Feldspars monoclinic

Figure 8.40 is a BF image of the exsolution nucrostructure commonly observed in cryptoperthites, together with its SAD pattern. The electron beam is approximately normal to (001). The single spots corresponding to the K-feldspar (monoclinic) and the pairs of spots corresponding to the... 

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... 

Hovis G. L. (1974). A solution calorimetric and X-ray investigation of Al-Si distribution in monoclinic potassium feldspars. In The feldspars, W. S. MacKenzie and J. Zussman, eds. Manchester Manchester University Press. 

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]. 

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. 

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).

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

Sodium is the most abundant of the alkali metals and is the sixth most abundant element in the Earth s crust, with an abundance of roughly 2.36 wt.%. Owing to its high chemical reactivity with water and, to a lesser extent, with air, sodium metal never occurs free in nature however, the element is ubiquitous and occurs naturally in a wide range of compounds. Sodium chloride is the most common compound of sodium and is dissolved either in seawater or in the crystalline form of halite or rock salt [NaCl, cubic]. However, it is widely present in numerous complex silicates such as feldspars and micas and other nonsilicate minerals such as cryolite [NajAlF, monoclinic], natronite or soda ash [Na COj, mono-clonic], borax [Na B O,. lOH O, monoclinic], sodium hydroxide or caustic soda [NaOH], Chilean saltpeter, and nitratite or soda niter [NaNOj, rhombohedral]. Obviously, the chief ore is the sodium chloride recovered from either brines or rock salt ore deposits. There are... 

Orthodase ) (common feldspar) KAlSijOg (Figure M66) Monoclinic. Grains are tabular and have nearparallel orientation. Blunted prisms A reddish, cream-yellow, or gray mineral. If green, it is called amazonstone or amazonite. If opalescent, it is called moonstone... 

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 ). 

These include celsian, Ba [Al2Si20g], and hyalophane, (K, Na, BaJ [Alj., Sig- Og], both of which are structurally very similar to the potassium feldspars. In celsian, which is truly monoclinic, the A1 and Si are ordered into alternate tetrahedral sites (Newnham and Megaw [I960]). 

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