Thus miscibility gap (solvus) is asymmetric. Find that K-feldspar is more tolerant of Na than v.v. Range of stable solid solution as a function of T given by diagram. On cooling high-T albite _ Na 'slows down' - framework collapses around the 'smaller' cation: ALBITE BECOMES TRICLINIC Na motion at high T makes the Na equivalent in size to K.What does the presence of twinning in a triclinic K-feldspar imply?Įxsolution example: Albite (twinned) in Kspar host.ĪLBITE STRUCTURAL CHANGES AS A FUNCTION OF TEMPERATUREĪt high temperatures: Albite is monoclinic: Symmetry change and twinning!: Cross-hatch twinning in microcline Varying degrees of Al-Si order - varying 'STRUCTURAL STATES' MICROCLINE: Al selectively on T1 sites -> TRICLINIC! (ordering destroys 2-fold and mirror.ORTHOCLASE: probability of Al on T1 not equal T2, but T1o=T1m and T2o=T2m.SANIDINE: equal probability of Al on all T sites-> MONOCLINIC.In monoclinic feldspars (space group C2/m), T1o = T1m and T2o = T2m because, while Al and Si are disordered, there is an equal probability of finding Al on T1o and T1m and an equal probability of finding an Al on T2o and T2m. In detail, T1 = T1o and T1m (related by the mirror) and T2o and T2m (also related by the mirror). where would triclinic K-feldspar (e.g., microcline) be found?Īl - Si ordering: Two distinct types of tetrahedral sites in monoclinic feldspar - known as T1 and T2.THUS: where would monoclinic K-feldspar (e.g., sanidine) be found?.(dG/dT)p = -S, where G = free energy, T= temperature, S = entropy Rearrangement requires breaking strong Al-O and Si-O bonds, so that it is possible to quench in disorder. Shape of the miscibility gap (can get more Na in Kspar than K in albite)Īl - Si order and disorder:(.thermodynamics.)ĭisorder increases with increasing temperature - applied to feldspars, find most Al-Si disorder at high temperature. While the same charge, the size of K and Na differ by ~ 30 %! Separates Na-K from Na-Ca feldspars = f(T). THUS: see LIMITED Ab-An-Kspar solid solution (sometimes termed Anorthoclase or ternary feldspar) NOT: K+ - Ca++ (different size and charge) In general, a mineral structure will accept a mixture of cations if the cations have the same charge OR are about the same size. Range of feldspar compositions found in nature determined by the sizes and charges on Na, Ca, K. Stable compositional range as a function of P and T defined by nature and experiments: triangular diagram. Other cations substituting into feldspar = Ba, Sr, Eu (some Fe, etc).Ĭcompositions are simple, basic structures are relatively simple, structures of minerals with compositions intermediate between Alb and An, and Alb and Kspar are KAlSi3O8 = K-feldspar (sanidine, orthoclase, microcline) Derivation of minerals from the silica frameworks by substitution of elements for Si.

Ternary diagram feldspar