Sericite from the Silverton Caldera, Colorado; correlation among structure, composition, origin, and particle thickness

“Sericite” is a petrographic term used to indicate highly birefringent, fine-grained, micaceous material that is viewed under the optical microscope. Other analytical techniques, including X-ray diffraction (xrd) analysis, transmission-electron microscopy (tem), and chemical analysis, reveal that sericite from the Silverton caldera is very fine grained muscovite or phengite, with a layer charge close to 1.0 equivalent per O₁₀(OH)₂ for the micaceous layers. The structure and chemistry of this sericite differ from that of coarser-grained micas, however, because the sericite particles are so thin that exposed basal surfaces comprise a significant proportion of the sample. The presence of these surfaces leads to swelling (expandability), to larger cation exchange capacities, and to smaller fixed cation contents in the structural formulae. These surface properties correlate with the thickness of the clay particles parallel to c*, in agreement with the theory of interparticle diffraction. Expandability also correlates with the Kubler index (thereby suggesting that this “crystal-linity” index is a function of particle thickness), with an intensity ratio (Ir), and with the wave number of an 824-834 cm^-1 infrared absorption band. The amount of octahedral Fe + Mg can be estimated from the wave number of a 528-540 cm^-1 infrared absorption band.

Based on xrd analysis and computer modeling, the sericites are classified as R ≥ 3, mixed-layer illite/smectites that range in expandability from > 4% to < 14%. The thickness of the expanding layers is related to interlayer chemistry and to relative humidity. Expandabilities measured by a conventional xrd peak-position method are systematically smaller than expandabilities obtained by particle-thickness measurements. Better agreement is obtained when xrd peak positions are corrected by deconvolution for a contribution from unarticulated illite particles. A new method for measuring expandability uses xrd peak-breadth measurements from very thin xrd sample preparations and the Scherrer equation.

Sericite from the Silverton caldera occurs in fractures that cut a Tertiary volcanic complex and appears to have been formed by the hydrothermal alteration of fault gouge. K-Ar isotopic ages and oxygen-isotope geothermometry indicate that sericite formed during two hydrothermal events that involved heated meteoric water. It formed at about 180°C during a 21-Ma event and at about 320°C during a 13-Ma event. Sericite crystal chemistry roughly reflects temperature of formation: the younger, higher-temperature sericites have thicker micaceous particles, which means that they are less expandable, contain more fixed cations per formula unit, and have smaller cation-exchange capacities, and they have larger 2M₁/1M polytype ratios.