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 Post subject: Colour shift in quartz over very short distances - why?
PostPosted: Mon Nov 02, 2015 10:41 pm 
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Joined: Wed Sep 02, 2015 10:52 pm
Posts: 1131
Location: Central Queensland, Australia
A short distance from my home there is an old amethyst dig site. The crystals are not of great quality generally, being mostly small and badly cracked. I have faceted a few clean ones. This is mostly about the fun of finding something yourself and it's a good way to get the kids interested in things gemmy and geological (except for mine on whom it didn't work). Plus it's close handy, the ground is soft and the digging is easy.

The crystals occur occur in a coarse, decomposed granite sand which is approximately three feet deep on average before you strike a hard floor of decomposing granite. They can be found right through from the surface to the hard floor but seem most concentrated in a thin layer of quartz chunks, partly-formed and fully formed crystals about four inches thick which is usually just above the floor.

The thing I find interesting is that the place seems to be composed of little micro-sites in which one colour of quartz dominates others. This material comes from one side of the track and in this spot, most of it is orange to orange-pinkish with the occasional pale purple piece.

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Move about 50 yards to the other side of the track and the ornage material dissapears, to be replaced by this...

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Move another 50 yards in the same direction and the crystals morph in purple with smokey tints...

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Keep going in the same direction and the purple dissapears and the crystals all become a "true" smokey quartz. There are also spots where the cyrstals have a pinkish hue as well as a sort of smokey citrine.

Why would different colours predominate in little micro-sites so very close together? Is it related to temperature ie, might different patches have cooled down at different speeds after formation?

This is not the original dig site, there are spots close by with better material but they are now off limits to digging. I'm still working on that one :)


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