f019253d_The_Delaware_Geological_Survey_inerals_of_the_Red_Clay_Valley_Description
[Rock Type] Mica [Description] 
A mineral with perfect basal cleavage that easily separates into sheets. The varieties are black biotite, white muscovite, bronze phlogopite, and green chlorite. Micas are common in all Piedmont rocks except the high-grade gneisses of the Wilmington Complex.
Common Rocks and Minerals of the Delaware Piedmont | The Delaware Geological Survey
Description