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Unknown mineral from Pikes Peak pegmatite
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Bob Carnein




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PostPosted: Jun 13, 2021 15:38    Post subject: Unknown mineral from Pikes Peak pegmatite  

A friend asked me to identify this crystal. He found it in a Pikes Peak pegmatite near Lake George, CO. The pegmatite has amazonite, smoky quartz, zircon, and abundant massive topaz surrounded by micaceous selvages. The crystal measures 9x5x3mm, appears to be orthorhombic, and has a hardness greater than that of a knife blade. It is semitransparent and colorless. Faint striations on the prism parallel to the long axis form a chevron pattern where they intersect, and the faces on the terminations are also faintly striated. In some places, the striations on the "vertical" prism are accentuated by the dark brown coating visible in the first photo.

There is no obvious cleavage perpendicular to the long axis. Unfortunately, I don't have a sensitive enough scale to measure the SG. I thought, at first, that it is topaz, based on morphology and hardness. However, the lack of obvious cleavage and odd striations make me question that. I plan to break the crystal and crush a part of it do do an immersion-oil test, but I'd like to know what FMF users think before I sacrifice it. I talked to the finder yesterday, and he doesn't remember giving it to me. Any suggestions would be appreciated.



IMG_9713 (2).JPG
 Mineral: Unknown
 Locality:
Crystal Peak area, Teller County, Colorado, USA
 Dimensions: 9x5x3mm
 Description:
View of unknown 9-mm-tall crystal showing faint striations intersecting along vertical edge (emphasized by black coatings on left side).
 Viewed:  1447 Time(s)

IMG_9713 (2).JPG



IMG_9730 (2).JPG
 Mineral: Unknown
 Locality:
Crystal Peak area, Teller County, Colorado, USA
 Dimensions: 9x5x3mm
 Description:
View of termination of unknown crystal. Dimensions shown are 5x3mm
 Viewed:  1455 Time(s)

IMG_9730 (2).JPG


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Peter Megaw
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PostPosted: Jun 13, 2021 15:50    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

Danburite maybe?
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Bob Carnein




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PostPosted: Jun 13, 2021 16:05    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

Thanks. I thought about danburite, but the striations still stump me.
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Pete Richards
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PostPosted: Jun 13, 2021 16:27    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

What about euclase? I have no idea what the local mineral assemblage is, so this is in that sense a shot in the dark.

The top view of the crystal suggests that it is really monoclinic, not orthorhombic, and if so we are looking down the unique two-fold axis. This observation is independent of the comment above - I have not looked up the crystallography of euclase.

Let us know what you figure out!

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Peter Megaw
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PostPosted: Jun 13, 2021 16:41    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

Good point on the monoclinic Pete...and euclase is monoclinic
But Euclase means "true/easy cleavage" and according to Bob it has no obvious cleavage (although it is unclear if he means basal or along the length)

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Pete Richards
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PostPosted: Jun 13, 2021 16:50    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

Your point about the cleavage is also a good one, Peter. I plead guilty to responding without research. The perfect cleavage of euclase is {010}, which would be perpendicular to the second view, if in fact it is looking down the two-fold axis.

Neither photograph suggests a cleavage, but it might be good to look for one with lighting from different directions. Of course, the frosted surface of the crystal makes seeing internal features difficult.

I am not wedded to the euclase suggestion - just suggesting.

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alfredo
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PostPosted: Jun 13, 2021 18:06    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

Have you tried fluorescence under UV? Danburite is often (unfortunately not always) fluorescent blue under shortwave UV. I know nothing about the fluorescence of euclase, but fluorescent response might provide some clue?
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Bob Carnein




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PostPosted: Jun 13, 2021 18:20    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

I did try fluorescence, and there is none, in either SW or LW. However, I played around with the lighting, and there is a single fairly planar crack that appears to be perpendicular to the long axis that shows up when the light hits it just right. Otherwise, the "base" and a broken side of the xl. show conchoidal fracture.
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John Betts




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PostPosted: Jun 13, 2021 20:18    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

Hardness?
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PostPosted: Jun 13, 2021 20:43    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

Harder than steel.
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alfredo
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PostPosted: Jun 13, 2021 20:49    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

"there is a single fairly planar crack that appears to be perpendicular to the long axis that shows up when the light hits it just right." ...sounds like an incipient basal cleavage? That, and the lack of fluorescence, brings topaz back into the game again.
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Joseph DOliveira




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PostPosted: Jun 13, 2021 20:52    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

Phenakite?
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PostPosted: Jun 13, 2021 20:55    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

Re: "Harder than steel"
That is true of euclase and danburite... and topaz and a number of other minerals.

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PostPosted: Jun 13, 2021 21:06    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

Joseph DOliveira wrote:
Phenakite?

Good thought. Right environment, right general color or lack thereof.

However, phenakite is trigonal and typically occurs as prismatic crystals with a hexagonal outline. The crystal shape of this specimen is inconsistent with phenakite, unless nature is playing tricks again.

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PostPosted: Jun 13, 2021 21:09    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

Harder than quartz? than beryl?
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PostPosted: Jun 13, 2021 22:02    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

How about Spodumene. I have a couple of pieces that have etching striations that run that way. There doesn't appear to be any from that area in the photos on Mindat. but maybe...
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PostPosted: Jun 13, 2021 22:10    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

Could it be Barylite? Barylite is monoclinic and is rare, but documented from the Crystal Peak area. Just a wild guess.
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PostPosted: Jun 13, 2021 22:13    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

Opps, make Barylite orthorhombic, my bad. Too many beers last night :)
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PostPosted: Jun 14, 2021 00:22    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

IF that is a phenakite, that is a 'whopper'!! I have collected extensively in the area mentioned, but don't remember seeing anything like that. Also never found phenakite that wan't tiny, tiny, tiny. I sense whatever it is, it may be very large and unusual find for that area, based on my digging there. Don't destroy it. Find someone with Ramen spectragraph or other non-destructive testing method.
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PostPosted: Jun 14, 2021 09:46    Post subject: Re: Unknown mineral from Pikes Peak pegmatite  

I thought of barylite too, but specimens shown on Mindat have a pretty consistent "look" to them, and none look like this crystal.
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