Mineral chemistry and physico chemical properties of amphibole asbestos form south Karnataka

Abstract

Formation of amphibole asbestos is related to the metamorphic changes in the host rock. The variety of amphibole asbestos produced depends on the chemistry of the rocks, which, in turn, is determined by the extent of contamination of the rock types prior to metamorphism. In Karnataka, anthophyllite and tremolite asbestos are found to occur in ultramafites and associated rocks. In the ultramafic rocks of Holenarasipur, antigorite–talc–olivine–magnesite–anthophyllite are found to coexist, which is a rather unusual paragenesis. This can be explained on the basis of metamorphic changes in upper greenschist to lower amphibolite facies, accompanied by CO? metasomatism. Anthophyllite asbestos is formed as veins during the waning stages of metamorphism through recrystallization in the fluid (rich in MgO and CO?) accumulated in structurally controlled loci. Tremolite in Sasivala is also found as veins; however, the CaO and SiO? were introduced from extraneous sources. The occurrence of veins is controlled by fractures and joints. The formation of tremolite asbestos, through the reaction of antigorite with CaO and SiO? in the fluid phase, occurs at a lower temperature than the general metamorphic conditions prevailing in the area. Crocidolite, amosite, and prieskaite are found only in the iron?ore formations. They are products of metamorphic reactions in the lower greenschist facies under medium pressures. Crocidolite along with magnesioriebeckite is produced in alkali?rich bands, while amosite is formed in bands containing magnesium but poor in alkali. Prieskaite is produced in basic rocks contaminated with ferruginous material. Conclusive explanations cannot be given for the development of asbestiform morphology in amphiboles. In general, two influential factors exist: (i) differential pressure and shearing stress, and (ii) minor chemical impurities during recrystallization. There is no difference in the crystal structure of both massive and fibrous varieties of the same amphibole. Asbestos fibre formation occurs through the development of lath?shaped crystals of micron to sub?micron diameter grown one over the other (lath on lath). The physical differences may be due to the special surface characteristics of amphibole asbestos. Different varieties of amphibole asbestos possess varying resistance to chemical reagents. Even within the same type of amphibole, massive, woody, and fibrous forms show different degrees of resistance. Acid resistance of amphibole asbestos is related to its surface properties since the amphibole structure is constructed such that the cations are protected by Si?O?? bands, making reaction comparatively difficult. Earlier explanations suggested that the cations are removed through exchange processes, leaving behind the silica framework which then acts as a protective coating. However, the present study shows that silica also enters the solution along with the cations, either as extremely small colloidal particles or as molecular complexes. Gas?adsorption studies and ion?exchange data do not indicate the presence of amorphous silica in the residue. The acid attack on amphibole asbestos proceeds through: (i) dissolution of surface?adsorbed ions originally present during crystallization, (ii) complete dissolution of fibres of extremely small diameter, and (iii) dissolution of imperfect regions on the surface, including pores partially or fully filled with amorphous material. These pore?filling materials vary in composition from metal hydroxides to hydroxylated metal silicates, with much higher cation content than the bulk material. The differing contributions of these factors explain the variation in acid resistance among different amphibole asbestos and among different morphological forms of the same amphibole. Gas?adsorption studies show the porous character of amphibole asbestos. Except for amosite, all varieties are microporous. The pore radii vary for different forms of the same amphibole. For example, asbestiform anthophyllites have pore radii around 7–8 Å, while untreated woody types have larger pore radii of about 27 Å, and even larger pores of 60 Å. These pores are originally filled with amorphous material. Clearance of these pores is shown by variation in adsorption isotherms using both nitrogen and water vapour. The absence of hysteresis at low P/P0P/P_0P/P0? in acid?leached samples indicates no retained amorphous silica. Water adsorption behaviour can be effectively used in characterising asbestos surface properties. Thermal studies show that the temperature of dehydroxylation (removal of OH groups from the bulk) varies for different varieties, being lowest for crocidolite and highest for tremolite. The temperature of dehydroxylation also depends on iron content and the oxidising conditions of the surrounding atmosphere, due to Fe²? ? Fe³? oxidation accompanying dehydroxylation. A key finding of this study is the removal of “extra water” below 500°C. Earlier explanations attributed this to inter?fibre hydrogen bonding. However, the higher temperature required for removal suggests stronger binding. Instead, the extra water is fixed as hydroxyl groups within the amorphous pore?filling material. Upon heating below 500°C, this material recrystallizes, releasing the extra water. Surface studies of samples degassed at various temperatures and the absence of extra water in acid?leached samples support this view. Reduction in tensile strength correlates not simply with water loss but with diminished inter?fibre binding due to recrystallization of pore?filling material. Surface properties of silicates have been little investigated except in quartz and its varieties. The present study indicates that many silicates may have surfaces containing pore?filling materials of composition different from the bulk. Therefore, compositional variations often attributed to bulk substitutions must be distinguished from those due to surface?adsorbed phases. Furthermore, surface properties control mineral crystallization and growth, especially in metamorphic rocks. This has been emphasised by Ramberg and DeVore. The relationship between chemical reactions and adsorption in silicate minerals plays a significant role in controlling mineral association. Hence, systematic surface studies may be of considerable value in mineralogy and petrology.