Occurrence of Turquoise Group Minerals in the Eastern United States
Henry Barwood
Indiana Geological Survey
611 N. Walnut Grove
Bloomington, IN 47405
Most collectors associate turquoise group minerals with the Western United States, where large and rich deposits of gem grade material have been mined for centuries. Mineral collectors are generally aware of the turquoise crystal locality near Lynch Station, Virginia, but seldom know about other eastern U.S. sites where turquoise, planerite and coeruleolactite have been found. Turquoise group minerals have been found abundantly in Pennsylvania, Virginia, Georgia, Alabama and Arkansas.
In Arkansas turquoise and planerite are found in the Arkansas Novaculite (with iron phosphates) and in the Bigfork Chert (with aluminum phosphates). Massive turquoise was mined from Novaculite at Porter Ridge (also known as the Mona Lisa or McBride Mine) in Polk County during the 1980's. Production figures are not available, but "many tons" of both hard and chalk turquoise were produced. The turquoise was processed and sold for lapidary use. Thin veins of planerite are common on Buckeye Mountain in Polk County and other iron phosphate localities as well as the type section of the Novaculite at Caddo Gap. At the Coon Creek Mine in Polk County, rare, tiny green crystals of a mineral, originally identified as "rashleighite", but now believed to be more like planerite, are found. Planerite occurs with wavellite and variscite in the Bigfork Chert at Mauldin Mountain, near Mt. Ida in Montgomery County. Most of the other wavellite and variscite localities in the Bigfork Chert also produce small amounts of planerite or turquoise.
Alabama and Georgia have a number of planerite occurrences, but two are exceptional. At Erin in Clay County, Alabama, black carbonaceous shales are cut by quartz veins that contain wavellite and planerite in some abundance. The planerite here is generally greenish and just approaches having distinct crystals. At the Brewer Mine east of Cedartown, Georgia, similar veins of green and blue-green planerite occur along with wavellite, crandallite, aluminum-strengite and cacoxenite. Rarely, the planerite from the Brewer Mine forms distinct green crystals and appears very similar to the material from the Coon Creek Mine in Arkansas.
Alabama and Georgia localities
Virginia is well known for the turquoise crystals from the Bishop Mine near Lynch Station. This long abandoned (and many times re-worked) manganese mine has produced magnificent deep sky blue turquoise crystals. The mine is in the Mount Athos Formation and was originally developed by a now collapsed shaft. Crystals coat a graphitic schist and quartz veins, with the best crystals on the quartz. Little is known about the exact paragenesis of the turquoise at this locality. Turquoise (planerite?) was found in residuum from the Shady Dolomite as vesicular masses coated with wavellite crystals at the Kelly Bank Mine near Vesuvius, Virginia. Specimens could be found in some abundance on the old dumps, unfortunately, Forest Service reclamation has obliterated this once productive mineral locality.
In Pennsylvania, turquoise (planerite?) has been found in residual clays west of Mt. Holly Springs in Cumberland County Pennsylvania. The mineral occurs here with iron and aluminum phosphates in a clay residual from the Tomstown dolomite. The turquoise is massive and light blue. The only U.S. occurrence of the mineral coeruleolactite, a member of the turquoise group, was General Timble's Mine near East Whiteland in Chester County, Pennsylvania. General Trimble's Mine was worked in the early 1800's and it is not known if any trace of it still exists. Green crystals of "turquoise" were also reported from the Bachman Mine near Hellertown, Northampton County, Pennsylvania, but never confirmed.
Virginia, Pennsylvania and New Jersey localities
The turquoise group, as redefined by Foord and Taggert (1986), presents some analytical problems for the Eastern U.S. specimens examined so far. X-ray diffraction and microprobe examination of specimens from many of these localities has revealed complex variations in both diffraction patterns and composition. The earliest formed planerites tend to be fibrous and low in iron, while later formed, and typically more crystalline material, has a higher iron content. Copper content in the planerites seems to parallel the iron content with higher copper content being associated with higher iron content. The general formula for the turquoise group is AB6(PO4)4(OH)8.4H2O, where A= vacant site, Cu, Fe, Ca or Zn and B= Al, Fe, Cr. Most specimens examined during this study have "A' positions occupied by combinations of vacant sites, copper and ferrous (?) iron and "B" positions occupied by variable aluminum and ferric (?) iron.
Turquoise group minerals in the Eastern U.S. are often associated with residual iron or manganese deposits with no obvious source of copper (with the exception of those in the Arkansas Novaculite where native copper is found in some units of the formation). The ability of manganese and iron oxides to selectively absorb metals such as copper, silver, nickel and cobalt is well documented and contents of these metals up to several percent have been reported. The possibility that turquoise and planerite in many of the Eastern U.S. occurrences derive their copper content from associated iron/manganese oxides, and not from distinct copper mineralization, cannot be discounted.