Polymorphism feldspars

However, it must be emphasized that interpretation of elemental diffusion in feldspars is complicated by the structural state of the polymorphs, which vary in a complex fashion with temperature, chemistry and re-equilibration kinetics. These complexities also account for the controversies existing in the literature regarding diffusion energy in these phases (see also, incidentally, figure 4.8). Elemental dilfusivity data for rock-forming silicates are listed in table 4.8. 

Table 5.63 lists the structural characters of the various feldspar polymorphs, according to Smyth and Bish (1988). Note duplication of cell edge c in anorthite and celsian and the progressive increase in volume that accompanies the lowering of symmetry in KAlSi30g. 

Table 5.63 Structural data of main feldspar polymorphs (from Smyth and Bish, 1988). ...

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... 

Figure 1. Representation of the ideal compositions of some major phyllosilicate phases in the MR - 2R - 3R coordinates. M = muscovite, paragonite P - phlogopite Py = pyrophyllite Kaol = kaolinite S serpentine T = talc Chlor = chlorite, 14 8 or aluminous 7 8 polymorphs Ce = celadonite F = feldspar.

The listed chemical formulae are ideal and most of these minerals contain trace and minor elements which undoubtedly affect the CL. Several of these minerals have polymorphic or compositional varieties which also may, or do, show CL (e.g. the silica polymorphs quartz, cristobalite, tridymite phosphate compositional varieties apatite, whitlockite, farringtonite, buchwaldite carbonate compositional varieties calcite, dolomite, magnesite). Glass and maskelynite (shock modified feldspar), although not strictly minerals, are relatively common. Below are described the CL observations for the most common phases including enstatite, feldspar and forsterite and they are related to their use for interpreting the mineralogy of meteorites. The observations for the other minerals are sporadic and many details have yet to be studied. 

Among the properties of feldspars, mention should be made in particular of their behaviour around the melting point. The polymorphic transformations are of less significance because alkali feldspars usually turn into melt and into glass after cooling. 

Smith JV, Brown WL (1988) Feldspar Minerals. 1. Crystal Structures, Physical, Chemical and Microtextnral Properties (2 Edition). Springer Verlag, Berhn, 828 p Smith JV, Steele IM (1984) Chemical substitution in sihca polymorphs. Neues Jahrb Mineral Mon 3 137-144... 

Feldspars. Like the silica minerals, feldspars are framework silicates with all tetrahedral corners shared. By replacing one in four of the Si atoms by Al, the anionic framework Si3A10g is obtained from Si02 If the framework charge is balanced by structural K, the formula of potassium feldspar, KSisAlOg, is obtained. This feldspar has two common structural polymorphs (different structures with the same chemical formula), orthoclase and microcline. If the charge is balanced by structural Na, the feldspar obtained is albite, NaSi3A10g. 

Based on the strong dilution trends of non-reactive species, such as chloride and sulfate, the extent of interaction of fracture condensate water with matrix pore water must have been very limited. However, reactive species, such as silica, show increasing concentrations owing to reaction with predominantly fracture-lining silica polymorphs and feldspars at higher temperatures. A precipitation zone of secondary mineral phases such as amorphous silica, calcite, and gypsum in... 

The most common phases obtained through recrystallization from the various zeolite types are feldspars, such as celsian and high- sanidine, feldspatoids, such as the above mentioned carnegieite and nepheline, and the high-temperature silica polymorphs (tridymite and cristobalite), which are usually the recrystallization products of high-silica zeolites, such as ZSM-5 [48]. 

Example In K-feldspar, Al replaces Si in the tetrahedral sites. In the high temperature from sanidine, AL has a disordered distribution in the Si02 network. Microcline, the low temperature polymorph, shows an ordered distribution of the two cations. Orthoclase, which is crystallized in intermediate temperature, has a degree of order in between the other two polymorphs. 

If the general feldspar formula is written as AB40g, then we can distinguish in the structure the A sites, occupied by the larger cations in six- to ninefold coordination, and the B sites with Si and Al in fourfold coordination. The polymorphism of feldspars (e.g., sanidine-... 

Laves, F., and J. R. Goldsmith, 1961. Polymorphism, order, disorder, diffusion and confusion in the feldspars. Institute Lucas Mallada, Cursillos y Conferencias fasc. 8 71. 

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