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AmericanMineralogist Volume70 pages1205 1210 1985 Chemicalvariationsin sodiumgeddtefrom Labrador JoNarnnu H Bnnc Departmentof Geology N orthern Illinois U niuersity DeKalb Illinois 60115 Abstract Sodium gedriteoccursin a xenolith of ferro aluminousgneissenclosedby granitic rocks of the Nain complex Labrador The sodium gedrite is zoned but the most Na rich compositionsapproximatethe formula NarRl Rf R Si6AI2O22 OH F 2 Theseare amongthe more Fe rich gedritesever reported X 0 344 52 and they are far more fluorine rich than any orthoamphibole ever reported Xp O 2434 493 The gedrites have a range of compositionsfrom Na A 0 48 A IV 1 44 to Na A 0 95 A IV 2 10 The most common compositionis Na A 0 80 and A IV 2 00 The magnitudeof the edenite coupled substitution relative to the tschermakite coupled substitution is much greater in thesegedritesthan in most other gedrites The factors that control the A A IY ratios or edenite tschermakiteratios in gedrites appear to be crystal chemical crystallographic in origin High X favors the tschermakiteexchangeat the expenseof the edeniteexchange perhapsby causinga slight distortion of the A site High X favors the edeniteexchangeat the expenseof the tschermakiteexchange perhaps becausethe hydrogen in the hydroxyl group slightly repelsNa from enteringthe A site or becauseF forcesFe2 into the M2 site Introduction Ever sincethe earlywork by Rabbitt 1948 and Sekiand Yamasaki 1957 orthoamphibolecompositionalvariation and the nature of the anthophyllite gedritesolid solution series have received considerablestudy Lal and Moorhouse 1969 Robinsonand Jaffe 1969 Robinsonet al 1971 1982 Papikeand Ross 1970 Stout 1971 1972 Kamineni 1975 Jameset al 1978 Spear 1980 Otten 1984 After reviewing their data as well as the data availablein the literature Robinsonet al 1971 suggestedthat endmember gedrite possesseda half filled A site principally Na and two Al atoms in tetrahedralcoordination yielding theformula Nao rRl Rl rR s si6Alrorr oH This

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