Smectite-illite-muscovite transformations, quartz dissolution, and silica release in shales

Quantitative analysis of the smectite-to-illite and illite-to-muscovite transformations indicates that 17-28 wt.% SiO2 and 17-23 wt.% SiO2, respectively, are liberated during these reactions, assuming that At is conserved. Dissolution of quartz silt in shales yields up to 6-9% SiO2 in the range up to 200 degrees C and a further 10-15% SiO2 in the 200-500 degrees C range. For muds altered to shales at 200 degrees C, 14-20 wt.% silica is evolved. From 200 to 500 degrees C, a further 18-28 wt.% silica is evolved. Additional small amounts of silica may be released in the alteration of feldspar to clay and by stylolitization of quartz silt. Thus, in the burial and temperature range of diagenesis into the epizone, major quantities of silica are released from clays and by quartz dissolution in shales. Within this range of alteration, concomitant decline of whole-rock Si/Al (SiO2/Al2O3) in the transformation of smectite to illite to muscovite suggests the liberated silica migrates from the source shale. As a result, the metamorphosed shales are more micaceous and less quartzose than their progenitor muds. In the diagenetic zone and anchizone, the evolved silica is probably a major source of quartz cement in sandstones. In the epizone, evolved silica is commonly present in quartz veins in the parent rocks. Fluid-inclusion temperatures in quartz overgrowths and fracture fillings in some sandstones suggest that some cements may have been derived from downdip basinal shales and pressure solution in sandstones.