Fe-Mg order-disorder reaction in orthopyroxene

This reaction has been investigated extensively by high-temperature geochemists. Maintaining the consistency to use capital K for equilibrium constants and lower-case k for kinetic properties, the exchange coefficient is denoted as Kj  

Some authors (e.g., Stimpfl et al., 1999) suggested denoting the above equilibrium coefficient as ku (lowercase k) because it is an intracrystaUine reaction (as opposed to intercrystalline reactions), and K as reaction rate constants of intracrystalline reactions. The suggestion is not adopted in this book because k is used to denote kinetic coefficients. 

The second method of measuring the site occupancy of Fe and Mg in Ml and M2 sites in orthopyroxene is by X-ray diffraction (XRD). In this method, singlecrystal X-ray reflection intensity data are collected. The positions (angles) of X-ray reflections are determined by the structure, but the intensities are influenced by concentrations of the individual elements in each site. Hence, with structure refinement, information on elemental distribution between Ml and M2 sites may be obtained. 

Data obtained by the XRD method were equally confusing. Many equations for the expression of Xp have been reported in the literature. The most recent results apparently still indicate thatXo is a step function of Xps (Stimpfl et al., 1999) For Xps between 0.19 and 0.75, Xq depends on temperature as follows  

The values of from the two expressions differ by about 20%. Most likely, this sudden jump of Xd atXps s 0.18 is an artifact. When applying the orthopyroxene geospeedometer, it is necessary to choose the appropriate relation of iC.  

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The above methods of investigating the order of the reaction with respect to each species independently, although simple and practical for many reactions (such as atmospheric reactions and aqueous reactions) studied by chemists and geochemists, is often difficult to apply to homogeneous reactions in a silicate melt or mineral because the concentration of each species may not be varied freely and independently. This will become clear later when the kinetics for the Fe-Mg order-disorder reaction in orthopyroxene and the interconversion reaction between molecular H2O and OH groups in silicate melt are discussed. 

Only two high-temperature homogeneous reactions have been investigated in detail for their kinetics by geochemists. One is the Fe-Mg order-disorder reaction in orthopyroxene, and the other is the hydrous species interconversion reaction in rhyolitic melt. The two reactions have been applied as geospeedometers in various geochemical and meteoritic problems. Because they are often encountered in geochemical kinetics literature, the two reactions are discussed in depth below. 

Figure 2-4 Kjj values for the Fe-Mg order-disorder reaction in orthopyroxene as a function of temperature for selected compositions. The line is a fit to the data for Xps = 0.011 and 0.162 (Equation 2-57). Data sources are as follows = 0.011and0.162 (Wang et al.,...

Box 2.2 Calculation of the equilibrium species concentrations of the Fe-Mg order-disorder reaction in orthopyroxene... 

Chemical reactions in a mineral. One example is the Mg-Fe order-disorder reaction in an orthopyroxene (opx) crystal ... 

Sykes-Nord J.A. and Molin G.M. (1993) Mg-Fe order-disorder reaction in Fe-rich orthopyroxene stmctural variations and kinetics. Am. Mineral. 78, 921-931. 

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