Specific surface area and ultramicroporosity in polymorphs of silica

Abstract

A detailed study of the specific surface area (SSA) and the volume of pores between 10 A and 400 A (ultramicroporosity -UMP-) in different types of silica phases is carried out in this paper. The samples were chosen from different locations and geological settings, with an emphasis on high silica content (above 80%) and mineralogical simplicity (one silica phase predominates in every sample). Some samples belong to continental siliceous rocks and silcretes of the Miocene age, and others to radiolarites transformed by thermal metamorphism. The results show that small proportions of other minerals in the siliceous rocks can alter the SSA and ultramicroporosity of the silica rock in a substantial way. The specific surface and pore volume are closely related to the crystalline structure of the silica polymorphs. Opal-A (diatomite) has 23.87 m 2 /g of SSA and 9.90% of UMP, opal-CT 7.76-14.05 m 2 /g of SSA and 6.68-8.50% of UMP, α-cristobalite 1.93-2.43 m 2 /g of SSA and 1.27-1.52% de UMP and α-trydimite 1.93-2.56 m 2 /g of SSA and 1.05-1.62% de UMP. There exists a loss of SSA and ultramicroporosity in the silica phases involved in the diagenetic transformation opal-A → opal-CT → opal-C (α-cristobalite). The host-rock in which the silica phase is formed can play an important role in the amount of the SSA. Thus, opal-CT formed in a sepiolite host-rock exhibits a greater SSA than opal-CT formed in a limestone host-rock. The pore volume distribution is characteristic of the silica phase although this distribution can vary in relation to the texture in the opal-CT