Pyroxenes exsolution

In pyroxenes, exsolutive processes proceed either by nucleation and growth or by spinodal decomposition (see sections 3.11, 3.12, and 3.13). Figure 5.30B shows the spinodal field calculated by Saxena (1983) for Cag sMgo sSiOj (diop-side) and MgSi03 (chnoenstatite) in a binary mixture, by application of the subregular Margules model of Lindsley et al. (1981) ... 

Exsolution in the pyroxenes. The variety of exsolution microstructures that develop in these minerals have been studied in great detail by TEM, and excellent reviews have been given by Champness and Lorimer (1976), Champness (1977), and Buseck, Nord, and Veblen (1980). Many of the exsolution features observed in the pyroxenes, such as the tweed texture, are essentially the same as those already discussed and, therefore, need not be considered further. However, some important characteristics of pyroxene exsolution microstructures warrant special mention. 

Independent of the type of exsolutive process, pyroxene exolutions usually develop on the (001) and (100) planes, probably because coherence energy is minimal along the axis perpendicular to these planes (development of compositional fluctuations is maximum along the direction that minimizes the value of G cf eq. 3.212 see also Morimoto and Tokonami, 1969, for an example of calculation). 

For a more detailed discussion on exsolutive processes in pyroxenes, there is an excellent synthesis in Buseck et al. (1982). 

McCallister R. H. and Yund R. A. (1977). Coherent exsolution in Fe-free pyroxenes. Amer Mineral, 62 721-726. 

Saxena S. K. (1983). Exsolution and Fe -Mg order-disorder in pyroxenes. In Advances in Physical Geochemistry, vol. 3, S. K. Saxena (series ed.). Berlin-Heidelberg-New York Springer-Verlag. 

Noncumulate eucrites originally formed as quickly cooled surface lava flows (unequilibrated noncumulate eucrites), but most were subsequently metamorphosed (metamorphosed noncumulate eucrites). Unequilibrated noncumulate eucrites, also referred to as the unmetamorphosed or least-metamorphosed noncumulate eucrites or Pasamonte-type eucrites, are surface lava flows (Figure 22(c)) that cooled quickly (Walker et al., 1978). As a result of their fast cooling, their pyroxenes (pigeonite of Mg —70-20) are zoned, and exsolution lamellae are only visible by TEM. These rocks have experienced only minor metamorphism (e.g., Takeda and Graham, 1991). 

Metamorphosed (equilibrated) noncumulate eucrites are also collectively referred to as the ordinary eucrites. They include the Juvinas type (main group) and the Stannem and Nuevo Laredo types. They are unbrecciated or monomict-brecciated, metamorphosed basalts (Figure 22(d)) and contain homogeneous low-calcium pigeonite (Mg —42-30) with fine exsolution lamellae of high-calcium pyroxene. The high abundance... 

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