INTRODUCTION

The term “glauconite” applies loosely to a group of green, three-layer clay minerals, all of which are chemically complex K-AI-Fe silicates containing more than about 15% total Fe2O3. Glauconites range in composition from K-poor smectites to K-rich glauconitic micas, with a general trend of increasing K with increasing age. These minerals commonly occur in sediments as small, rounded grains or peloids. Glauconite grains are particularly abundant in Middle Cambrian to Early Ordovician and Middle Cretaceous to Early Cenozoic rocks (Van Houten and Purucker, 1984). They are common also in many modern environments, such as the continental shelf off Vancouver Island, British Columbia, Canada; Queen Charlotte Sound, British Columbia; Monterey Bay, California; the Atlantic coastal shelf off the United States; the East Australian continental margin; and in bottom sediments in various parts of the Atlantic, Pacific, and Indian oceans.

The origin of glauconite is not completely understood. Most workers believe that it forms authigenically on the sea floor by alteration of substrate materials such as skeletal debris, fecal pellets, and various kinds of mineral grains, particularly mica and feldspars (e.g., Boggs, 1992, p. 152; Chaudhuri et al., 1994). Some glauconites may have formed by alteration or transformation of mixed-layer clays by adsorption of K and Fe. Others likely form by precipitation of dissolved material in the pores of a substrate that is progressively altered and replaced (Odin and Matter, 1981). Precipitation can take place also in the cavities of microfossils to produce internal molds. Berner (1971) reported that glauconite forms slowly at the sediment/water interface, where it is associated with organic matter under generally positive, but fluctuating, Eh conditions.