FMF - Friends of Minerals Forum, discussion and message board

[Peter Megaw]

My daughter Lauren found a really superb adularia minature in the drawers at Arkenstone's opening last weekend. It's a minature with very sharp clean white crystals, no cleavages, and none of the chlorite I am used to seeing on/in such pieces. It looks multiply twinned...but could be intergrown sharp bladed crystals. I tis a bit hard to see in the pictures so I tweaked the contrast to see if I could bring it out more.

All Rob had for locality was "Goschener", which as I understand it covers a lot of real estate. He said he'd had it for a while. Lauren would be very happy to get more specific information on the piece. It looks so distinctive to me I suspect someone out there might even recognize the find.

[Harjo]

Peter, the label should read Göscheneralm (Göscheneralp) . I can't be more specific about the locality as Adularia is a common mineral in the Alpine clefts of the region.
Here's a photo that Joe posted on Mindat: https://www.mindat.org/photo-84177.html it's also from the Göschenen area.
It's a nice specimen you've got there, congratulate Lauren on that one!

Cheers

Harjo

[Montanpark]

Peter,
i am not sure but it may be that i formerly owned this specimen (at least it is very similar)... please send me a PM ... i will have a look in my records. If it is the same, it is an old specimen from the Krantz stock, coming from the Göscheneralp area,

cheers
Roger

[strahler]

Peter,

I would venture to say it is Goschneralp, beyond that, I do not think one could define it to Winterstock/Gletschorn/Galenstock. Any closer Identification, one would have had to known the strahler, to be specific.

Here is one I have from Madernertal, same twin habit, although matrix of Madernertal is very distinctive.

[Duncan Miller]

Hello Peter

This won't help with the Swiss adularia locality, but the attached drawing may help diagnose the twinning in your daughter's specimen. This is from an article I wrote some years ago about multiple twinning in adularia specimens from a quarry near Vredendal, a town some 300 km north of Cape Town, South Africa. I will try to attach a photograph of one of the twinned crystals, about 10 cm across.

Duncan

Illustration of the formation of a complex polycyclic twin in adularia feldspar. A: Diagram of an idealised monoclinic adularia single crystal viewed obliquely from the right, showing the narrow side pinacoid b, the broad prism faces m, the basal pinacoid c, and the second-order pinacoid x. B: Diagram of the adularia crystal viewed from the side, showing the crystallographic axes. The c-axis is vertical, the b-axis emerges perpendicularly from the page, while the a-axis is inclined by 116º to the vertical plane bearing the b and c-axes. The basal pinacoid c is parallel to the inclined a-axis, and the prism faces m and side pinacoid b are parallel to the vertical c-axis. C: Drawing of the adularia single crystal viewed obliquely from the right, showing the approximate trace of the {021} Baveno twin plane, as a thick line, on the crystal faces. The right hand portion of the crystal is mirror reflected through the twin plane to produce the pyramidal twin illustrated in D, viewed from above. Polycyclic twinning, involving four twin pairs like those in D, can be visualised as four-fold rotation of the Baveno twin around the shared a-axis, represented by the line joining the upper two c faces. This produces the complex cyclic twin sketched in E, viewed from above, from a real example found by Lesley Bust. The corner fishtail grooves can be on either the upper or lower surface, depending on the relative development of the basal pinacoid c and the second-order pinacoid x faces. This particular example also has some evidence of reflection twinning across the {100} plane, parallel to the vertical plane containing both the b and c-axes of the untwinned crystals, so three different twin laws are involved here.

[Peter Megaw]

Duncan...very cool thanks. Lauren and I will sit down and decipher her crystal with this in hand.

[Jordi Fabre]