Iridescent anthophyllite-gedrite from Simiuttat, Nuuk district, southern West Greenland" composition, exsolution, age

• Main Authors: K. A. Rodgers, P. D. Kinny, V. R. Mcgrfgor, G. R. Clark, G. S. Henderson
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Akulleq; Greenland; Nuuk; Simiuttat
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.628.2582; http://www.minersoc.org/pages/Archive-MM/Volume_60/60-403-937.pdf

Golden iridescent, <1-100 mm crystals of alternating lamellae of anthophyllite and gedrite constitute the bulk of orthoamphibolite pods within quartz-cordierite gneisses of the Akulleq terrane at Simiuttat, SW Greenland. X-ray powder diffraction powder gave a = 18.526(7), b = 17.979(15), c = 5.285(23) A; a single crystal has a = 18.546(7), b = 17.950(16), c = 5.280(1) A, space grou ~ P. with some reflections being notably broader than others. Spot EMPA yielded composite analyses: A1 = 0.89-1.3, Mg/(Mg+Fe 2+) = 0.57-0.61, Na/Al wv = 0.22-0.26. AFM imaging of {210} cleavage surfaces, showed a uniform corrugated morphology parallel to [001]; wavelength was 190-350 nm, mean 250 nm, amplitude 3 nm. A plan view resembles TEM images of (010)-parallel exsolution textures of orthoamphiboles. A second set of corrugations may crosscut he [001]-parallel ridges at 20-25 ~ akin to reported lamellar intergrowths developed parallel to both (010) and (120). Unequivocal evidence linking topography with lamellae is absent. In contrast o the conventional multi-layer reflector model, the ridged surface pJrovides an additional origin for iridescence, acting as a diffraction grating. Included zircons, 50-10 ~m, have-ttf/Zr = 0.008-0.012, ttI-+Fe n c. 0.16.2~176 ages are from 2690 to