FMF - Friends of Minerals Forum, discussion and message board

[Bob Carnein]

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.

[Peter Megaw]

Danburite maybe?

[Bob Carnein]

Thanks. I thought about danburite, but the striations still stump me.

[Pete Richards]

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!

[Peter Megaw]

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)

[Pete Richards]

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.

[alfredo]

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?

[Bob Carnein]

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.

[John Betts]

Hardness?

[Bob Carnein]

Harder than steel.

[alfredo]

"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.

[Joseph DOliveira]

Phenakite?

[Pete Richards]

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

[Pete Richards]

[John Betts]

Harder than quartz? than beryl?