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Jasper?

Posted by Reiner Mielke  
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Reiner Mielke July 16, 2017 12:22PM
https://www.mindat.org/photo-832685.html why is this jasper?
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Owen Lewis July 16, 2017 03:10PM
Good spot! ;-)
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David Baldwin July 16, 2017 04:04PM
Looks like 'bloodstone' to me, which is otherwise known as green chalcedony/jasper or heliotrope.
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Paul Brandes July 16, 2017 06:40PM
Reiner Mielke Wrote:
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> why is this jasper?
>
That's a good question. Looks like a piece of the basalt if you ask me....
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Becky Coulson July 16, 2017 07:32PM
On my monitor it looks dark green with conchoidal fracture - more like chalcedony than basalt?
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Alfred L. Ostrander July 16, 2017 09:51PM
Is there a problem with jasper or chalcedony of some variety being associated with pillow lavas or a problem with the specific locality given as the pillow lavas found in Alamedilla? Or should the jasper been listed as a variety of chalcedony or variety of quartz. I have to agree with David and Becky in that it appears to be a roughly broken out chunk of a jasper like material.

Reiner, I think your question is valid but what, specifically, do you find wrong with the caption?
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Reiner Mielke July 16, 2017 09:54PM
I don't see any difference between this specimen and say basalt but maybe there is? Would be nice to know how it was determined that this is jasper.
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Doug Daniels July 16, 2017 10:05PM
I agree it looks more like basalt, but then there's a little shell-shaped chipout, about the center of the lower edge of the piece.
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Ralph Bottrill July 16, 2017 10:25PM
It does at first glance look basaltic, especially on weathered areas and with its rough fracture, but a close look at thin fracture edges shows more translucency than expected. Maybe a highly silicified rock, which is what most jaspers are. I don't know the area, but inter-pillow material is often silicified basaltic sediment, usually grey to greenish cherts, and this would fit.



Edited 1 time(s). Last edit at 07/16/2017 10:30PM by Ralph Bottrill.
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Reiner Mielke July 16, 2017 10:47PM
Hello Ralph,

That is what I thought as well, except I didn't know jasper was a term that could be applied to such silicified rock.
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Alfredo Petrov July 16, 2017 11:20PM
I don't know what the rest of you see on your screens, but it certainly looks like a dark green jasper to me. As for translucency, I agree with Ralph: blow up the screen and see how light shines through thin flakes of the rock adjacent to fractures. Not basalt.

All my life I've called fine-grained silica varieties "jasper" if they were impure and opaque, "chalcedony" if they were translucent. Pretty much what Wikipedia says about jasper: "Jasper, an aggregate of microgranular quartz and/or chalcedony and other mineral phases, is an opaque, impure variety of silica, usually red, yellow, brown or green in color; and rarely blue. The common red color is due to iron(III) inclusions." Modern attempts to redefine jasper and chalcedony based on microscopic features of the silica fibres composing them, while scientifically noble, ignore the much looser historical usages.

And Mindat's listing of some obscure Australian locality as "First Recorded Locality" for jasper seems quite wrong, considering that Europeans have known jasper much longer than they've known of the existence of Australia.
(Edit: This seems to be a programming bug caused by the jasper subvariety "darlingite" being first recorded from Australia, and that FRL migrating up the heirarchy and becoming the FRL for Jasper too.)



Edited 2 time(s). Last edit at 07/16/2017 11:25PM by Alfredo Petrov.
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Reiner Mielke July 17, 2017 01:56AM
Thanks for clarifying that Alfredo, Jasper it is then.
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