Amazonite with Smoky Quartz (#42)
|North America||Yucca Hill, near Lake George, Park County, Colorado, USA||2" x 1 ½" x 1 ¼"|
The most common mineral group in the crust of the earth is feldspar, but the term "feldspar'' includes several distinct minerals. The most abundant mineral in the crust, taking the oceanic and continental crust together, is plagioclase feldspar, which has the chemical formula (Ca,Na)(Al,Si)4O8 - meaning that Ca and Na (calcium and sodium) can substitute freely for each other, as can Al and Si (aluminum and silicon), the only constraint being that overall charge balance must be maintained. The other major group of feldspar minerals is called alkali feldspar. Sodium and potassium are the common alkali elements, and the formula is (K,Na)AISi3O8. Together, plagioclase feldspar and alkali feldspar make up half of the volume of the earth's crust. Quartz is the third most abundant mineral - the most abundant in the continental crust, but virtually absent from the oceanic crust.
Depending on the details of how the Al and Si are arranged in the crystal structure and the proportion of K and Na, there are different species of alkali feldspar, each of which has a distinct mineral name. One of those distinct types of alkali feldspar is called microcline. Amazonite is a variety of microcline characterized by a green to blue-green color. The color is caused by the presence of lead, Pb, as a very sparse impurity in the crystal structure, which generates a color center (see the Learn More for #4 and #9 for a discussion of color centers).