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[Pete Richards]

Quartz has relatively low symmetry - much lower than would be suggested by its most common habit composed of a hexagonal prism terminated by what appears to be a hexagonal pyramid on each end. The lower symmetry is sometimes revealed by the presence of a modifying face such as the familiar "x" face. In an ideal perfectly formed crystal, x faces would be found on six of the 24 corners at the tops and bottoms of the prism faces, as shown in the left drawing below (only two of the six x faces are visible in this orientation).

Because of the low symmetry, quartz has "handedness" which is also revealed by the x face. The drawing is of a right handed crystal; a left handed crystal would be the mirror image, with the x faces on the left corners of the top of the prism faces.

Quartz is often twinned. One mode of twinning, the Dauphiné twin, can be thought of as two crystals of the same hand superimposed on each other. An ideal perfect right-handed Dauphiné twin would look like the right drawing below. However, Dauphiné twins usually don't look like this, because the twinning usually occurs in small domains. What one usually sees is one extra x face or maybe two.

Recently I obtained a smoky quartz crystal from Moat Mountain in New Hampshire which corresponds to the ideal model of a Dauphiné twin. It has x faces at the right top of all six prism faces, though some are better developed than others. In addition, three of the prism faces are completely developed, and these have x faces on the bottom left as well. The other three faces are covered at the bottom by contact with the matrix or with other crystals. Images of the six prism faces are included.

Several of the x faces are interrupted or terminated by a segment of the prism face on which they are located. These may represent regions of Brazil law twinning, which combines a left handed crystal with a right handed one.

Because such things are rather esoteric and not understood by many collectors and dealers, this remarkable crystal was also a bargain at a price of $10!

[Peter Megaw]

Great find...and explanation Pete. Thanks!

Might be worth mentioning that quartz handedness has a morphological manifestation (that you illustrate nicely) and a structural manifestation, which is the opposite of the morphological one. So, a structurally left-handed quartz crystal is morphologically right handed. Think of it like Left-handed indicate right-brain dominance.

John Anthony (UA Mineralogy prof) used to maintain that a high proportion of folks with good skills at structural crystallography are left-handed. Are you Pete? (no tell-tale watch in your picture!)

[Pete Richards]

[Josele]

An interesting piece and an amazing story of Brazilian quartz, I imagine Conselheiro Mata quartz is especially prolific in x faces.

Let me ask a question I have from long time in mind: How rare is to find a quartz prism showing x faces? or, in other words, which percentage of quartz prisms show his handedness? I got some dozens of quartz in my collection and no one of them show any x face.

In your specimen, only the bigger prism shows x faces or the smaller ones show x faces too? In other words, in a cluster where one prism have x faces, most of the rest of the prisms will have x faces too?

Thanks

[Peter Megaw]

Josele...good question! Although all quartz is structurally handed, X faces (and related faces) that let you determine morphological handing (and from that structural handing) show a wide range of development within and between localities. Many Brazilian localities show them commonly, Arkansas shows them rarely. I have collected pockets in Arizona where perhaps 10% showed X faces and unpacked barrels of Brazilian quartz where we were separating them because the metaphysical crowd liked right handed crystals better and would pay a premium for them...and well under 10% were left over without X faces.

During WWII when untwined quartz crystals (or at least largish untwined domains) were needed for making radio oscillators, the USGS ran a systematic examination of all the quartz mines operating there at the time to see if they could find a link between morphology and freedom from twinning. It was run by Charles Palache (see Quartz volume III of the System of Mineralogy for more) and executed by F. Earl Ingerson, one of my professors at UT Austin. Ingerson said the program was a complete failure other than giving him the opportunity to visit nearly 300 quartz mines. He gave me a suite of his samples...all with X and other faces neatly labeled. One of the pieces has two joined crystals, one left and the other right handed. The rest of his collection vanished sometime after his death.

I suspect this program was the basis for Palache's famous comment that "one of the rarest things in the mineral kingdom is an untwined quartz crystal"

[jorgedavid]

Hi Peter

In Taxco-Zacualpan Area in México are many points with this Quartz crystals X faces, but too are many faden associated to metamorphic belt one , Taxco Viejo Green Schist , you know if is possible to find in this terraines Xfaces Crystals ? There are some negative relation between metamorphic environments and development of this crystals Peter?

Saludos , Jorge Díaz de León

[Pete Richards]

Peter Megaw said it right - the frequency of x faces on quartz varies greatly from location to location. In my experience, they are often seen and particularly nicely developed on smoky quartz from the Alps.

I don't recall ever having seen x faces on faden quartz, but the faden habit would make them less obvious, I would think. It may also be that the alpine vein environment in which faden crystals develop is not conducive to the development of x faces for some reason of temperature or chemistry.

By contrast, most of my limited collection of gwindels show obvious x faces, but they are mostly from the Swiss alps where many of the normal crystals also have x faces, so the fact that they are gwindels may be irrelevant.

Josele, all of the prism faces on my crystal are modified by x faces, though some of them are larger and easier to see than others. In fact, I just looked at the specimen again. It consists of five crystals with varying degrees of completeness, and all of them have x faces on each prism face! Some are right handed and some left handed, but all like the main one in showing complete Dauphiné twinning. Suggests there was an environmental cause at work....

[Josele]

After that this thread had awakened my curiosity, this afternoon I was reviewing again my quartz specimens and I found two pieces showing small x faces that I had not seen before. Also I found a piece with some s faces in form of elongated parallelogram and some pieces showing faces of steep rhombohedron. I had to watch carefully to see them!

If I'm not wrong, in last photos of Pete's specimen both s faces and steep rhombohedron faces are also present.

Thank you for sharing your knowledge, is easy and funny learn with you all!

[Pete Richards]

[Elise]

[Pete Richards]

Hi Elise,

Thanks for your contribution summarizing Sunagawa's writings. He writes some interesting stuff!

One thing that would be interesting to know, and I don't know, is how frequently quartz in igneous rocks crystallizes initially as high- or beta-quartz, then inverts to low quartz on cooling. Since inversion often produces Dauphiné twinning, this would indeed be a mechanism for promoting Dauphiné twinned overgrowths in veins etc.

Unfortunately, as you know, Dauphiné twinning cannot be detected by normal optical examination, so looking at routine thin sections of granite and other igneous rocks would not answer the question....

[Elise]

There is a paper entitled "Growth and morphology of quartz crystals synthesized above the transition temperature" which may lend some insights into that question (Hosaka, et al 1995 Journal of Crystal Growth). Sunagawa does write that "High-temperature quartz formed in acidic magmas, transforms its structure to low-temperature quartz at temperatures lower than 573 [degrees Celsius], while keeping the external form unchanged." But he doesn't mention Dauphine twinning in conjunction with that.

[Pete Richards]

Some igneous rocks contain phenocrysts of once-high quartz, with the typical bipyramidal habit with little or no prism. Most examples of "high-"quartz that I have seen are these phenocrysts which have weathered out of the rock.

Frondel, in Dana's System 7th edition Vol. 3 (p. 117) says that Dauphiné twinning may be acquired when high quartz inverts to low quartz. However, it apparently does not happen systematically. Frondel also says that the presence or absence of Dauphiné twinning cannot be used to infer that a crystal was initially high quartz, though some forms of twinning e.g. the lamellar twinning in amethyst indicate that it did not form as high quartz.

[Roger Warin]

Hi, Pete,
A right-handed quartz Crystal, with a Dauphiné Twin.
See my shot 073-Apatite-Goscheneralp-1_R (03 sep.) in the post http://www.mineral-forum.com/message-board/viewtopic.php?p=41166#41166 with 2 apatite xls on this specimen.
Roger.

[Elise]

Pete and other RMS'ers,
It would be fun to have a quartz workshop sometime at the Rochester Mineralogical Symposium (or a crystallography workshop), even if it's in the Hospitality Room around the chips & dips table! Thanks so much for creating this instructive thread - I look forward to more!
Cheers!
Elise

[Jordi Fabre]

Moved to the Featured Columns of FMF section. To know more about this section please read: What is "Featured Columns of FMF"?

[Duncan Miller]

It might assist some people following this thread to read Amir Akhavan's description of twinning in quartz and how to identify the various twins.
http://www.quartzpage.de/crs_twins.html
(link normalized by FMF)

[Elise]

It has been such a pleasure to watch The Quartz Page website grow over the years.

[Elise]