Introduction

    This guide is written to help potential buyers protect themselves on eBay and the Internet in general. As a mineral scientist, I am puzzled and often downright offended by the number of uninformed ads touting feldspar gems with names such as andesine, labradorite, andesine-labradorite, sunstone, red andesine, and other similar titles.  This guide will explain what a feldspar is, how andesine and labradorite fit into the feldspar group, and how to spot a dubious ad. 

The Science of Feldspars: What's in a Name?

    There seems to be a lot of confusion over what a feldspar is, and how andesine and labradorite fit into that nomenclature.   First, let me establish that minerals are not proper names, and are therefore not capitalized unless part of a title or at the beginning of a sentence.  Now that we have that out of the way... the feldspars are a family of framework aluminosilicates composed of groups of aluminum, silicon, and oxygen, with metal ions of either calcium, sodium, or potassium attached.  Two important notes: first, this is a very basic explanation; second, there are some rare feldspars with either barium, rubidium, and even ammonium attached!  However, while there are about 20+ members of the feldspar group, only about 9 of them are common.  These common feldspars are divided further into two groups, the alkali feldspars, and the plagioclase feldspars.   The plagioclase feldspars include andesine and labradorite, which we will discuss further.

    The feldspar group is the most common family of minerals in the earth's outer crust, comprising about 60% of our rocks.  They are well known and fairly well understood.  As far as the plagioclase series of feldspars, they form a compositional range from pure calcium (Ca) feldspar to pure sodium (Na) feldspar, with continuously varying compositions in between.  For example, a plagioclase may be (90% Ca, 10% Na), (75% Na, 25% Ca), or any similar ratio adding up to ~100% (allowing a little room for other occasional metal ions to substitute in the mineral).   Now, in the past, such compositions were given intermediate names such as albite, oligoclase, etc., as shown here in terms of  percent Na composition: albite, 100-90% Na; oligoclase, 90-70% Na; andesine, 70-50% Na; labradorite, 50-30% Na; bytownite, 30-10% Na; anorthite, 10-0% Na. 

    The previous list seems to be a great cause of confusion for people who describe gemstones made from plagioclase feldspars.  As an example, you will often seen ads titled Andesine-Labradorite or Andesine Plagioclase, among others.  What many folks don't understand is that feldspars are often zoned, that is, they may contain areas of andesine alternating with areas of labradorite.  This is a phenomenon of how the crystal grows in the magma.  Of all the feldspars I have analyzed, most were zoned.  It is simply a convention to average the ratios of Na to Ca and report that as the composition, while noting the zoning in your report.  I should also mention that there is a trend to retire such names, and simply report a mineral as, say, plagioclase  (34% An), where the An represents the sodium end member.

    Before we continue, take note that this is a simplified explanation.  However, everything here, and more, can be found on the Internet.  Many professors post their mineralogy notes on line--you can find more than you ever wanted to know about feldspars by searching!

    Now, let us return to naming.  Take heed of ads that declare they are selling a new, rare feldspar composed of andesine and labradorite, or similar words. Well, there is nothing special about that zoning.  The feldspar should simply be listed as the average composition.  In the end, it doesn't matter; what matters is the gem quality of the specimen.  Also beware of vendors trying to give their own names to minerals, or claiming that "a new name is being proposed for this material."  Well, unless that name is being proposed by a bona fide mineralogist with credentials acceptable to the International Mineralogical Association (IMA), no one is naming anything.  The IMA has a commission governing new minerals, with specific rules including the requirement that a new mineral have both a unique crystal structure and chemical composition; since the plagioclase feldspars have been known and studied for a long time, we can be reasonably certain there aren't any new minerals in that series.  Sellers who list names such as Labrasine, Labrasite, or Andorite are simply fabricating names and causing confusion.  As an mineral scientist I will state it bluntly: such activities are in contradiction to the rules of mineral nomenclature, possibly in contradiction of Federal Trade Commission rules regarding the uniqueness of a trade commodity, and a sure sign that the seller is not quite informed about the material being sold.  Again, all of this knowledge can be gained by reading scientific information about mineralogy posted on the Internet.  Don't just take my word for it!

Where Did You Say You Were From???

    There is another matter of concern.  As far as anyone has been able to document, the transparent, richly colored, and sometimes dichroic feldspars come from two mines in Oregon, USA.  These localities have been documented in Gems and Gemology, among other respected publications.  Yet we see such gems advertised as originating from Congo, Brazil, and India, among other vague localities.  If you have any doubts, ask the seller about the exact locality.  If you get a vague answer, or no answer at all, you need to decide if you are comfortable with the advertised locality of origin.  It may not matter to you, but Oregon sunstone is a recognized brand, and the miners do not appreciate the value of the product being diluted.  If there is another locality where these deep red, green, and copper-bearing materials are being mined, the world needs some documentation so all doubts can be removed.  Until then, who can be certain?  Extraordinary claims require extraordinary proof.  As of 5 August 2007, I have received none such; I will update this guide as new information is revealed.

Conclusion

    These brightly colored plagioclase feldspars make wonderful gemstones.  However, consumers should be aware of the confusion regarding names and origins.  You should always bid what an item is worth to you, and not be swayed by verbal voodoo in advertisements.  Good luck!