The Role of Axinite and Boric Acid in Pegmatite Formation

Abstract

Analytical tests such as physical property tests, Raman spectroscopy, and X-ray diffraction are not only helpful, but oftentimes essential for both discovering the identity of and gaining structural information about distinct minerals. In this case, these tests can be used to discover the chemical composition of a ferroaxinite-tinzenite blend. Pegmatite formation in the fourth stage of mineral evolution represents the first time minerals such as axinite formed as well as a substantial mineralization event. Research suggests that boron in the form of boric acid may play a previously unknown yet crucial role in the formation of pegmatites, and therefore axinite, and creating the necessary conditions that resulted this crystallization event. 

Background

Axinite {(Ca,Fe,Mg,Mn)3Al2BSi4O15(OH)} is a subgroup of minerals with slightly varying composition Ford 1903. The triclinic mineral often grows in very flat, tabular crystals with sharp axe-like edges, giving it its name Ford 1903. A few variations of the same chemical formula are considered to be axinites. They include ferroaxinite, magnesio-axinite, manganaxinite, and tinzenite Ford 1903. These minerals share a base formula of Ca2XAl2BOSi4O15(OH), where "X" can vary in composition. For example, ferroaxinite replaces the "X" with a Fe2+ ion. On the other hand, magnesio-axinite and manganaxinite contain magnesium and manganese ions in place of the "X," respectively. Finally, the last variation of axinite, tinzenite, contains both Fe2+ and Mn2+ ions, and only one calcium ion. These different compositions result in slightly different properties for the individual minerals, such as color. Axinites form in very obscure conditions and, as a result, are therefore relatively rare. They are nearly exclusively found across the world in granitic, near-surface pegmatites and areas of hydrothermal activity Stability of tourmali....

Axinite Crystal