Moonstone
From Rabb Park, West slope of the Black Range
September 2025
Moonstone is a sodium potassium aluminum silicate ((Na,K)AlSi3O8) in the feldspar group. A mineral may have several different forms. In the case of moonstone it may be composed of sodium or potassium (Na,K). Minerals are arrayed in a taxonomic tree something like the Linnaean system used in biology. Moonstone is a tectosilicate mineral in the feldspar group, alkali series, orthoclase variety.
Tectosilicate minerals include the quartz group (the silicas like chalcedony, agate, amethyst, “normal quartz”, etc.), the feldspar group (which includes things like plagioclase and orthoclase), the feldspathoid group, the scapolite group, and the zeolite group. Many of these groups are further divided into subgroups. Moonstone is an orthoclase in the feldspar group.
Moonstone is known for its adular-escence (or schiller/shiller). This is the bluish cast, best seen in the bottom photograph, which appears to emanate from inside of the rock. This is an optical effect, it is a product of the crystal structure of the mineral, not of any particular type of inclusion.
