Abstract: Identification and Quantification of Silica Phases in the Monterey Formation Using Infrared Spectroscopy
Don Harville, Allen Britton
The Miocene Monterey Formation contains a variety of silica phases that evolve in response to burial diagenesis. The alteration of diatomite (amorphous Opal-A) to porcelanite (Opal-CT) and ultimately to quartz-chert with depth has been extensively documented in the literature. X-ray diffraction (XRD) techniques have traditionally been used to identify the presence of crystalline silica phases. XRD traces of amorphous opal-A are very broad and may go undetected in amounts up to 20 wt. %, especially in mixtures with opal-CT, cristobalite, and quartz.
The (transmission) infrared technique is a measure of a mineral's molecular composition. Minerals absorb infrared energy because of internal inter-atomic bonds. Infrared absorption bands occur when a change in the dipole moment of the internal vibration exists. The mass of the atoms, bond strengths, and molecular structures all influence the position (wave number or frequency) of these absorption bands. Mineral phases of silica differ in their crystal structure and ordering of distinct silica tetrahedra. These differences allow for the discrimination of these phases by
infrared measurements. According to Beer's Law, the intensity of an IR absorption band is directly related to the concentration of that particular mineral. Calculation of silica phases in mixtures is accurate to ±5% by this technique. This procedure has been applied to field cases where the diagenetic boundaries of opal-A/opal-CT and opal-CT/chert were accurately delineated.
AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994