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Improving Mindat.orgJasper?

16th Jul 2017 12:22 UTCReiner Mielke Expert

https://www.mindat.org/photo-832685.html why is this jasper?

16th Jul 2017 15:10 UTCOwen Lewis

Good spot! ;-)

16th Jul 2017 16:04 UTCDavid Baldwin

Looks like 'bloodstone' to me, which is otherwise known as green chalcedony/jasper or heliotrope.

16th Jul 2017 18:40 UTCPaul Brandes 🌟 Manager

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....

16th Jul 2017 19:32 UTCBecky Coulson 🌟 Expert

On my monitor it looks dark green with conchoidal fracture - more like chalcedony than basalt?

16th Jul 2017 21:51 UTCAlfred L. Ostrander

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?

16th Jul 2017 21:54 UTCReiner Mielke Expert

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.

16th Jul 2017 22:05 UTCDoug Daniels

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.

16th Jul 2017 22:25 UTCRalph S Bottrill 🌟 Manager

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.

16th Jul 2017 22:47 UTCReiner Mielke Expert

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.

16th Jul 2017 23:20 UTCAlfredo Petrov Manager

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.)

17th Jul 2017 01:56 UTCReiner Mielke Expert

Thanks for clarifying that Alfredo, Jasper it is then.

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