Microstructures

First Previous Section 17 Next Last

Myrmekite in Deformation Zones

Click image to enlarge

55. Myrmekitic intergrowths on K-feldspar - The distinctive, cauliflower-texture of myrmekitic replacement reaction is evident on the margins of a K-feldspar grain (black, lower left of image) from deformed adamellite of the alpine Maggia Nappe, Ticino, Switzerland. Deformation was at epidote-amphibolite facies. Recrystallized quartz grains fill the upper right part of the image. The reaction of K-feldspar to plagioclase and quartz results in a volume decrease and is favored on high normal stress sites around the grains.

FOV 3.2 mm, Nicols Crossed.

56. Recrystallized myrmekitic intergrowths on microcline - Relict myrmekitic quartz/oligoclase intergrowths are present along the foliation-parallel edge of this large microcline crystal (upper half of image) from the Ponaganset augen gneiss in the Hope Valley shear zone of eastern Connecticut. This microcline grain is over 5 cm long and has well-developed tails of recrystallized K-feldspar (not illustrated) parallel to the weak foliation and strong mineral lineation, which are aligned horizontal to the image. Deformation occurred at temperatures of 440°-445°C. Quartz relict vermicules (small, bright blue grains, all the same lattice orientation) tend to be subrounded near to the microcline host crystal but are more polygonal in outline with distance from it, toward the bottom of the image. The oligoclase forms polygonal, straight-sided grains of random orientation, also indicating significant post-formation recovery. Compare the morphology of these myrmekitic intergrowths with those of image # 55, which shows the more classic bulbous vermicular texture.

FOV 1.5 mm, Nicols Crossed + Gypsum Plate.

57. Myrmekitic intergrowths on K-feldspar - Myrmekitic intergrowths here occur on the margins of a K-feldspar grain between S-planes in an S-C mylonite from the Santa Rosa Mylonite zone in the Eastern Peninsular Ranges of southern California. S planes are oriented from top left to lower right, and C-planes are not visible in this image. Deformation was in the temperature range of 450°-550°C. The K-feldspar grain in the center of image is bordered by biotite and quartz ribbons with some muscovite and recrystallized feldspar grains. Myrmekitic intergrowths of oligoclase and quartz are only observed on the two sides of the K-feldspar grain that parallel the S-planes, which face the shortening direction. The reaction of K-feldspar to plagioclase and quartz is volume-reducing and is favored on high normal stress sites around the grains. Myrmekites do not occur on the two ends of the K-feldspar that parallel the extension direction, where there are recrystallized new, equant, K-feldspar grains.

FOV 1.5 mm, Nicols Crossed.

58. Myrmekitic intergrowths on K-feldspar - This example of myrmekite development occurs in a mylonitic granite from the Catalina metamorphic core complex of Arizona. Foliation is oriented top left to bottom right. The K-feldspar porphyroclast sits in a matrix of fine-grained white mica, epidote, quartz and iron oxides. Reaction to myrmekite is apparent along the uppermost margin of the K-feldspar.

FOV 1.5 mm, Nicols Crossed + Gypsum Plate.

Click here to view Flash animation in a new window.

Myrmekite in Deformation Zones	Click image to enlarge
55. Myrmekitic intergrowths on K-feldspar - The distinctive, cauliflower-texture of myrmekitic replacement reaction is evident on the margins of a K-feldspar grain (black, lower left of image) from deformed adamellite of the alpine Maggia Nappe, Ticino, Switzerland. Deformation was at epidote-amphibolite facies. Recrystallized quartz grains fill the upper right part of the image. The reaction of K-feldspar to plagioclase and quartz results in a volume decrease and is favored on high normal stress sites around the grains. FOV 3.2 mm, Nicols Crossed.
56. Recrystallized myrmekitic intergrowths on microcline - Relict myrmekitic quartz/oligoclase intergrowths are present along the foliation-parallel edge of this large microcline crystal (upper half of image) from the Ponaganset augen gneiss in the Hope Valley shear zone of eastern Connecticut. This microcline grain is over 5 cm long and has well-developed tails of recrystallized K-feldspar (not illustrated) parallel to the weak foliation and strong mineral lineation, which are aligned horizontal to the image. Deformation occurred at temperatures of 440°-445°C. Quartz relict vermicules (small, bright blue grains, all the same lattice orientation) tend to be subrounded near to the microcline host crystal but are more polygonal in outline with distance from it, toward the bottom of the image. The oligoclase forms polygonal, straight-sided grains of random orientation, also indicating significant post-formation recovery. Compare the morphology of these myrmekitic intergrowths with those of image # 55, which shows the more classic bulbous vermicular texture. FOV 1.5 mm, Nicols Crossed + Gypsum Plate.
57. Myrmekitic intergrowths on K-feldspar - Myrmekitic intergrowths here occur on the margins of a K-feldspar grain between S-planes in an S-C mylonite from the Santa Rosa Mylonite zone in the Eastern Peninsular Ranges of southern California. S planes are oriented from top left to lower right, and C-planes are not visible in this image. Deformation was in the temperature range of 450°-550°C. The K-feldspar grain in the center of image is bordered by biotite and quartz ribbons with some muscovite and recrystallized feldspar grains. Myrmekitic intergrowths of oligoclase and quartz are only observed on the two sides of the K-feldspar grain that parallel the S-planes, which face the shortening direction. The reaction of K-feldspar to plagioclase and quartz is volume-reducing and is favored on high normal stress sites around the grains. Myrmekites do not occur on the two ends of the K-feldspar that parallel the extension direction, where there are recrystallized new, equant, K-feldspar grains. FOV 1.5 mm, Nicols Crossed.
58. Myrmekitic intergrowths on K-feldspar - This example of myrmekite development occurs in a mylonitic granite from the Catalina metamorphic core complex of Arizona. Foliation is oriented top left to bottom right. The K-feldspar porphyroclast sits in a matrix of fine-grained white mica, epidote, quartz and iron oxides. Reaction to myrmekite is apparent along the uppermost margin of the K-feldspar. FOV 1.5 mm, Nicols Crossed + Gypsum Plate. Click here to view Flash animation in a new window.