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Identity HelpNeed help with feldspar classification

2nd Oct 2016 17:11 UTCEch Noch

Found this material in a pegmatite pocket with large smokey Quartz crystals and I want to be sure that I list the correct mineral names when adding the new locality.

2nd Oct 2016 17:20 UTCEch Noch

-- moved topic --

2nd Oct 2016 19:59 UTCErik Vercammen Expert

That's an intresting find. The first photo represents baveno twins of microcline: mostly, these twins form a more or less quadratic prism, but sometimes, flattened crystals form an angle of about 90°, like in these picture from Mont-Saint-Hilaire: http://www.mindat.org/photo-177630.html, and that is unusual.

3rd Oct 2016 02:47 UTCEch Noch

Very interesting, I looked everywhere trying to find a proper classification for these, I narrowed it down to car. microcline but that's as close as I got. Evidently because I'm not a vetted member I can't add or edit localities, so at some point I will add this to the database.

3rd Oct 2016 03:16 UTCEch Noch

3rd Oct 2016 03:17 UTCEch Noch

The is a taper towards the bast of the formation, like an inverted pyramid.

3rd Oct 2016 03:31 UTCAlfredo Petrov Manager

Congrats, Jobadiah! Very interesting feldspar.

3rd Oct 2016 07:19 UTCMatt Neuzil Expert

Someone can add a location for you. Just leave in this post the information for where it is located, such as county, state, etc. One of mindat admins will add it for you and you can upload your finds.

I had added a new spot recently myself. It had been a while, so the process changed a little since the last time.

3rd Oct 2016 11:19 UTCPeter Slootweg 🌟

Nice find! It looks like albite feldspar to me. The crystals on the large specimen seem to overgrew some larger more blocky crystals. It might also explain the shape of the very nice box crystals. It looks like a replacement of albite on/after k-feldspar.

3rd Oct 2016 13:56 UTCEch Noch

Oooo, I had not thought about albite, it is found in the area, and would go well with the very large Quartz specimens. Crystal stucture would match... Food for thought!

3rd Oct 2016 14:20 UTCHarold Moritz 🌟 Expert

Not possible to tell from these photos what type of feldspar is showing. Albite may overgrow microcline in pockets. To determine albite, look for the evidence of polysynthetic twinning on the crystals surfaces or on some cleavage faces. All albite shows this but microcline does not. The twinning shows up usually as tiny parallel and very straight straiae that reflect light at two slightly different angles. You may need magnification to see this.

Here is a megascopic example on some cleavages (look at all photos): http://www.mindat.org/photo-766430.html

Here is a crystal showing them on the surface: http://www.mindat.org/photo-750914.html

Microcline on the other hand will likely show a perthitic internal texture caused by exsolved albite. These look like subparallel, slightly irregular veins inside the microcline. They do not cause alternating reflections on the surface and are not nearly as regular as the albite twinning. You can see the perthitic veining in the background of the second linked photo. It is most obvious in some amazonite: http://www.mindat.org/photo-261616.html

It takes some practice to get to know these features and you likely have a complicated example, but once you get it, you will have no difficulty telling these feldspars apart.

3rd Oct 2016 14:23 UTCAlfred L. Ostrander

Jobadiah,

Based on the somewhat platy nature of the crystals my first thought was cleavelandite, a variety of albite. From the last photo, I will add that the associated quartz

is rather impressive!

Best Regards,

Al O

3rd Oct 2016 15:08 UTCEch Noch

Another example, I would say that these are tabular, micro clinic, and parallel, they do not reflect at different angles and clearly site was one that I had looked through but had been unable to come up with a match. I will do my best to get some higher resolution photos with better magnification if that would help. In a 6'D x 2'W x 1.5'H pocket, I only found 7 of those squared formations.

3rd Oct 2016 15:51 UTCHarold Moritz 🌟 Expert

Nice stuff! Generally, in a pocket setting, albite tends to be the dominant, finer-grained feldspar forming masses of crystals with lotsa voids in between and as overgrowths on things inside. Microcline tends to line the walls of the pocket only and as individual, prismatic crystals rather than fine-grained aggregates. Based on this I'd say your feldspar is almost certainly albite. With crystals this small you will need a hand lens or microscope to see the twinning to confirm.

3rd Oct 2016 20:59 UTCEch Noch

I'm sorry the pictures look so distorted, click on them to see the actual picture... Obviously. Also, I just want to give a sincere thanks to everyone who has given input, really appreciate it!

Best I can do with an iPhone an no magnification while at work.

16th Oct 2016 17:15 UTCEch Noch

...

17th Oct 2016 21:05 UTCIan Nicastro

These are impressive finds! That smoky is huge! There is likely a mix of feldspar species from your pockets. Some of that does look like albite to me. We find albite, microcline and orthoclase in pegs down in southern california, usually the albite is limited to pocket zones and often the crystals are slightly translucent, the other species tend to be opaque. If you want me to geiger or raman that orange crystal from your other thread, shoot me a private message and I can ship it back to you. I really like that attractive peppering of the muscovite ontop of your feldspars, and the boxy shapes of the feldspars.

17th Oct 2016 23:22 UTCEch Noch

Some more of the float material, the wall material is much different and is solid, whereas the float reminds me of clouds. Neat stuff! The crystals were very nice from this pocket. Very dark! Let me see what I can do about sending the other specimen to you.

16th Feb 2017 18:11 UTCEch Noch

So, I hate to revive such an old thread, but I recently took one of these specimens by a rock shop in Fairfield CA and one of the employees there identified this as (if I recall correctly) Rhombohedral Calcicte... Can anyone elaborate, I've tried finding information to cooberate this ID but am having a hell of a time. Without seeing it, can anyone add any input? Some of these specimens sat for a week or more in Oxalic and biflouric acids with no effect. I'll try vinegar later today.

16th Feb 2017 19:24 UTCAlfredo Petrov Manager

Calcite? :-S Well, the rock shop employee needs to learn some basic mineralogy. ;-)

Feldspar would seem to be far more likely, and neither oxalic nor vinegar will affect it. Calcite, on the other hand, would be destroyed by either of those acids.

16th Feb 2017 19:51 UTCEch Noch

My thoughts exactly!

24th Feb 2017 05:42 UTCIan Nicastro

It's totally feldspar. We just can't ID the species from photos alone, it would need Xray or Raman spec, or thin section w/ polarized microscope. If you want send me a private message and I can try to get some analyzed by Raman. Sometimes pegmatite pockets, alpine cleft deposits and miarolitic cavities can have soft stuff like calcite in them, but it's uncommon and calcite would not survive acid. Also please wear gloves with oxalic acid, that stuff is a mild toxin capable of penetrating skin (in addition to being acidic) and chronic exposure can cause issues.

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