Feldspar and albite and oligoclase and andesine and bytownite and labradorite and anorthite

Once you spend a lot of time patiently learning terminology, the study of minerals begins to make sense. And, the study of minerals then provides a window into gemology, petrology, geology, vulcanology, chemistry and other disciplines. The key to learning these disciplines is to realize that they are defined by linguistic choices. Once you learn the words and learn to what they point, you not only are able to understand what some people are talking about but you have the necessary resources to undermine the field, if that is your interest.

That is, once you learn that amazonite was the name given (by mistake, actually) to a kind of mineral (which scholars now think was jade or nephrite) allegedly given by women warriors (again, probably mistakenly called Amazons because the "myth of the woman warrior" was on the brain of the hearty male conquistadores in Brazil in the mid-sixteenth century), you are offered windows not only into the world of feldspars, of which amazonite is one, but into the world of the naming of the River, the question of what the first Europeans actually met when they were floating down the Amazon and into the human capacity to impress an inherited reality on new phenomena that actually have little to do with that inherited reality. Phew. I think that was the longest sentence I have ever written. But the goals of this mini-essay are much more modest than redefinition of fields. Before you redefine you have to learn it as it is. So, let's learn the basic langauge of feldspars today, a mineral that supposedly occupies more than 50% of the earth's surface.

Divining (and Dividing) Feldspars

As any of 1,000 web sites and dictionaries will tell you, feldspar is a word formed from the German, meaning "lustrous field mineral." It is a group of "alkali aluminosiolicates" because potassium and/or sodium and/or calcium is added to aluminum and silicon in these minerals. More than 98% of the earth's surface is composed of just eight chemical elements: oxygen, silicon, aluminum, iron, calcium, sodium, potassium and manganese. If I were teaching this material, then, I would have students memorize those and their chemical symbols within the first five minutes of class.

Thre are about 20 kinds of feldspars which mineralogists divide into three groups: plagioclase feldspars (plagioclase, derived from Greek, means "slanted breakage" and this refers to the way these minerals "fracture." They are not orthoclase, or breaking at right angles; they break more irregularly), potassium feldspars (sometimes also known as orthoclase feldspars because this is the dominant member of the subgroup), and "other feldspars." One web site says that only 9 of the feldspars are plentiful (six plagioclases and three potassiums), but at least one of the "others," perthite, is of significant interest. I think I'll return to perthite on the next page.

There are at least five gemstones that are feldspars. Gemstones are subsets of minerals but differ from them in that they are durable, rare and beautiful (at least that is the definition that everyone gives). The feldspar gemstones include labradorite, which is its own "brand" of plagioclase feldspar, spectrolite, moonstone (also known as adularia because found in Adula, Switzerland, and moonstone is a, get this, "microperthite orthoclase"--I'll help you out, be patient), sunstone, which is a brand of oligoclase (in the plagioclase group) and amazonite, a microcline (potassium feldspar).

Creating the Categories

So, in this and the next mini-essay I will only talk about the first two categories of feldspar, as well as perthite. The first, the plagioclase feldspars, consist of a series (this is a technical term to be exposited below) whose chemical symbol is (Na,Ca) AlSi3O8. What this means is that all six of the members of the group have varying percentages of Sodium (Na) and Calcium (Ca) ranging from Albite, which has 100% Sodium and 0% Calciium, to Anorthite, which is the reverse. So, let's get this table out, even though scholars tell us that three really aren't precise percentages of Sodium and Calcium for the six plagioclases.

Albite 100% Sodium and 0% Calcium; Oligoclase 90% Na, 10% Ca; Andesine 70% Na and 30% Ca; Labradorite 30 % Na and 70% Ca; Bytownite 10% Na and 90% Ca; Anorthite 0 % Na and 100% Ca. You can also see these six brands of plagioclase feldspar differentiated by the Albite/Anorthite ratio, but I don't need to fill your head with lots of numbers at this point.

Most will say that differentiation among the six is impossible to the naked eye, and even the catgegores are not completely fixed. As far as I can tell, identification is sometimes associated with the kind of rock formation in which these plagioclases are found, though this doesn't at first seem to make much sense. Should a piece of Lydian pottery cease to be Lydian pottery because it is found in Troy Level 17 rather than Troy Level 3?

Let's continue our journey.