Feldspar

The name is derived from the German term Feldspat. For centuries, miners generally referred to minerals and rocks that exhibited an easy (or complete) cleavage as “spat.” This perfect cleavage was often evidenced by visible cleavage traces and the fact that small flakes (“spatten”) would detach from the mineral or rock body upon impact with a hammer. The term feldspar emerged around the mid-18th century, as mineralogists began to distinguish between various mineral types with greater precision.

Various theories exist regarding the precise etymological origin of the name. For instance, in 1783, Urban Breckman questioned whether “feldspar” might be named after its specific mode of occurrence—appearing as “fields” (Felder) or patches within granite and other rock types, rather than as distinct veins, massive bodies, or complete outcrops. Conversely, René-Just Haüy, in his 1804 work Traité de minéralogie, suggested the name might have been chosen because feldspar fragments were systematically found in “fields.” Thus, feldspar is understood as “fragments of fields,” serving as an indication that feldspar gradually weathers into arable soil.

Petrological Classification

In petrology, this group is divided into two primary subgroups:

• Alkali Feldspars: These are calcium-poor feldspars, predominantly enriched in potassium. At high temperatures, the alkali feldspar series ranges from sanidine to high albite; however, this series is incomplete at lower temperatures. Some researchers include barium feldspars (celestine and hyalophane) and potentially the rubidium-bearing feldspar, rubicline, within the alkali feldspar group.


• Plagioclase: The series ranging from albite to anorthite.

Feldspars may contain trace amounts of hydroxyl ($\text{OH}^-$) groups, which are considered structural defects (e.g., Young et al., 2015).