Plagioclase Feldspar Thin Section

Plagioclase is a continuous series of solid solutions known as the plagioclase feldspar series rather than a specific mineral with a particular chemical composition.

Plagioclase feldspar thin section. Anorthite ca 90 100 na 10 0 from greek for not straight because of its triclinic symmetry. Rather than referring to a particular mineral with a specific chemical composition plagioclase is a continuous solid solution series more properly known as the plagioclase feldspar series. Small 250x250 max.

The series ranges from albite to anorthite endmembers where sodium and calcium atoms can substitute for each other in the mineral s crystal. The presense of parallel striae on cleavage surfaces distinguishes plagioclase from k feldspar. In thin section or grain mount the general lack of pleochroism and the twinning are good indicators.

Can be confused with quartz if there is no twinning however the optic axis figure will resolve this. This was first shown by the german mineralogist johann friedrich christian hessel in 1826. Add tags comment rate.

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Plagioclase is series of framework silicate minerals in feldspar group. Plagioclase series of minerals in the feldspar group from greek for oblique and fracture in reference to the oblique angle between its best cleavages. The series ranges from albite to anorthite endmembers with respective compositions naalsi3o8 to caal2si2o8 where sodium and calcium atoms can substitute for each other in the mineral s crystal lattice structure.

Often a grey color in comparison to other feldspars. In thin section plagioclase commonly shows the characteristic albite polysynthetic twinning.