Microstructures

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Undeformed Rock Microstructure

Click image to enlarge

1. Undeformed feldspar and quartz grains – Undeformed granite from the Lucerne Pluton in coastal Maine shows euhedral crystals of plagioclase (blue, with two sets of albite twins), anhedral K-feldspar (yellow, with minor alteration showing as darker patches along cleavage planes), and anhedral quartz (dark red, in extinction, at base of image). With a high resolution monitor you can see that where the central plagioclase grain abuts the K-feldspar grain it has a fringe of symplectite intergrowths. These intergrowths are of quartz and K-feldspar, with the latter in close to optical continuity with the plagioclase crystal.

FOV 1.5 mm, Nicols Crossed + Gypsum Plate.

2. Annealed quartz grains – Metaquartzite from the Witwatersrand Formation in the collar of the Vredefort Dome, near Johannesburg, South Africa, shows a well-developed foam texture as a consequence of annealing recrystallization (recrystallization in the absence of a directed stress) above 550°C. The majority of the now polygonal quartz grains are undeformed, show flat-field extinction and have 120° triple point grain boundaries. There is a wide range of grain size and original sedimentary grain boundaries have been destroyed. Grain boundaries are not absolutely planar, indicating that there has been minor late-stage grain boundary migration.

FOV 0.8 mm, Nicols Crossed.

3. Undeformed hornblende and plagioclase – Undeformed diorite from a small lens within the Borrego Springs section of the Eastern Peninsular Ranges Mylonite Zone, southern California, shows euhedral, compositionally-zoned amphibole (green, mainly hornblende with pargasitic overgrowths) and plagioclase crystals.

FOV 1.5 mm, Plane Light.

Undeformed Rock Microstructure	Click image to enlarge
1. Undeformed feldspar and quartz grains – Undeformed granite from the Lucerne Pluton in coastal Maine shows euhedral crystals of plagioclase (blue, with two sets of albite twins), anhedral K-feldspar (yellow, with minor alteration showing as darker patches along cleavage planes), and anhedral quartz (dark red, in extinction, at base of image). With a high resolution monitor you can see that where the central plagioclase grain abuts the K-feldspar grain it has a fringe of symplectite intergrowths. These intergrowths are of quartz and K-feldspar, with the latter in close to optical continuity with the plagioclase crystal. FOV 1.5 mm, Nicols Crossed + Gypsum Plate.
2. Annealed quartz grains – Metaquartzite from the Witwatersrand Formation in the collar of the Vredefort Dome, near Johannesburg, South Africa, shows a well-developed foam texture as a consequence of annealing recrystallization (recrystallization in the absence of a directed stress) above 550°C. The majority of the now polygonal quartz grains are undeformed, show flat-field extinction and have 120° triple point grain boundaries. There is a wide range of grain size and original sedimentary grain boundaries have been destroyed. Grain boundaries are not absolutely planar, indicating that there has been minor late-stage grain boundary migration. FOV 0.8 mm, Nicols Crossed.
3. Undeformed hornblende and plagioclase – Undeformed diorite from a small lens within the Borrego Springs section of the Eastern Peninsular Ranges Mylonite Zone, southern California, shows euhedral, compositionally-zoned amphibole (green, mainly hornblende with pargasitic overgrowths) and plagioclase crystals. FOV 1.5 mm, Plane Light.