Aluminum peridotites

Secondary alteration processes are also responsible for element deviations, particularly the strong depletion of calcium and sodium relative to aluminum in some abyssal peridotites, as well as part of the scattered variations of silicon, magnesium, calcium, and sodium. Serpentinized, orogenic and ophiolitic peridotites may also be strongly depleted in calcium and sodium. However, these samples are rare in the selected database for peridotite massifs, which includes only moderately serpentinized samples. 

In a given facies, the sodium and titanium content of clinopyroxene shows a negative correlation with depletion (or chromium content) and can increase substantially in metasomatized xenoliths. The substimtion of sodium, chromium, and aluminum has a complex relationship with T, P and degree of depletion but in general, clinopyroxene from higher P samples contains more sodium. Representative analyses for different peridotite facies are shown in Table 4. 

The role of chemical weathering has long been recognized in economic geology. Tropical bauxites, which account for most of world s aluminum ores, are typical examples of residual concentration of silicate rocks by chemical weathering over long time periods (Samma, 1986). Weathering of ultramafic silicates such as peridotites forms residual lateritic deposits that contain... 

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