Overview

Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Essexite, also called nepheline monzogabbro is a dark gray or black holocrystalline plutonic igneous rock. Its name is derived from the type locality in Essex County, MA. Modern petrology identifies rocks according to mineralogical criteria. Utilising the IUGS QAPF diagram of Streckeisen (1974) essexite is more formally known as nepheline monzodiorite or nepheline monzogabbro depending on the ratio of orthoclase to plagioclase and the abundance of nepheline. In order to produce a magma composition suitable for forming essexite the partial melting of the source rocks must be restricted, generally to less than 10% partial melting. This favors producing a melt rich in large-ion lithophile elements (LILE) such as K, Ba, Rb, Cs, Sr. The source melts of essexites contain more aluminium and alkali ions than available silica tetrahedra, which is why essexites crystallise nepheline instead of plagicolase. Higher than normal potassium favors the production of orthoclase, which is usually absent from most mafic igneous rocks.

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