Feldspars isomorphic also?

[ger100]

Under "pyrope garnet" topic, there was a link to an article on "isomorphic" gemstones. This listed along with garnets, tourmaline and topaz as being isomorphic. The article explained how several changes of minerals affected color and name of garnet and how subtle the changes could be.

I''ve noticed quite a variation in color for what is being called Andesine/Labradorite (feldspars). I believe that there is some ratio of calcium to sodium which determines which is which. But most people seem to be ignoring this ratio since it takes an expensive test to distinguish.

My question:
In "Andesine/Labradorite" I have seen green, red, red-orange, green-red, yellow, yellow-orange, orange, yellow-green, pale yellow, and all of these, at times, with streaks of color or clear veining. There are some gorgeous pale peach stones which flash green "lights." At one time JTV called these "honey." GOK what they''re calling them now. But MOST of what is being called green has some element of red in it...most of it. Some is a very nice pure teal green. Same goes for the red: some is a gorgeous vivid red, although a lot has some muddling of green in it.

Would these unusual colorings, sometimes with subtle variations, make the Feldspar group ALSO isomorphic??? Even though apparently only one element is the thing detemining which is Andesine and which is Labradorite, don''t the different colors suggest other mineral changes as well, similar to the garnets in the article about isomophic gems?

Thanks
Ger

 

[Richard M.]

Date: 5/19/2007 7:25:08 PM
Author:ger100
Would these unusual colorings, sometimes with subtle variations, make the Feldspar group ALSO isomorphic??? Even though apparently only one element is the thing detemining which is Andesine and which is Labradorite, don''t the different colors suggest other mineral changes as well, similar to the garnets in the article about isomophic gems?

Yes -- not the feldspar group as a whole, but the plagioclase feldspars (which include the species andesine and labradorite) are examples of the isomorphous mixing of two endmembers, albite (NaA1Si 3 O and anorthite (CaAl 2 (SiO 4) 2) in all proportions and with no sharp line between the several subspecies. Albite and anorthite exist in a chemical state known as a "solid solution" series.

The point to remember here is that the divisions into separate species was purely arbitrary, for convenience in classification. So based on the relative percentage of anorthite to albite, we have albite, oligoclase, andesine, labradorite, bytownite and anorthite. That''s why I''m amused to see such commercial hype lavished on the distinction between andesine and labradorite -- they''re basically the same stuff with very tiny variations.

Color is a different matter. Feldspars of the kind you mention are colored by impurities called chromophores. Labradorite -- an extremely confusing mineral designation -- can also be pseudochromatic, ie. "false-colored," as a consequence of its crystal structure. I''m referring to the optical effect called "labradorescence" which is not seen in the types of plagioclase you mention. But the reds, greens etc. are, to the best of my knowledge, the result of chemical impurities like copper for the red color.

There is also research currently taking place on the effects of irradiation on some kinds of feldspars. To my knowledge the results aren''t in yet.



Richard M.

 

[ger100]

Thanks Richard! Very interesting. My chemistry knowledge is many moons old, but this nice explanation is quite helpful.

When will your link be functional? Are you a cutter too??

Ger