12.109 Lecture Notes September 13, 2005 Rock Forming Minerals II Structure and composition of: FELDSPARS Feldspars The “meat and potatoes” of crustal igneous rocks (most abundant igneous mineral in the crust) Feldspar from the German “feldspat,” crystals found in the field Alkali feldspars are any mixture of Albite, NaAlSi3O8, and Kspar, KAlSi3O8 Kspar has three polymorphs: Sanidine high T Orthoclase Microcline low T monoclinic monoclinic triclinic C2/mC2/mC 1 Structure Based on 4 member ring/square consisting of SiO4 and AlO4 tetrahedra Link the squares in a “double crank shaft” Symmetry of these ring structures makes the perfect cleavages, 88º and 92ºTectosilicate (framework silicate) – silicate where all Si tetrahedra are linked together at apices Ring often notated “T4O8”, tetrahedral anion, alkali cations in holes balance charge Calcium feldspar = Anorthite Plagioclase Plagioclase twinning Polysynthetic (repeated) Simple (two crystals related by a twin operation) Review of mineralogy: Twin operation – symmetry operation not part of symmetry group of crystal Symmetry operations elements Reflection mirror plane Rotation axis Twin happens when a symmetry element not part of the crystal structure operates on two halvesSanidine & Orthoclase monoclinic, Microcline triclinic variation in atomic arrangement of feldspars due to changes in temperature we can measure this ordering using optics or xrays Æ give you T of equilibrium Al/Si distribution Kspar KAlSi3O8 in crystal structure are 2 crystallographically distinct tetrahedral sites, T1 and T2 At higher T, Al and Si are randomly distributed over T1 and T2 So average site occupancy is .25 Al, .75 Si As T goes down, T1 prefers Al Si – O bond 1.61 Å Al – O bond 1.73 Å So unit cell length along c axis changes with motion of Al atoms Anorthite has 2x the amount of Al, different Al/Si arrangement Æ anorthite always triclinic, unit cell of symmetry 2x length of alkali feldspars Plagioclase series Albite—Anorthite Polysynthetic twinning can be used to determine An content to ~2-4 mole% From extinction angle, see
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