Bill Long 11/08/04
Before getting into the potassium feldspar group, I want to mention something more about the concept of a series, of which the plagioclase feldspars are an example.
Bowen Reaction Series
About 100 years ago, N. L. Bowen, a scientist at the Geophysical Laboratories in Washington, D.C. began studying the process of crystallization of silicate materials from magma. The assumption seemed to be that as the earth cooled certain different specimens of magma would become concretized. Thus, he discovered that as magma cools from 1400 to 800 celsius specific minerals form. At the higher temperatures, associated with mafic (pertaining to dark-colored minerals high in iron and magnesium and relatively low in silica and oxygen. Look up the word. It is right after "mafia" in the OED. As a matter of fact, if you don't look up the word, I will make some mafic shoes for you and throw you into Lake Michigan) or intermediate magmas, the general progression of mineral production is separated into two branches, one of which is the plagioclase feldspars.
Thus, as the magma cools, one moves from a more calcium rich (i.e., anorthite) to a more sodium rich (i.e., albite) stone. As the magma cools, then, we have the next member of the series created. While geologists still adopt this approach to plagioclase feldspars, one engaging internet geology teacher warns us that, according to Strickler's Rule # 1, that all regional theories break down at the local level. This is the geological equivalent of the notion that if you look at something closely enough, it disintegrates.
Back to the Feldspars--the Potassium Feldspars
The other group of feldspars is called the potassium or orthoclase feldspars. These four types of feldspar are orthoclase, sanidine, microcline and anorthoclase. Significant for this group is that they all have the same chemical structure. They are potassium aluminum silicates--KAlSi3O8. Because the elements are the same for each of the members of the group, they have to be differentiated by other things, in this case crystal structure, and are called polymorphous. This is to be differentiated from isomorphous, which means "having the same crystal form, meaning that the molecular arrangement is identical, except for the fact that different elements are present." That is, the plagioclase feldspars are isomorphous while the potassium feldspars are polymorphous.*
[*Oops. Do we need a brief footnote on crystal structure? There are six of them, which can be easily learned. Orthoclase and sanidine, for example, are monoclinic, microcline is triclinic, and anorthoclase can be either. A monoclinic crystal may be defined as where two of the three axes make a right angel with each other, but the first and third intersect at other than a right angle. Triclinic crystals have all three reference angles of different lengths, and all intersect at angles other than 90 degrees].
Perthite and Microperthite
But we left a term hanging from the previous essay and that is perthite. It is one of the "other feldspars," and is interesting because it is defined as the intergrowth of two different feldspar minerals in one rock. Perthite is usually composed of veins or strips of one feldspar running through a different feldspar. It can be striped, veined or almost zebra-pattered from the lamellar growth within a crystal.* Perthite can then give birth to a whole
[*A lamella is a thin plate, scale, layer or film, especially of bone or tissue, according to the OED.]
range of terms based on the size of the specimen (cryptoperthite is something only seen with help of a powerful microscope, while microperthite and macroperthite exemplars can be recognized by the naked eye). Ah, remember how I said in the previous essay that moonstone (adularia) is a "microperthite orthoclase?" Now you understand it. Then, we have antiperthite and mesoperthite, depending on whether the postassium-dominated mineral forms first (microcline or orthoclase) before the sodium (albite or oligoclase) as in antiperthite or the reverse, as in mesoperthite. I have only been doing this about three weeks, so you can learn it too!
There are lots more technical things you can learn about feldspar, to be sure, but this will suffice for an introduction. You begin to see that if you are patient with words and learn to associate words with other words, the conceptions make sense after you think about them. The fact that they make sense does not mean, of course, that the terminology is anything other than a convention agreed upon by geologists to communicate with each other. There are uncertainties at the edges, to be sure, and all of it is based on an understanding of geologic formation of the earth.
But, what if the Fundamentalists are right, or take over geology, in the end? After all, they just elected a President of the United States. What if they then convince the world that geology is all just the science of the devil because we know for sure that everything is less than 7,000 years old? Then, all our terms change. But, I don't think it will take a Fundamentalist to shake up the field. The enemy is much more insidious.
Copyright © 2004-2010 William R. Long