Aluminosilicates with Sheet Structures that Form Fibers

ALUMINOSILICATES WITH SHEET STRUCTURES THAT FORM FIBERS 

The third mineral group included in the category of biopyriboles is the micas. The micas are indicated by bio, which comes from biotite, one of a large number of compositionally diverse species that make up the micas, a commonly encountered group of minerals. 

The micas are characterized by extended silicate sheets rather than chains. Their structures resemble the serpentine mineral group in that they are dom- 

The 2-3 subscript for the B site in the formula expresses the fact that there are two families of mica structures, the dioctahedral and trioctahedral micas, based on the composition and occupancy of the intralayer octahedral sites. The trioctahedral micas have three divalent ions—for example, Mg or a brucitelike [Mg(OH)2] intralayer, and the dioctahedral group—two tri-valent ions—for example, Al or a gibbsitelike [AlfOHfa] intralayer, between the tetrahedral sheets. In the dioctahedral micas, therefore, one-third of the octahedral sites are vacant or unoccupied (Fig. 2.12C). 

The trioctahedral micas can be distinguished by x-ray diffraction from the dioctahedral type. The dioctahedral micas characteristically show distortions that are detected as variations in the bond angles of the hexagonal pattern, Fig. 2.12E (Bailey, 1984). Natural mineral samples often exhibit an occupancy of the B site of greater than 2 and less than 3, producing many variations that cannot be detailed here. 

---