THE RELATIONSHIP IN PHYLLOSILICATES AND POLYSOMATIC BIOPYRIBOLES TO FORMATION OF ASBESTIFORM HABIT VIA TETRAHEDRAL/OCTAHEDRAL FIT COMPENSATION AND THE MOLECULAR ACTION OF WATER

In efforts to understand why certain minerals form in the asbestiform habit, mineralogists still cannot fully explain the cause-and-effect of this phenomenon. Although we know that there are chemical variances and pressure or temperature regimes that are conducive to the formation of asbestos, a complete and absolute picture of how and why amphibole forms fibers, or serpentine forms chrysotile scrolls remains elusive. Research indicates however that there are two primary ways that sheet silicates compensate for the fundamental misfit between their tetrahedral silica layers (T) and their octahedrally-coordinated cation layers (O) that is by either tetrahedral rotation /stretching or by bending or modulation of the layers in concert. Rotation or stretching occurs in both the 1:1 layer silicates (T-O) such as serpentines, and the 2:1 phyllosilicates (T-O-T) such as vermiculite or talc. The other primary means of misfit compensation is structural bending, with the obvious examples of antigorite or chrysotile. Although it was originally hypothesized as early as the 1950s that this curving or bending of the sheet structure was entirely due to the T-O misfit, more recent research points to the importance and variances of hydroxyl bonding in the chrysotile structure. A secondary mode of compensation for the fundamental misfit is by the addition or subtraction of silica tetrahedra or octahedral cations in modulated fashion, which affects the overall chemistry of the mineral as a whole. In polysomatic hydrous biopyriboles we see the importance of hydration alteration reactions in the transformation of chain zippers. Thusly, a wide variety of intergrowth microstructures appear in Mg-rich 1:1 modulated layer silicates, analogous to the hydrous biopyriboles as is common intimate intermixing in a polysomatic series. It is therefore common that the means by which all of our regulated asbestos minerals form is through the combined action of T-O misfit compensation and the action of water in the crystallizing or re-crystallizing process.