Gabbro rock feldspars

Figure 13. A plot of 5 0(fdsp) vs. 5 (pyroxene) for various gabbros. If equilibrated at magmatie temperatures, these minerals would form a linear array with small fractionation and a slope of 1. The steeper slope and highly variable fractionations shown here indicate that the rocks have been altered by infiltration of hydrothermal fluids and that feldspar has exchanged more completely than pyroxene, (from Criss et al. 1987).

Fig. 3.21 The classification of plutonic igneous rocks by Streckeisen (1967, 1976) is based on the modal abundances of quartz (Q), alkali feldspar (including albite, An < 5%) (A), and plagioclase (An >5%) (P). The fields have been numbered in order to identify the rock types that correspond to them 1. quartz, 2. alkali granite, 3. granite, 4. granodiorite, 5a. quartz diorite (trondhjemite), 5b. quartz diorite (tonalite), 6. alkali syenite, 7a. quartz syenite, 7b. syenite, 8a. quartz monzonite (adamellite), 8b. monzonite, 9a. monzodiorite, 9b. monzogabbro, 10. diorite/gabbro (Adapted from Streckeisen 1967)...

I assumed that did not change with thermal variations, but this is not the case. A student of Buettner, R. W. Dana, published an extremely important master s thesis in 1969 on emissivity in the 8-14 band. He showed that e changes with real temperature. For example for rocks such as quartz, granite, gabbro, dunite and albite, and for water when T>0°C, a< 1 when T=0°C, e= 1 and finally when 7 <0°C, > 1 (Dana, 1969, p. 40). Quartz, olivine, potassium feldspar, and microcline tend to reduce the e value while this value increases when darker rock shades are due, among other things, to the presence of augite, labra-dorite, and hornblende. 

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