 110 in the UK (Cornwall has 35)

Kilchoanite is a polymorph of another mineral called Rankinite, itself quite rare, but not as rare as Kilchoanite. All three Kilchoan TL minerals are Calc-silicates, that is they contain Calcium, Silicon and Oxygen. The other two have a small amount of water in their structure, Kilchoanite does not. There are many other calc-silicates, some are of great commercial importance as they form in concrete, so this group has been much studied. Rustumite and Dellaite were discovered in laboratory test of man made substances, but were not know to occur in nature until they were discovered here.

So what is special about Kilchoan? and where can you see Kilchoanite?

The second question is not easy to answer, I have been looking for some time now, and have sent samples to be tested, but with only negative results so far. The site is reported to be on the north side of Glas Bheinn, the hill/ridge to the north of the Kilchoan House Hotel.


Glas Bheinn north aspect.

The Kilchoanite started life as a limestone, uplifted and heated by the intrusion that now forms Glas Bheinn. It would not have been at the surface at the time, but subject to considerable heat and pressure. The Glas Bheinn rocks are associated with centre 1, the earliest phase of the Ardnamurchan 'volcano'. Much later centre 3 was intruded nearby, and the limestone (or whatever it was at the time),  was subjected to another bout of heating and pressure, probably at somewhat different levels. This double metamorphism in differing conditions are what caused the unusual minerals to be formed.

So where is it? I thought I had found the location recently


But I was wrong! So I need to revisit the site and check again. How do I know I was wrong? That will be the subject of another blog entry.

To be continued.....




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