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Identity HelpMystery of an Enigma
29th Mar 2012 05:06 UTCMatt Ciranni OP
Okay, this is a strange one.
This rock LOOKS like calcite...but it's not: For one thing, it is too hard! A steel point will streak against it but will NOT scratch it. The hardness of this rock would be at least 6, so it can't be flourite either (that would be an obvious second guess) A few more characteristics:
-It has a waxy luster and a greyish white translucent appearance. Most calcite on the other hand, is either pearly white or glassy in appearance.
-rock is around 7 cm long.
-looks like it crystallized in the orthorhombic system similar to calcite
-it was found in the mountains east of Belleview and Hailey, Idaho. There is a lot of agate here, in fact I went to this area specifically to collect agate and found quite a few decent pieces of agate along with this.
-Most of the rock in the area this was found is volcanic basalt/rhyolite.
As I said, it has a vaguly waxy, tranclucent greyish white appearance which is very similar to the agates you can find around the area. Which makes me wonder, could this rock be some kind of agate pseudomorph after calcite? Can agate actually replace crystalline calcite (or flourite) in some circumstances? I have never heard of agate actually crystallizing by itself, except to form drusy quartz crystals in cavities.
This rock LOOKS like calcite...but it's not: For one thing, it is too hard! A steel point will streak against it but will NOT scratch it. The hardness of this rock would be at least 6, so it can't be flourite either (that would be an obvious second guess) A few more characteristics:
-It has a waxy luster and a greyish white translucent appearance. Most calcite on the other hand, is either pearly white or glassy in appearance.
-rock is around 7 cm long.
-looks like it crystallized in the orthorhombic system similar to calcite
-it was found in the mountains east of Belleview and Hailey, Idaho. There is a lot of agate here, in fact I went to this area specifically to collect agate and found quite a few decent pieces of agate along with this.
-Most of the rock in the area this was found is volcanic basalt/rhyolite.
As I said, it has a vaguly waxy, tranclucent greyish white appearance which is very similar to the agates you can find around the area. Which makes me wonder, could this rock be some kind of agate pseudomorph after calcite? Can agate actually replace crystalline calcite (or flourite) in some circumstances? I have never heard of agate actually crystallizing by itself, except to form drusy quartz crystals in cavities.
29th Mar 2012 06:11 UTCJenna Mast
If it's not fluorite then I would say that possible chalcedony that grew over fluorite and then came off, leaving a mold of the fluorite.
I could be wrong.
I could be wrong.
29th Mar 2012 07:24 UTCRock Currier Expert
A quartz or chalcedony cast after?
29th Mar 2012 08:26 UTCGeorge Eric Stanley Curtis
It is a cast, possibly formed around and on flourite or pyrite cubes which have since vanished, either dissoled or left behind whenthe sample you have became separated.
The sample looks exactly like many found on dumps here in Cornwall UK,
It is probably quartz var. chalcedony./ agate
,
Best Regards
The sample looks exactly like many found on dumps here in Cornwall UK,
It is probably quartz var. chalcedony./ agate
,
Best Regards
29th Mar 2012 14:46 UTCD Mike Reinke
Matt,
I found something very similar in Colorado on a steep slope, a big 2" cube-hollow in a quartz rock that was also full of smaller pyrite bits.. I would love to have the pyrite that is long gone. Now I call it my "air after pyrite" pseudomorph!
Mike
I found something very similar in Colorado on a steep slope, a big 2" cube-hollow in a quartz rock that was also full of smaller pyrite bits.. I would love to have the pyrite that is long gone. Now I call it my "air after pyrite" pseudomorph!
Mike
29th Mar 2012 16:02 UTCPaul Pohwat
In his now classic U. S. National Museum Bulletin 131, The Minerals of Idaho (1926), Earl V. Shannon describes the chalcedony from Blaine County. Quoting from page 180 of this publication:
Usually the first lining of the cavity has been calcite followed by the chalcedonic silica and the nodules show on their outer surface the gash-like molds of the acute rhombohedral crystals of calcite.
Hope this helps out.
Usually the first lining of the cavity has been calcite followed by the chalcedonic silica and the nodules show on their outer surface the gash-like molds of the acute rhombohedral crystals of calcite.
Hope this helps out.
29th Mar 2012 23:58 UTCMatt Ciranni OP
actually, that makes a lot of sense; sounds like the chalcedony sort of forms around the calcite. (and this piece was, in fact, found in Blaine County.)
Thanks!
Either way, it is an interesting piece.
Thanks!
Either way, it is an interesting piece.
6th Apr 2012 09:54 UTCEligiusz Szełęg Expert
negative crystals after calcite or fluorite within qyartz(chalcedony)
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To cite: Ralph, J., Von Bargen, D., Martynov, P., Zhang, J., Que, X., Prabhu, A., Morrison, S. M., Li, W., Chen, W., & Ma, X. (2025). Mindat.org: The open access mineralogy database to accelerate data-intensive geoscience research. American Mineralogist, 110(6), 833–844. doi:10.2138/am-2024-9486.
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