I picked up a pretty cab a couple of weeks ago - it was labeled blue fire feldspar, which I wasn't familiar with, but the stone was pretty. I took it home and checked it out - it does appear to be a feldspar, but when I searched on this type of feldspar, I came up with lots of rocks for sale, but no real information.
Does anyone out there know anything about this particular feldspar? At a guess from looking at it under a scope, I would say it might be plagioclase with microcline....but would just like some facts. thanks much! Jennifer
Jennifer, Sounds like Moonstone. Google it and see if it fits what you have. The following is a Moonstone cab from rock collected at the Ray Mica Mine: Mike
Or, it might be Laboradorite . . . a variety of plagioclase.
I have seen some material called Blue fire feldspar, so I think I know what you are talking about. This is a fine grained dark material with larger and randomly oriented lath like crystals (phenocrysts) that contain the "fire". If that seems to fit, I'll give a shot in the dark. Let's compare it to larvikite. Larvikite is classified as monzanite, an igneous rock with nearly equal plagioclase and alkali feldpspars and less than 5% quartz. The cooling history allows for larger and more equigranular texture without the phenocrysts that are present in the blue fire feldspar. This suggests a finer texture rock more like a latite or andesite. The phenocrysts are more lath like than blocky than usual for a phenocryst but that isn't a real problem. The composition of the feldspars is what I am after here. Schiller, the play of light, in feldspars is produced by alternating layers of alkali and plagioclase feldspar. So, this part of the rock could be labradorite or oligoclase layered between the alkali spar. Anyway, it does produce schiller. Keep in mind that schiller can be produced in more than one kind of feldspar. That is why you can have schiller in both oligoclase and labradorite. If I am right on this speculation, you could think of it as similar to larvikite. They have very similar ingredients that just got cooked up and cooled differently. I know this answer has a lot of petrology (study of rocks) rather than mineralogy(study of minerals) in it. The real answer to most rocks is the study of both: how minerals come together to become rocks. And, this answer is a lot of hypothesizing based on what I can see with my 10x loupe in blue fire feldspar compared to known studies of larvikite. Still, it is where I would start if I intended to do a real study on blue fire feldspar.
If the stone is from Ontario Canada, it could be peristerite. When I was at Quadville, Ont. a few years ago we found a lot of the stone they called peristerite. Some even had rainbow flashes in them, but mostly they had a bluish sheen. Ken
Thanks for the information and sorry for my getting back so late to all of you - my dad, another rockhound, has been in the hospital for months and I do nothing but drive back and forth from IL to OH
Anyway, it definitely isn't moonstone, or labradorite or peitersite -I"m very familiar with those. I think Al O is probably right about it - the stone is dark and has large randomly oriented crystals. In any case, the one I have is pretty and will make a nice piece of jewelry once I get done driving every week. Thanks again!