Which one of the following is sodium and calcium-rich feldspar with twinning striations? Sanidine Microcline Orthoclase Plagioclase

The correct answer is Plagioclase.<br /><br />Plagioclase is a group of feldspar minerals that are rich in sodium (Na) and calcium (Ca). They are characterized by the presence of twinning striations, which are fine parallel lines or bands that can be seen under a microscope. These striations are caused by the alignment of the twinned crystal structures within the mineral.<br /><br />Sanidine, microcline, and orthoclase are other types of feldspar minerals, but they do not have the same level of sodium and calcium content or the characteristic twinning striations as plagioclase.