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Identity HelpFeldspar crystals- possible localities

22nd Jul 2018 22:39 UTCHolger Hartmaier 🌟

The attached photos are of two specimens included in a collection from the late John Casper Lund, a Canadian geologist that worked primarily in British Columbia and possibly into the USA as well in the 1960's to early '80's. There was no locality data provided and I was hoping someone might recognize the mineralogy. The main crystals are microcline, pale pink to pinkish-buff in colour, exhibiting micro-perthitic structure and Baveno twinning. The host rock is a porphyritic quartz monzonite-granite in composition, composed dominantly of alkali feldspar (about 65% of feldspars) and about 20% quartz. There is less than 5% mafics, as disseminated biotite. It looks similar to material posted on Mindat from Beaverdell BC (labelled as orthoclase- not microcline), however based on BC Minfile descriptions, the Beaverdell host rock is highly chloritized and this is not. Also there is no mention of perthitic texture in the Beaverdell material. The other BC possibility is Shingle Creek, west of Penticton, however the host rock there is a quartz-feldspar porphyry.

I realize this type of material may be relatively common, however I'm hoping it may be distinctive enough that someone might recognize the locality.

23rd Jul 2018 01:03 UTCJohn Sobolewski 🌟 Expert

I believe this comes from Shingle Creek, near Penticton, British Columbia, where I have collected similar specimens in a similar matrix about 30 to 40 years ago. The specific roadcut where I collected had both simple crystals and Baveno twins. It was called Orthoclase when I collected it but apparently it has been reanalyzed to Sanidine. John S.

23rd Jul 2018 01:53 UTCHolger Hartmaier 🌟

Thanks John,

If the matrix looks the same then it may be a good bet it came from there. In the Mindat photos there is only one which shows the matrix rather poorly. The rest are loose crystals. Do your crystals have perthitic texture too?

Holger

23rd Jul 2018 03:25 UTCRuss Rizzo Expert

Holger,

I don't mean to butt in... But I thought that you might find this helpful.

I checked my Beaverdell specimen and the matrix bears no resemblance to the matrix of your specimen. I would be confident labeling yours as Shingle Creek.

Russ

23rd Jul 2018 04:30 UTCHolger Hartmaier 🌟

Thanks Russ,

No problem- thanks for confirming that. The Mindat photo of the Beaverdell material isn't that clear, so I have to rely on feedback from other collectors with matrix pieces from known localities. Based on some reference material I've been reading on Shingle Creek, there is a wide variety of matrix material in the area that contains porphyritic crystals, so some variation in composition is expected even from the same locality.

Holger

23rd Jul 2018 08:01 UTCJohn Sobolewski 🌟 Expert

Holger,

Your matrix looks similar to mine. The mindat photo showing the matrix from Shingle Creek is a specimen I collected and I took the picture with an El Cheapo $99 camera when I was still learning how to use it. At that particular location, I found both simple crystals and Baveno twins. Most of the time, the matrix was more friable than the crystals, so it was easier to extract whole crystals than to get matrix pieces. I have moved my collection twice since taking the photo so I cannot put my hands of this specimen now and cannot tell you of it has a perthitic structure.

Note however that I was collecting at a location right next to the road with parking for my car less that 10 feet away. I was told that was the easiest place to collect and that there were several other locations nearby but none as convenient. It is possible that those other locations may have sharper crystals in a harder and less friable matrix.

I hope this helps. John S.

23rd Jul 2018 14:21 UTCDavid K. Joyce Expert

I've collected at Shingle Creek and have seen many dozens of sanidine specimens from "Beaverdell". All Shingle Creek specimens that I've seen have had rougher crystal faces than all Beaverdell crystals. Never saw any chloritized rock from the Beaverdell location. Perhaps minfile is refering to the rock that surrounds the porphyritic rock? I have no doubt that there must be other places in the area that have such crystals but I haven't actually seen any. The twins that I have seen in both places were all carlsbad twins? Guess there could be manebach twins!?

David K Joyce

24th Jul 2018 15:54 UTCHolger Hartmaier 🌟

Thanks John and David,

Attached is a link to a Geological Survey of Canada publication on the Shingle Creek porphyry by Bostock, which should be added to the Mindat references in the locality data. The report is an upgraded version of Bostock's thesis at UBC, which I've also attached for reference.

https://geoscan.nrcan.gc.ca/starweb/geoscan/servlet.starweb?path=geoscan/fulle.web&search1=R=101491

No doubt there could be other places where similar crystals could be found. The geologist that collected these specimens worked in all areas of BC. Based on the information I've gathered so far, they seem most likely to be from Shingle Creek, unless someone has other suggestions.

Thanks,

Holger

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