2 Isomorphous Substitution Substitution, during formation, of one ion for another of similar SIZE (but not necessarily the same charge) in an ionic solid without changing the structure (shape, morphology) of the crystal.Isomorphic = “same shape”
5 Oreo cookies serve as models for layer minerals (phyllosilicates) One Oreo can be a mica or smectite-type mineralFirst proposed by Jerry Irvin at UCR
6 Two Oreo cookies can be split and reconstructed to form a 1:1 mineral KaoliniteTwo Oreo cookies can be split and reconstructed to form a 1:1 mineral
7 Pyrophyllite (no sub’n) [Si8]IV[Al4]VIO20(OH)4 (8 x Si4+) + (4 x Al3+) = 44+(20 x O2-) +(4 x OH1-) = 44-= 0Net charge = 0Muscovite (Tet’l sub’n)K2[(Si6, Al2)]IV[Al4]VIO20(OH)4(6 x Si4+) + (6 x Al3+) = 42+(20 x O2-) +(4 x OH1-) = 44-= Net charge = -2Satisfied by 2 x K1+ ions that are “fixed” or tightly held in interlayer
12 High-charge 2:1 mineral properties Source of K+ in soils as they weatherNon-expansive, non-sticky, non-plasticS.A. = m2/g (mostly external)CEC = cmol/kgc-spacing = 1.0 nmInterlayer held tightly together by K+ fit in ditrigonal cavities of tetrahedral sheet
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