Triclinic feldspar

Polymorphic transitions render the thermodynamic description of feldspar end-members rather complex. According to Helgeson et al. (1978), the transition between the monoclinic and triclinic forms of NaAlSi308 may be regarded as the overlap of two high-order transitions (cf section 2.8), one associated with displacive structural modifications and the other with ordering on tetrahedral sites. The analysis of calorimetric data by Holm and Kleppa (1968) concerning the enthalpy... 

Merkel G. A. and Blencoe J. G. (1982). Thermodynamic procedures for treating the mono-clinic/triclinic inversions as a high-order phase transition in equations of state for binary analbite-sanidine feldspars. In Advances in Physical Geochemistry, vol. 2, S. K. Saxena (series ed.). New York Springer-Verlag. 

Thompson J. B. Jr. and Hovis G. L. (1978). Triclinic feldspars Angular relations and the representation of feldspar series. Amer. Mineral, 63 981-990. 

Whenever a crystal transforms (usually on cooling) to a lower symmetry structure in which a number of orientational variants are possible, transformation twins are an almost invariable consequence if the new phase is nucleated at several different places in the original structure. The formation of Dauphine twins at the jS-to-a transformation in quartz is a classic example, which has already been mentioned briefly in Section 8.6. We now consider the twins and modulated structures associated with the monoclinic-to-triclinic transformation in the K feldspars, KAlSisOg. 

More complex exsolution microstructures are observed in cryptoper-thites of similar composition when the K-feldspar lamellae become triclinic (see, e.g.. Brown and Parsons 1984 Champness and Lorimer 1976 Willaime, Brown, and Gandais 1976). 

The pure members of the feldspar group are (refer to Table 7) sodium feldspar (albite, triclinic) — Na20. AI2O3. 6 SiO ... 

The symmetry-breaking strains in the monoclinic-to-triclinic transition for feldspars... 

Plagioclase A variety of feldspar a group of triclinic silicates with the general formula (Na, Ca)Al(Al, Si)Si20g. 

Albite (sodium feldspar) NaAlSijOg Triclinic. Plates and rods. A lamellar, leaf-like variety of albite is called cleavelandite A colorless or milky-white mineral... 

Microcline (potassium feldspar) KAlSijOg (Figure M67) Triclinic. Oblique blunted crystals White to gray, brick red. If green, it is called amazonite or amazonstone... 

Anorthite (calcium feldspar) CaAljSijOg Triclinic. Parallelepipedic prisms Grayish, greenish, or reddish white... 

Thompson (1969). Margules theory is applied in principle to homogeneous crystalline solutions, with the alkali feldspars, Fe(ll)-Mg orthopyroxenes, Na-K-Ca-Fe-Mg amphiboles and Ca-Mg carbonates used as examples. For an expansion of this theory and an application to Al-Si ordering as a function of temperature in monoclinic and triclinic feldspars, see Hovis (19T ) and Thompson, Waldbaum and Hovis (19T ). 

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