FMF - Friends of Minerals Forum, discussion and message board

[Duncan Miller]

There is an easy way to test a piece of rose quartz to see if it is a single crystal or not. I have just viewed a 679 g piece of gem rose quartz through crossed polarizing filters. On 360 degree rotation the stone blinks on and off (extinction) coherently four times = single crystal. (If you happen to view the stone down the c axis this won't work.) You can rotate the filters (together) instead of the stone, but finding filters large enough to test a 10 ton chunk might prove difficult. A photo of the sophisticated test set-up is attached.

Duncan Miller

[John S. White]

Elise, I would be interested in being reminded of what the other mechanism for coloring rose or pink quartz is that was determined by George Rossmann. I am familiar with dumortierite having been found in some rose quartz, and I am aware that rutile is often cited as being present in rose quartz, but what did George come up with?

[Elise]

[Duncan Miller]

Elise,

Strain can induce biaxial optical behaviour in otherwise uniaxial and isotropic crystals, so a biaxial figure like your elegant example in elbaite tourmaline does not necessarily indicate an intergrowth or polycrystallinity. I think it likely that gem rose quartz, at least that showing asterism, is monocrystalline, and not "massive" in the common sense of the word indicating polycrystalline intergrowth. Otherwise, as you have pointed out, how would you get regularly orientated fibres or crystallites that produce the asterism? And in at least some examples, there is clear evidence of single crystal behaviour, as demonstrated in the symmetrical extinction in my piece of rough and those gems that produce a clear uniaxial interference figure. Large, fractured, and opaque masses of rose quartz from pegmatite bodies may well be massive. The test under crossed polarizers obviously is applicable only to gem quality material. As I wrote in my private e-mail to you, perhaps "anhedral" would be a better term to describe rose quartz, which tends not to form euhedral crystals. The reason(s) why not seem unclear, but interesting. (We have hijacked the thread -> https://www.mineral-forum.com/message-board/viewtopic.php?t=901, and perhaps should be apologetic.)

Duncan

[Jordi Fabre]

Another thread (Quartz and family) is related with this thread (where this thead grew) -> https://www.mineral-forum.com/message-board/viewtopic.php?t=901

[Pete Richards]

[Elise]

I didn't mean to confuse strain and anomalous figures with any seen in the rose quartz, only that one might see confusing figures in different areas of a massive material (and quartz also can show anomolous biaxiality). Unfortunately, the large cab is frosted on the base obscuring that kind of observation unless perhaps a very strong light and using the larger microscopes might help ( I don't think I want thin sections made out of it). With some smaller Madagascar tumbled pieces I have, the asterism is visible but the optic figures are confused.

It was in regards to my presistant and lengthy questions to this that in one communique from Dr. Rossman he explained (briefly) regarding the areas of crystallinity that produce the asterism: Pink quartz is a rare single crystal variety colored by radiation induced electron hole centers involving Al - P substitutions; it does not form stars. "Within the massive material there are regions of single crystals which can span many centimeters (Rossman personal comm, 2008)." Hence, despite its aggregate nature, the regional crystal symmetry environment allows the formation of a six ray star. I've been chewing on that for 2 years. Doesn't it imply smaller areas of crystallinity (with the star coming through) rather than the idea of only relatively large single crystal areas as discussed above? In talking with Dr. Bassett about this at the time, he was of the opinion that the effect could be produced by the single crystals amongst the others and coming through as a single sharp star....which still sets me back where I was.

Although this all in regards to massive rose quartz, are there other varieties of "massive" quartz which would fit into the tree and would they be exceptions to the micro/crypto?

Best wishes,
Elise

[Pete Richards]

Maybe a useful way to think of it is to think of the quartz deposit as a rock, not a massive mineral. Rocks are made up of mineral grains. Each grain is (typically) a single crystal of a mineral, and as such has consistent optical and crystallographic properties within the grain. But the grains (typically) have no particular orientation or relationship to each other within the rock. In a granite, the grains are quartz, feldspar, and mica, and a few other goodies. In a pod of rose quartz, the grains are quartz, quartz, and more quartz. Each grain is a single crystal, and has that same internal consistency, permitting, among other things, uniform extinction directions, and asterism. But the grains are related to each other in the "rock" at random. Whether the grains are huge, baseball sized, or marble sized, this basic texture remains true. If the grains were sand sized, you would not see asterism (and I sort of doubt if even the pink color would be apparent).

[Elise]

[alfredo]

I might be wrong here but, just for the record, I think it was Japanese mineralogist and dealer Dr. Hidemichi Hori who first proposed using the name "pink quartz" as a varietal name distinct from "rose quartz".

As was pointed out above, we don't normally invent different names when two different causes produce the same varietal phenomenon, like emerald being the result of colour contributed by vanadium or chromium (or both together). But the case of pink quartz vs rose quartz is quite different, as the different causes lead to quite different physical properties and morphologies. Because of these differences, crystal faces are frequent in pink quartz, but unlikely in rose quartz, with asterism being the reverse - much more likely in rose quartz than pink quartz. Furthermore, pink quartz could be light-sensitive, becoming paler on long exposure, whereas rose quartz should be stable in sunlight.

[John S. White]

Excellent points Alfredo, so where do we go from here? It would be helpful if the inclusions that are responsible for the reflections which generate stars could be identified, and it seems to me that this should be relatively easy. I note that many references attribute this to rutile, yet quartz containing "titanium" (presumably rutile) from a locality in Virginia is blue. Could be anatase I suppose. Rutile-rich quartz from Itabira, Brazil, is not pink or red.

[Amir Akhavan]

The titanium is probably inside that dumortierite look-alike (that has a different unit cell than dumortierite)
Elise has already pointed that out.

[Elise]

[Elise]

[Elise]

[Dean Allum]

Duncan,

I created that setup and saw 4-extinctions per 360 degree rotation when the Z-axis was perpendicular to the light. This setup does not appear to work if the surface is frosted or there are excessive internal ghosts.

When you place thin sections of mica between the polarizers, you get a Psychedelic Party!!!

I am going to put this onto an overhead projector for the next rock talk that I give to an elementary school. I am also wondering what this would look like with those orthogonal polarized lenses now used in 3-D theaters.

Thanks,
Dean Allum

[Duncan Miller]

Dean

See Olaf Medenbach's superb web site <homepage> for ideas about working with polarized light and crystals. Go to "How to play with polarized light", download the file, and save it for future use. He explains how to you use a flat screen monitor or cell phone as a source of plane polarized light. Most overhead projector lamps are not bright enough but you can use an ordinary slide projector (if you can find one) as a light source for demonstrations. I haven't tried using a digital data projector and don't know their polarization characteristics.

My understanding is that the REALD 3D spectacles used in most movies operate with circularly polarized light. You can get paradoxical effects if they aren't orientated correctly. Try using them mutually reversed to obtain extinction. This is because the plane polarizing filter has an incorporated 1/4 wave plate on one side (as explained on Olaf's site), which converts plane to circular polarization, and vice versa if the right way around.

Duncan