Microstructures

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Lattice Preferred Orientation (LPO)

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51. LPO and GSPO in quartz - In this example of deformed granite from the Pinaleño Mountains in Arizona, recrystallized quartz forms three distinct bands in a matrix of fine-grained relict feldspar, white mica, and epidote. Foliation is horizontal in the image. The obliquity of quartz grain long axes (GSPO) indicates a left-lateral shear. The tendency for many of the new quartz grains to be in extinction indicates that there is also a lattice preferred orientation (LPO) with many of the quartz C-axes pointing straight up the microscope barrel. The absence of any grain shape preferred alignment in the matrix is due to grain boundary sliding - the sliding of these very small grains along the boundaries that separate the different phases.

FOV 1.5 mm, Nicols Crossed.

52. LPO and GSPO in quartz - Another sample of deformed granite from the Pinaleño Mountains shows a well-developed GSPO and LPO in the quartz-rich zones. Foliation is horizontal and quartz grain long axis obliquity indicates right-lateral shear in the image. If there were a random alignment of lattice directions then there would be a random speckled pattern of red, blue and yellow grains with the gypsum plate inserted. It is clear that the majority of the grains in this specimen have their optic axis (the C-axis in the case of quartz) in close alignment. Measurement of this statistical distribution of quartz C-axes using a Universal Stage attachment to the microscope can give information about the shear sense and deformation conditions.

FOV 1.5 mm, Nicols Crossed + Gypsum Plate.

53. LPO in quartz - The quartz-rich zone across the center of this example of mylonitic pegmatite from the Roses granodiorite, southern edge of the Cap de Creus Peninsula, Spain, shows a well-developed GSPO and LPO. Deformation was at lower greenschist facies and the macroscopic foliation of the shear zone in the image runs from lower left to middle right. Although there are no other obvious shear sense indicators in this sample, GSPO in the quartz-rich band indicates right-lateral shear. A strong LPO is obvious, as the majority of the quartz grains show blue with the gypsum plate. Feldspar grains (large clasts at upper left and lower right of image) have deformed by brittle failure and show no preferred alignment of long axes or lattice orientations.

FOV 6.4 mm, Nicols Crossed + Gypsum Plate.

54. LPO and GSPO in quartz - Metaquartzite from the Witwatersrand Formation in the collar of the Vredefort Dome, near Johannesburg, South Africa, shows a single, well-developed foliation (aligned from upper left to lower right in image) and a strong LPO (most grains are blue or red with the gypsum plate inserted). Deformation was at upper amphibolite facies. The elongate quartz grains are parallel to the main foliation in the rock and their boundaries are decorated and 'pinned' from migration by tiny mica platelets (not visible in image).

FOV 3.2 mm, Nicols Crossed + Gypsum Plate.

Lattice Preferred Orientation (LPO)	Click image to enlarge
51. LPO and GSPO in quartz - In this example of deformed granite from the Pinaleño Mountains in Arizona, recrystallized quartz forms three distinct bands in a matrix of fine-grained relict feldspar, white mica, and epidote. Foliation is horizontal in the image. The obliquity of quartz grain long axes (GSPO) indicates a left-lateral shear. The tendency for many of the new quartz grains to be in extinction indicates that there is also a lattice preferred orientation (LPO) with many of the quartz C-axes pointing straight up the microscope barrel. The absence of any grain shape preferred alignment in the matrix is due to grain boundary sliding - the sliding of these very small grains along the boundaries that separate the different phases. FOV 1.5 mm, Nicols Crossed.
52. LPO and GSPO in quartz - Another sample of deformed granite from the Pinaleño Mountains shows a well-developed GSPO and LPO in the quartz-rich zones. Foliation is horizontal and quartz grain long axis obliquity indicates right-lateral shear in the image. If there were a random alignment of lattice directions then there would be a random speckled pattern of red, blue and yellow grains with the gypsum plate inserted. It is clear that the majority of the grains in this specimen have their optic axis (the C-axis in the case of quartz) in close alignment. Measurement of this statistical distribution of quartz C-axes using a Universal Stage attachment to the microscope can give information about the shear sense and deformation conditions. FOV 1.5 mm, Nicols Crossed + Gypsum Plate.
53. LPO in quartz - The quartz-rich zone across the center of this example of mylonitic pegmatite from the Roses granodiorite, southern edge of the Cap de Creus Peninsula, Spain, shows a well-developed GSPO and LPO. Deformation was at lower greenschist facies and the macroscopic foliation of the shear zone in the image runs from lower left to middle right. Although there are no other obvious shear sense indicators in this sample, GSPO in the quartz-rich band indicates right-lateral shear. A strong LPO is obvious, as the majority of the quartz grains show blue with the gypsum plate. Feldspar grains (large clasts at upper left and lower right of image) have deformed by brittle failure and show no preferred alignment of long axes or lattice orientations. FOV 6.4 mm, Nicols Crossed + Gypsum Plate.
54. LPO and GSPO in quartz - Metaquartzite from the Witwatersrand Formation in the collar of the Vredefort Dome, near Johannesburg, South Africa, shows a single, well-developed foliation (aligned from upper left to lower right in image) and a strong LPO (most grains are blue or red with the gypsum plate inserted). Deformation was at upper amphibolite facies. The elongate quartz grains are parallel to the main foliation in the rock and their boundaries are decorated and 'pinned' from migration by tiny mica platelets (not visible in image). FOV 3.2 mm, Nicols Crossed + Gypsum Plate.