Ray E. Ferrell¹, Per Aagaard², Henning Dypvik²
(1) Louisiana State University, Baton Rouge, LA
(2) University of Oslo, Oslo, Norway

Abstract: How many illites are there?

The informal mineral group name, illite, may apply to several types of clay-sized materials in a single sedimentary rock. Numerous published reports based on major element chemistry, isotopic composition, XRD determinations of mineral composition and electron microscopy often indicate that three or more illites are present. These illites may be extrabasinal detritus or a product of authigenesis and postdepositional alteration. Identifying these components is the key to unraveling their origin and the geohistory of fine-grained clastic sediments.

One XRD technique that is being used to identify multiple illites in a single sample employs mathematical decomposition procedures to remove single peaks from overlapping bands. Then, the individual minerals are identified by whole-pattern fitting of actual or simulated patterns to the sample being analyzed. Further distinction of the illite and illite/mixed-layered clay varieties can be achieved by K- and Mg-saturation of the clay fractions and the effects of glycerol and ethylene glycol on the expandable materials.

Results from the Louisiana continental shelf and the Haltenbanken area of offshore Norway often indicate two or three discrete illites and as many illite/mixed-layered minerals are present. The illite peak widths vary from broad to sharp, and their d-values occur within the 10 +/-0.5 A range. The broad peaks may represent the incipient stages of diagenesis or highly weathered detritus. The procedure clearly provides a more detailed characterization of the illite in a sample, but questions of origin are not answered unambiguously.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana