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rock, indicating a concealed or hidden crystal habit (albeit viewable in cross -polarizing light), <br />and forms in hydrothermal or geothermal settings while opalitic chert forms in surface <br />environments, usually in water during fluctuating pH conditions. Opal, by definition, contains <br />a small amount of water and has a Moh's hardness in the range of 6.5 as opposed to 7.0 for true <br />chalcedony. It should be noted, however, that after opalitic chert is deeply buried and affected <br />by geothermal heating, vugs or fractures in the rock are usually filled with chalcedony. <br />Opalitic chert is variegated and translucent to some degree, even in thick samples. <br />Miocene opalitic chert ranges in color from almost clear to milky gray to white (often referred <br />to as chalcedony), and brown to red; formed in shallow, ephemeral lakes; and almost always <br />contains ostracodes (clam shrimp) and stromatolitic (i.e. algal) banding. "Local" sources <br />include Troublesome and Browns Park formations (Miocene), east and north, respectively, <br />from the Steamboat Mesa area, but a useful secondary source exists in nearby river gravel, in <br />the present bed or in terraces. The primary opalitic chert comes from Burro Canyon Formation <br />(Jurassic -Cretaceous); it is white to cream, yellow, and pink to red in color, and co-occurs as <br />interbeds with the orthoquartzite and porcellanite from the same formation. <br />Other names are commonly applied to opalitic chert types. "Pumpkin" chert, an orange - <br />to -red chert with manganese dendrites is usually derived from Mississippian, Pennsylvanian <br />and Permian rocks. A local type is imported from quarries in the Morgan Formation located <br />along the Yampa River. Similarly, "pigeon blood" chert is white or clear opalitic chert with <br />blebs of hematite and probably ferrihydrite and is formed in karst in Paleozoic limestones, but <br />also in Miocene playa lakes. The name "root beer" chert is often applied to any dark brown <br />chert formed in terrestrial environments in perennial and ephemeral lakes or, less commonly, <br />during subaerial erosion of limestone over hundreds of millions of years. A type regionally <br />observed has been identified in quarries found in Sand Wash Basin. "Jasper" is applied to any <br />red, orange or yellow chert, but is more specifically spherulitic felsite which originates in <br />rhyolite source rocks such as those found on the Flat Tops and Grand Mesa. Banded opalitic <br />chert from the Green River Formation (Eocene) is referred to as "tiger" or "shavetail" chert and <br />was formed on the bed of lakes Gosuite and Uinta. The material is usually dark brown or black <br />with tan banding, and inclusive invertebrates, especially ostracodes, are commonly replaced by <br />light blue opalitic chert or chalcedony. The banding represents lake varves — alternating <br />opalitic chert and porcellanite — representing clastic deposition during monsoons (porcellanite) <br />and silica precipitation in the dry season during deposition. <br />Orthoquartzite and porcellanite are silica cemented clastic rocks, the former sandstone <br />and the latter mudstone (i.e., siltstone or claystone). Mesozoic clastic rocks are identifiable by <br />mineral composition which is almost exclusively quartz and black chert grains due to <br />30 <br />