Feldspar structure

Figure 5.53 Idealized feldspar structure (topologic symmetry). (A) Projection from axis a, showing conformation of four-member rings composed of two nonequivalent upward-directed TI-T2 tetrahedra and two nonequivalent TI-T2 downward-directed tetrahedra. (B) Double gooseneck chain seen along axis a. Periodicity along axis a is 4 (about 8.4 A).

So far, only normal diffraction (with sharp maxima at the points of reciprocal lattice) has been mentioned. Investigation of background scattering can provide extended information on various kinds of crystal defects (well beyond stacking faults and the like) as has been demonstrated e.g. on metal and feldspar structures. 

The occurrence of minerals which show CL is highly dependent on the type of meteorite. Possibly the most common phase which occurs is feldspar. Because this mineral accepts very little Fe into the structure, quenching is not a problem however, because the feldspar structure is quite open, the Na- and K-rich feldspars are easily damaged by electron beams. In contrast anorthite, the Ca rich variety, is quite stable. Pyroxene and olivine are common phases in meteorites but because they both usually contain iron, most do not luminesce. Only in the primitive meteorites do nearly pure enstatite and forsterite occur and both show brilliant CL. Other minerals are rare but include ... 

Celsian. One of the compounds important for ceramic materials science is monoclinic celsian, BaAljSijO, a compound with a feldspar structure. Flowever, its formation during solid phase synthesis occurs in several stages with the formation of a very stable hexagonal modification of BaAljSijO as an intermediate phase. Its subsequent transition into thermodynamically stable monoclinic celsian is hindered and requires calcination at elevated temperatures ( 1600°C) either the hydrothermal treatment, or the use of mineralizers [12-14]. 

Figure 8.28. Diagrams illustrating the relationship between the lattices of the monoclinic feldspar structure with c 1.4 nm (CT), body-centered anorthite (iT) and primitive anorthite (Pi).

A second chemical factor affecting mineral weatherability is the position of ions in the structure. The tetrahedra of Ca feldspars contain half Al3+ and half Si4+. At room temperature, Al3+ is more stable in octahedral coordination, The charge deficit created by the Al3+ substitution is made up by Ca2+ ions between the tetrahedra. The structural strain, the charge deficit in the tetrahedra, and concentrated Ca2+ counter charge weaken the anorthite feldspar structure with respect to weathering relative to Na and K feldspars. In Na and K feldspars, only one-quarter of the tetrahedral positions are occupied by A1 and that charge deficit can be locally neutralized by Na+ or K+. Calcium feldspars are, therefore, the least stable feldspars under soil conditions. Potassium feldspars are more stable than Na feldspars, because K fits better between adjacent tetrahedra. 

Fic. 96. Polyhedral packing diagram of [Cu(Lios)2lBF1 (a) and schematic view of the feldspar structure viewed down the a-axis (b). (From Fig. 2 in Keller, S. W. Angew. Chem., Int. Ed. Engl. 1997,36, 247.)... 

The celsian structure (Fig. 3b) [17] is similar to the feldspar structure in which all four vertices of the silica tetrahedral are shared, forming a three-dimensional network. As in hexacelsian, A1 substitutes for Si with charge compensation by Ba in the larger interstices of the structure. Gay [18] and Newnham and Megaw [17] considered the formation of a superlattice in celsian associated with ordering of the Al-Si atoms. 

De Vore [1959] assumes transformation of feldspar into mica by a solid-state reaction consisting of an alteration of chemical bonding at the surface of the feldspar structure leading to the formation of metastable ionic groups by partial dissolution and to subsequent polymer-... 

A recent review of feldspar structure by W. H. Taylor has appeared as Chapter 14 of llte Crystalline State. Vol. IV. Crystal Structure.of Minerals, edited by Sir Lawrence Bragg. London G. Bell and Sons [1965]. 

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