FMF - Friends of Minerals Forum, discussion and message board

[Mathias]

This time, not long ago but very recently, I purchased a specimen containing not twinned but tripled Enargite Crystals, with pyrite, Quiruvilca mine Peru.

Although I have no doubt about the minerals on the specimen, nor the location of origin, I would like to know if anybody else has ever seen this crystal form?

Thanks,

Mathias

[rfarrar]

According to Dana's 7th edition, Vol.1, p.390 - Common twin plane {320}, but according to Stepanovic, 1903 {120}. "Sometimes in star-shaped trimmings, with adjoining parts meeting at nearly 60 degrees." There is an illustration of a trilling or sixling twin cross-section from M. Caudaloso, Huancavelica, Peru.

[Pete Richards]

These are wonderful little twins! I've never seen examples of this twin relationship in enargite. I'm just back from three blissful weeks in Maine, including a visit to the Emmons Mine, and I want to comment further on these twins but I need to do some more research. There are indications of confusion in the literature due to changes in the axial settings used to describe enargite and these twins over the years....

More, hopefully, sometime in the coming week.

[Justin Hickok]

That is amazing stuff right there. Wow!
I would be interested in purchasing some. If that's allowed here.
Thanks.

[Mathias]

Hi Justin this is the only specimen I have, I recently purchased it from a befriended dealer/collector who bought an old collection from an elder collector who finally decided to stop. I got the chance to buy a few pieces from this collection before it goes to the market. There was only 1 such specimen in this collection. If I knew there were more, I'd sure bring you into contact with him. But there aren't.....
best regards,
Mathias

[Jordi Fabre]

[Justin Hickok]

Jordi, thanks for the info. Im still learning and I have a habit of stretching the rules a bit.
I'll keep what you said in consideration from now on.
I do appreciate and respect this forum and everyone's professionalism.

Mathias, thanks for your reply. Totally understand.
Im definitely going to be looking for that particular mineral at next months' rock, gem, mineral show at the Denver coliseum.
Danka.

[Pete Richards]

These exciting little stellate twins were something new for me to see, though later I realized that I had seen a drawing of such twins in Dana’s System 7th edition. I decided to try to draw an example using SHAPE, and that is where the fun began. I chose a single crystal bounded by {230} and {001}, the first indices chosen to give a crystal with a moderate wedge shape; this crystal is shown on the left in the figure. The choice of the form that bounds the flat platy surface of {001} is not critical: it only gives a crystal of about the right shape to be part of the observed twin.

Then I told SHAPE to compute a trilling by interpenetration on {320} and {-320}, analogous to the classic aragonite trillings. This is the twin law reported in the literature - Mindat and the Handbook of Mineralogy (www.handbookofmineralogy.org)) are web-accessible sources. The result, shown in the middle, was totally unsatisfactory!

On a hunch, I tried a trilling on {230} and {-230} (note the switch of the 2’s and 3’s from the previous trilling). This is shown on the right, and is quite satisfactory. The angles between the primary crystal, oriented vertically, and the second and third individuals, to the left and right, are close to but less than 60°. The angle between the second and third individuals, generated by the twinning, is a bit more than 60°. This shows that this trilling only approximates perfect hexagonal symmetry, and reflects the fact that the boundary between the second and third individuals is not a twin boundary, but just one where individuals already twinned to the first crystal meet each other.


So what happened? Why did the published twinning indices did not work? This is an example of a fairly common error in the propagation of crystallographic information through time. It involves a change in the fundamental crystallographic framework that describes the geometry of the mineral, but that is not adequately carried through in all the information derived from or related to the new framework.

Both Mindat and the Handbook of Minerals give the unit cell as orthorhombic with a=7.407Å, b=6.436Å, and c=6.154Å. Both describe crystals as tabular on {001}, and both describe twinning on {320}, sometimes as star-shaped twins.

Dana’s System of Mineralogy, 7th edition, does not give unit cell dimensions for most minerals, because these parameters had not yet been determined when it was written. Rather, morphological studies had established unit cell ratios, which described the shape of the unit cell but not its absolute size. This source gives a=0.8713, b=1.0, and c=0.8277.

If we convert the modern unit cell parameters into ratios by dividing by a=7.407Å, we get a=1.0, b=0.869, c=0.831. This is in good agreement with the ratios from Dana’s System, except that the a and b ratios are switched. This indicates that, somewhere along the line, the a and b axes were switched in the official mineralogical description of this mineral.

But the indices of the trilling given in Dana - {320} - are identical to the ones in the modern literature. When the axes were switched, the indices should have been switched as well, but they weren't.

This failure, all too common, resulted in a morphological description that is not reproducible - in fact it has become wrong. Relative to the modern framework, this twin should be described as a twin, not on {320}, but on {230}.

[Bob Carnein]

Thanks, Pete, for the clear and elegant explanation of the enargite twin. This is why I continue to enjoy this forum.

[Justin Hickok]

Mathias, im really hoping to get your take on something im getting ready to post.
Something that's been haunting me for two years.
Dang rocks anyhow!

[Mathias]

Dear Pete,

Thanks for your crystallographic analysis.

Although I studied chemical engineering (30 years ago…..) and I have basic knowledge about crystal structures, unit cells etc. I think I missed the point; would you please be so kind to explain it in laymen terms?

As far as I understand, your modeling program was not able to produce triples?


Thanks,
Mathias

[Pete Richards]