FMF - Friends of Minerals Forum, discussion and message board

[Pierre Joubert]

[marco campos-venuti]

Dolomite in quartz is very common in Brazil. Here is an example.
Quartz can crystallize in a wide range of temperatures. But the crystal shape of feldspars from the Celsian-Hyalophane series is very similar to rhombohedral carbonates.

[Jordi Fabre]

[cascaillou]

Hi,
concerning the identity of those white crystals, you could try:
-LW and SW UV (if any fluorescence, report intensity and color of fluorescence, and also check in complete darkness if there's any residual phosphorescence once the UV have been cut)
-check the transparent ones for doubling of their inclusions/edges (with a good microscope)
-check for cleavages or any characteristic twinning under the microscope
-you could also try crossed polars under the microscope, checking for lamellar twining, if any.
-if one of the included crystals has decent transparency, you might even try conoscopy between crossed polars under the microscope (check for optical axis under crossed polars, and then use the conoscope)

[Pierre Joubert]

[cascaillou]

Christmas is coming! :-)

[Pierre Joubert]

[Pierre Joubert]

Hi Mike and Vic. I would like to continue our discussion on inverted crystals under this topic as the specimen comes from Tafelkop area, near Brandberg. Perhaps Peter, Duncan and other experts can chip in. As far as I know, 'inverted' crystals are hollow and formed slower than other gas inclusions. Sometimes these have water with an air bubble trapped inside.

[Michael Shaw]

Wonderful piece Pierre, and great photos.
Michael

[Don Lum]

Pierre,

Great specimen and photographs.

Don

[Mike Wood]

Incredibly clear and sharp photo's of the quartz Pierre!

[Fiebre Verde]

Excellent photos Pierre!

[Duncan Miller]

Hello Pierre

Don't you mean 'negative crystals', rather than 'inverted crystals'? I think they form as the trapped solution cools and the dissolved silica crystallises on the inner walls of the host quartz, necessarily following the crystallographic structure and orienation of the host. The gas bubble forms as the gas too comes out of solution. Gentle heating can cause the gas to redissolve.

Duncan

[vic rzonca]

Likewise, Pierre, I have usually termed them negative crystal inclusions, but I suppose inverted works, although when I think of inverted it is as up-side-down, not out-side-in. It amazes me that these inclusions match perfectly the exterior geometry of the host crystal, but surely makes sense. It seems the Brandberg quartz is rich in very interesting inclusions. Am I wrong, or does quartz exhibit these negative - invert crystal forms more offen than most minerals?

[Pierre Joubert]