504 MINERALS/Amphiboles i Monovalent ion (Na or K) for vacancy (u) at A ii Monovalent (Na,K) for divalent Ca at B (M4 sites) iii Trivalent (Al) for tetravalent (Si) at T sites iv Trivalent (Al,Fe3ỵ) for divalent (Mg, Fe2ỵ, etc) at C (M1, M2, M3 sites) To maintain charge balance, it is necessary for more than one of these substitutions to take place, e.g., in pargasite, NaCa2 Mg; Fe2ỵ ị4 AlẵSi6 Al2 O22 OH; Fị2 , where the Na ion is in the A position (type substitution), Al is in the M sites (type substitution) and the excess charge is balanced by the substitution of 2Al in the T site (type substitution), the maximum substitution allowed for Si At the same time, there may be complete substitution between Mg and Fe2ỵ to give ferro-pargasite A complete range of possible amphibole substitutions was published by Leake and others, and also Deer and others in 1997 There are, thus, four main groups with boundaries as follows: i Iron-magnesium-manganese-lithium amphiboles: (Ca ỵ Na)B < 1.00 ii Calcic amphiboles: (Ca ỵ Na)B ! 1.00 and NaB < 0.50 iii Sodic-calcic amphiboles: (Ca ỵ Na)B ! and 0.50 NaB 1.50 iv Sodic amphiboles: (Na)B ! 1.50 Figure Double chain of linked (Si,Al)O4 tetrahedra character istic of all amphiboles Because of excellent argon retention properties and the common incorporation of potassium in their structures, the amphiboles are particularly useful for K–Ar dating Hornblende K–Ar ages have been used to date a wide variety of metamorphic and igneous rocks Leaving aside the lithium-bearing holmquistite, the two orthorhombic amphiboles are anthophyllite, Mg; Fe2ỵ ị7 ẵSi8 O22 OH; Fị2 , and gedrite Mg; Fe2ỵ Þ Figure The crystal structure of an amphibole as viewed along z Pairs of chains are linked by cations M 1, M 2, M and M4, and in some cases A