THE Feldspar croup

摘要

In Part I, I explained why the term feldspar refers to a group of minerals rather than a single species. Enough feldspars differ chemically that scientists have created two major groups and a couple of minor ones. One 1 group, based on chemical content, contains members of the potassium feldspar group: orthoclase, microcline and lanidine. These were described in Part I. Here. I'll discuss the second major group, the plagioclase feldspars, a much larger part of the feldspar species. The group includes albite. oligoclase, anorthite. andesine, labradorite, bytownite, and anorthoclase. Within some of these species are useful varieties. Oligoclase, for example can occur as sunstone. moonstone and peristerite. These three varieties depend on light phenomena for their special features. What distinguishes these mineral species from the potassium feldspars is their chemistry. While potassium is the main cation in the potassium feldspars, the plagioclase group is sometimes called the soda feldspars and the lime feldspars, since sodium or calcium, and sometimes both, is their main metallic ingredient. A third smaller group has barium as the main metallic element, but there are only a couple of unimportant barium feldspars. To add to the confusion, combinations of sodium/calcium, potassium/barium or potassium/sodium may act in concert. This is possible because those metal ions have similar valences and their atoms are close to the same size.