ALTERATION MECHANISM OF BIOTITE IN GRANITIC ROCK CAUSED BY CRUSTOSE LICHEN “PORPIDIA”

Abstract

The alteration mechanism of biotite in granitic rock caused by crustose liche “Porpidia” was investigated by means of X-ray diffraction, optical microscope, EPMA and electron microscope with EDX.
Vertical thin sections of lichen-rock interface were observed by a scanning electron microscope to make clear the physico-chemical phenomenon caused by lichens. The contact surfaces between lichen and a biotite were physically observed with this section, and were further investigated chemically by EPMA. As a result, physical destruction of biotite grains by lichen thallus was observed, and the formation of iron oxide or hydroxide was recognized.
The chemical compositions of some parts of the biotite crystal were analyzed by EPMA, such as altered parts closed to lichen thallus, altered parts far from lichen thallus and fresh parts. Consequently, comparison of the different parts revealed increase in Si0₂ content on one hand and decrease in FeO and K₂O on the other hand from the fresh biotite crystal throughout the licheni alteration. Due to the lower solubility of Si0₂ and higher solubility of Fe₂O₃ and Al₂O₃ from minerals under strong acid condition, which is formed by lichen thallus, it seems that Si⁴⁺ content in tetrahedral site is increased by cation exchange. Also due to the low pH and activity of high molecular weight polymers, the possible weakening of the iron-xygen bonds will lead to the dissolution of Fe ³⁺ ions. Consequently, it is likely that, due to the increasing negative charge of biotite layers with dissolutio of Fe ³⁺, dissolution of K⁺ will be disturbed. Vermiculite-like minerals in biotite formed with the actio or effect of lichen were identified by quantitative chemical analysis as wt% of oxides and calculation of structural formulae on the basis of 22 oxygen atoms per unit cell.