Dear Rock 'n' Gem,
Issue 61 Autumn 2013 received. Barry Taylor's first letter makes the false claim that temperature differences come into agate formation. This cannot be the case. The second letter repeats the claim.
Our letter writer was asked to draw attention to the colloidal origin of agates. It is inevitable that excess fluid is produced by such an origin. Silica in solution is not involved in agate production. This would involve different temperatures. These do not come into the picture.
Agates are all the same, being formed at 50'C this needs to be understood.
To continue about Mexican Coconuts(Issue 59) these were obtained in large numbers and were marketed as geodes especially if they provided amethyst. The agate fractions were included in the isotope study. The result agreed with the others but were not published...

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rock and gem magazine issue 62 winter 2013


The geodes may have clearly visible eyes (spherulites) on the outer surface. They are derived from obsidian, the original rock being entirely glass, to begin with.
Dryhead agate should be classified but not as agate. Abrief note(from the publishers) states that authors disagreed on this point.
This should not have been possible; the colloidal origin has been accepted for many years (based on laboratory work). The silica particles are small enough to be supported in moving waters, but are vastly bigger than in true solution. Dryhead material comes from solution.

Yours faithfully
J.Jocelyn


Dear Editors & Compilers,
In my last letter, for some reason I claimed my colleague refused to review the copy received of AGATES published by the Natural History Museum in 2006 this is not true, the review by John Faithful was published in 2007 in the Scottish journal of Geology!
It was a short review Dr Faithful is the curator of Mineralogy at the Hunterian Museum Glasgow.
…Geodes with amethyst are presently available from Ardownie Quarry in Scotland. I have a piece. The amethyst in these is also smokey having been exposed to radioactivity for a long time, as other older amethyst.

Yours faithfully
J. Jocelyn.


 

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