Li and F-bearing alkali amphibole from granitic pegmatite at Hurricane Mountain, Carroll County, New Hampshire

U. S. Geological Survey, Denver Federal Center, Lakewood, CO, United States

At Hurricane Mountain, Carroll County, New Hampshire, bodies of granitic pegmatite in riebeckite granite contain large (up to 10 cm long and 2 cm across) primary crystals of Li-bearing fluor-arfvedsonite in miarolitic cavities, grading to euhedral Li- and F-poor arfvedsonite. Fine-grained, fibrous, light blue-gray riebeckite occurs as a late-stage hydrothermal filling in the miarolitic cavities. The early, Li-rich, fluor-arfvedsonite has: a 9.836(5), b 17.997(7), c 5.316(4) Aa, beta 103.735(4) degrees , V 914.20(6) Aa ³ ; Z = 2, D (sub meas.) 3.34 g/cm ³ , D (sub calc.) 3.353 g/cm ³ ; biaxial (-), 2V (sub meas.) 44(1) degrees , 2V (sub calc.) 46 degrees ; alpha 1.681(2), beta 1.692(2), gamma 1.694(2), inclined dispersion, r>v; X / c-7 degrees , Y = b, Z / a+7 degrees ; X dark blue, Y lavender gray, Z pale yellowish brown; X>Y>Z; X is opaque at 0.03 mm thickness. A structural formula, on the basis of 24 (O, OH, F) atoms is: (Na (sub 0.86) K (sub 0.25) )Na ₂ (Fe (super 2+) (sub 2.54) Fe (super 3+) (sub 1.485) Mn (sub 0.10) Zn (sub 0.02) Li (sub 0.49) Ti (sub 0.07) ) (Si (sub 7.71) Al (sub 0.07) )O ₂₂ (F (sub 1.34) OH (sub 0.63) ). Arfvedsonite within the miarolitic cavities contains less Li and F than that of the earlier generation, and the still later riebeckite contains only 0.09 wt.% Li ₂ O and 0.3 wt.% F. The Fe (super 3+) :Fe (super 2+) ratio of the early Li-bearing fluor-arfvedsonite and that of the euhedral arfvedsonite crystals within miarolitic cavities is 0.58. The late, fibrous, cavity-filling riebeckite has an Fe (super 3+) :Fe (super 2+) ratio of 0.99. The total iron content of the three amphiboles increases with continued crystallization. These amphiboles are products of peralkaline pegmatites locally derived from peralkaline granite.

This record provided courtesy of AGI/GeoRef.

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