Mineralogy and Petrography of the Ordovician “metabentonites” and Related Limestones*

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Read 2nd November, 1951. A series of samples of metabentonite, here referred to as K.bentonite, and related limestones were collected from the Upper Black River and Lower Trenton formations of the Ordovician System in Central Pennsylvania. The K-bentonite occurs in yellow and gray beds which range from half an inch to ten inches in width. X-ray diffraction patterns of the 22 K-bentonite samples are similar to those for illite and have the (060) reflection at 1-50~ indicating that the clay mineral is of the dioctahedral variety. However, the value of the (001) reflection ranges from 10.2 to 10.6 .~, suggesting that the material is a mixed layer clay composed of expanded and non-expanded layers of hydrous mica (montmorillonite and illite).t This possibility was tested by saturating the clay with H, Ca, Na, K, NH 4, and ethylene glycol and noting the effect of the treatment on the position of the (00l) spacings in the X-ray diffraction patterns. Using the curves published by Brown and MacEwan (1950) for the analysis of mixed layer complexes, it was found that approximately 20~ of the layers are expanded and 80~ are not. This ratio is supported by chemical analyses which show that of the exchangeable cations 20~ are calcium, sodium, and magnesium and the remaining 80~ is potassium. In addition to X-ray diffraction effects due to the mixed layer mineral, eight of the 22 K-bentonite samples gave very weak chlorite patterns. The chlorite reacted in the manner described by Brindley and Ali (1950) in that when the material was heated to 600~ for 24 hours the 14 ~ line shifted to 13.4 ~ and increased in intensity while the (002) and (003) reflections were considerably weakened. Differential thermal analysis curves of two of the K-bentonite samples are shown in Figure 1. The lower curve is typical of 14 of the K-bentonites. The presence of a single hydroxyl-endothermal peak at 725~ suggests that there is little variation in the composition of the octahedral sheets in the two types of layers. The upper curve, * This paper represents some of the results of research on the morphology, structure and origin of fine-grained minerals conducted in the Division of Mineralogy of The Pennsylvania State College and supported by the Office of Naval Research. t The nomenclature used in this paper is based on a classification of clay