Petrographic Microscopy Questions and Answers

Answer: D

Brown cathodoluminescence of quartz suggests a regionally metamorphosed rock source.

Cathodoluminescence colour reflects the temperature of crystallisation and the trace elements and defects in quartz. In the provenance scheme widely used in sandstone petrography, violet to blue luminescing quartz comes from plutonic and volcanic rocks that crystallised at high temperature, brown luminescence is typical of quartz from regionally metamorphosed (especially low-grade) rocks, and authigenic quartz does not luminesce.

So plutonic and volcanic quartz are linked with blue-violet colours rather than brown, and pegmatitic quartz is not a separate brown-luminescing class.

Answer: A

The microstructure is myrmekite.

Myrmekite is a worm-like (vermicular) intergrowth of quartz in sodic plagioclase, typically growing as bulbs into K-feldspar along its margins. It forms by replacement of K-feldspar, and it is especially common in deformed granitic rocks, where strain at the feldspar boundaries promotes it.

Migmatite is a mixed rock of partly melted gneiss, not a microstructure. Metapelitic describes metamorphosed shales, and idioblastic describes metamorphic crystals bounded by their own faces, neither of which is an intergrowth of quartz and plagioclase.