About This Item
Share This Item
Mineralogic and chemical studies of interbedded red and greenish-gray sandstone, siltstone, and mudstone from the Upper Devonian Catskill facies in New York indicate that the red coloration is a result of the presence of diagenetic red hematite pigment. Thin sections of red sandstones show hematite partly replacing rock fragments composed of fine-grained mica and chlorite, as well as coating sand grains and forming patchy interstitial matrix material. Hematite is absent at many sand-grain contacts in red rocks.
X-ray diffraction studies of powdered rock samples and oriented clay aggregates show that the greenish-gray rocks are illite poor and chlorite rich relative to interbedded red rocks. X-ray diffraction intensity data indicate that chlorite is more iron rich in green or drab rocks. There is no significant difference in the total iron content of interbedded red and greenish-gray rocks; however, small green patches in otherwise red rocks have significantly lower amounts of total iron than do the surrounding redbeds.
These results suggest that hematite in these redbeds has formed diagenetically under near-surface oxidizing conditions, largely controlled by the level of the groundwater table. Chlorite is believed to be the major source of the iron that has been precipitated as hematite. Except for localized green patches where reducing conditions prevailed because of the presence of organic matter, the greenish gray rocks are believed to be composed of detrital sediments that have undergone little alteration.
End_of_Article - Last_Page 360------------