PETROLOGY AND CHEMISTRY OF "BLUESTONE" IN NEW YORK STATE

Sandstone dimension stone, known commercially as "bluestone", has been quarried in southeastern New York for more than a century. The industry is currently centered in Delaware County in the southwestern Catskills. In 2002, New York State implemented a new mining law specific to this industry in the form of an "exploration" permit with reduced requirements relative to a standard mining permit. This action led to a sharp increase in the number of applications for "bluestone" mines. The definition of bluestone carried in the law, "feldspathic sandstone of Devonian age that splits easily along bedding" is insufficiently precise. Petrographic and chemical analyses were performed to characterize the material. Bluestone, as the term is currently applied to the actively mined stone over a wide geographic area, is a sublitharenite of relatively uniform composition comprised of subangular grains averaging 0.5mm. In the primary mining region, it is quarried from the Slide Mountain and Upper Walton formations where sandstone is interbedded with gray and red shale. The rock is dominantly quartz (43%) and rock fragments (15%) with interstitial "sericite" (22%), chlorite (8%), muscovite (4%) and plagioclase, K-feldspar, biotite, and opaque phases (<2% each). Apatite, calcite, ferromagnesian silicates, and zircons are present in trace amounts. Pyrite, and less commonly chalcopyrite and galena, occur is proximity to plant fossil (coal) detritus. There is evidence of pressure solution and recrystallization, burial depth being estimated at >4 km. Chemical composition (n=6, wt %) is SiO2 - 80.25, TiO2 - 0.99, Al203 - 10.99, total Fe as Fe2O3 - 4.03, MnO - 0.06, MgO - 1.10, CaO - 0.58, Na2O - 0.97, K20 - 2.19, P205 - 0.09, loss-on-ignition - 2.23. Trace elements present (ppm) are Sc- 7, V - 80, Cr - 52, Ni - 38, Zn - 66, Ba - 302, Rb - 93, Sr - 59, Zr - 489. The color of the stone, one of the most economically important properties, varies from greenish gray (5GY6/1) to grayish red purple (5RP4/2). The color does not depend on the total Fe content, which is quite consistent in the analyzed samples, but rather on the ferric/ferrous ratio. That ratio varies from 1.38 to 2.14 in the example cited above. Increased ferric iron content is associated with a change in color from gray green to less valuable pinkish and purple hues, called "lilac" stone.