f019253d_The_Delaware_Geological_Survey_inerals_of_the_Red_Clay_Valley_Description
[Rock Type] Feldspar [Description] 
In weathered rocks or granitic pegmatites, feldspars occur as milky white or pink porcelain-like minerals that often break into rectangular shapes with shiny flat surfaces. In fresh, unweathered amphilbolites or gneisses, the feldspars are glassy and transparent. In the 18th and 19th centuries, feldspar was quarried in the Red Clay Valley for use in porcelain, china, and glazes. Orthoclase and plagioclase are two types of feldspar found in the Delaware Piedmont.
Common Rocks and Minerals of the Delaware Piedmont | The Delaware Geological Survey
Description