Abstract
Sericite is a common alteration product found in modem and fossil hydrothermal systems (McDowell and Elders, 1980; Lowell and Guilbert, 1970); it consists of mixed-layer illite/smectite (I/S) and has a high proportion of illite layers (Horton, 1985). Sericite from the Silverton caldera of the San Juan volcanic field in southwestern Colorado has recently been characterized with a variety of analytical techniques (Eberl et al., 1987). Eberl et al. (1987) should be commended for their multidimensional approach; however, we disagree with two critical interpretations made in their study. First, we think that the Silverton samples consist of two separate mineral phases, illite and illite-rich I/S. Second, we think that illitic I/S minerals contain layers with two distinct chemical compositions, layers with a smectite charge and layers with an illite charge. Thus, according to definitions in Zen (1962) and Garrels (1984), illitic I/S should be considered to be two thermodynamic phases, illite and smectite. Both issues are discussed below.
