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Andrew Harvey




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PostPosted: Nov 22, 2017 11:54    Post subject: What is this  

Hi everyone - my first post! The photo shows a crystal on a piece of massive quartz. The field of view is about 14mm. The quartz was recovered from a small cliff fall at Pentireglaze Haven near New Polzeath, Cornwall. The bedrock in the area of the fall is a killas with occasional quartz veins. On the same stretch of coastline and fairly near is the Pentireglaze lead and silver mine. I guess the crystal is about 1mm x 0.5mm and although it has is a fine lustre, due to its size it is hard to tell if this is metallic, vitreous or somewhere in between. Is it more quartz or something else? Thank you


171122150203252005859.jpg
 Locality:
Pentire Glaze Mine, Polzeath, St Minver, Wadebridge District, Cornwall, England, United Kingdom
 Dimensions: 1mm x 0.5mm
 Description:
 Viewed:  1064 Time(s)

171122150203252005859.jpg


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Pete Modreski
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PostPosted: Nov 22, 2017 12:34    Post subject: Re: What is this  

I got a smile out of seeing your reference to the "killas" host rocks, which is a term I haven't much seen or thought about, since I once visited mines in Cornwall, several decades ago! A classic, local-to-Cornwall geologic term.
Back to your crystal (or cluster thereof), as you say, it's hard to be certain if it is metallic or not. My guesses are, perhaps arsenopyrite if it is metallic, or something in the mica family (muscovite or chlorite) if it is not. A suggestion, under the microscope take the point of a needle and gently pick on it; see if it indents or flakes apart as a mica would, or if it is hard, then it is probably a sulfide or an arsenide.
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Andrew Harvey




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PostPosted: Nov 22, 2017 12:56    Post subject: Re: What is this  

It flakes, so presumably Mica. I have always wondered about testing to identify a species. I haven't done it yet because establishing hardness or streak atre likely to cause damage while ascertaining the cleavage may cause total destruction? I suppose if one has a handful of specimens, losing one for id purposes isn't too bad.

Love the site and this facility in particular. I have a couple more that I can't identify. I'll post them today but if there is a daily limit then please set them aside for another day.

Many thanks for advice
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Pete Modreski
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PostPosted: Nov 22, 2017 13:20    Post subject: Re: What is this  

No, no limits here, though we always appreciate it if someone does not post 10 or 20 unknowns one after the other!
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