In Hark diagrams, MgO, FeO, CaO, TiO2 and P2O5 decrease as SiO2 content increase, but Al2O3, Na2O and K2O contents also increase, but the petrographic observation of a large amount of the alkali feldspar phenocrysts, how to explain this phenomenon?
Dear friend Yang Wenjian
The Harker variation diagram is a useful tool for examining the compositional variations in rocks and minerals, and the relationship between different chemical elements. It is a plot of the concentration of a given chemical element against another element, typically SiO2. The pattern of the plot can provide insight into the origin and evolution of the rock or mineral.
In the case of the Harker diagram for alkali feldspars, the decrease in MgO, FeO, CaO, TiO2, and P2O5 content as SiO2 content increases can be explained by fractional crystallization, which is the process by which minerals solidify from a cooling magma. As the magma cools, minerals begin to crystallize out of the melt in a specific order, with minerals that crystallize at higher temperatures having a different composition than those that crystallize at lower temperatures. In the case of alkali feldspars, as the SiO2 content increases, the proportion of other chemical elements decreases due to the crystallization of other minerals from the melt.
However, the increase in Al2O3, Na2O, and K2O content can be explained by the fact that alkali feldspar is itself a mineral that crystallizes at high temperatures from the magma, and its composition is different from the minerals that crystallize at lower temperatures. As the magma cools, the alkali feldspar crystals continue to grow, incorporating more Al2O3, Na2O, and K2O into their structure, which results in the observed increase in these elements in the Harker diagram.
It is important to note that the specific pattern observed in a Harker diagram is dependent on the composition of the magma and the conditions under which the minerals crystallize. The observed patterns may differ for different types of magmas and for different minerals.
References:
- Blundy, J. D., & Cashman, K. V. (2008). Petrologic reconstruction of magmatic system variables and processes. Reviews in Mineralogy and Geochemistry, 69(1), 179-239.
- Holland, T. J. B., & Powell, R. (2011). An introduction to igneous and metamorphic petrology. John Wiley & Sons.
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