Framework Silicates Framework Silicates
• Feldspars
• Feldspathoids
• Silica polymorphs
Feldspars Feldspars
• Simple chemistry
• Substitution of Na for K or NaAl for CaAl
• Crystal systems
– Monoclinic (orthoclase, sanidine)
– Triclinic (plagioclase, microcline)
• Tabular habit
Feldspar Twinning Feldspar Twinning
• Simple twins (two parts)
– Carlsbad
– Common in monoclinic feldspars
• Polysynthetic twins
– Albite and others
– Common in triclinic feldspars
K-spars spars
• Orthoclase, Sanidine, Microcline
• All optically negative
• All have n ~ 1.53
• Distinguished by 2V
– Sanidine 2V = 0-30?
– Orthoclase 2V = 30-70?
– Microcline 2V = 70-90?
• Microcline has Scotch plaid twins