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Since opening in 1955 the Hunting Hill quarry in suburban Rockville, Maryland has been a popular collecting area. Fine specimens of red grossular, green diopside, lustrous orange botryoidal prehnite, clinochlore and clinozoisite have been found, as well as rare crystals of desautelsite, rare tochilinite crystals over 1 cm, the only known crystals of the magnesium-iron carbonate, coalingite, and at least 52 other species. A range of interesting pseudomorphs has also been found.
INTRODUCTION
Since it was opened in 1955 for aggregate stone for asphalt and road building by the Rockville Crushed Stone Company, Inc., the Hunting Hill quarry has been one of the most popular mineral collecting sites in the Washington, DC metropolitan area. The quarry was sold to London and Northern in 1985, and then to several other companies in succession until it was purchased in 1989 by Bardon Aggregates Industries, who recently sold out to Aggregate Industries-Mid Atlantic. The quarry is still a very active commercial operation despite the encroachment of urbanization, and was 137 meters deep (120 meters below sea level) in August 2002 (John Croney, personal communication).
The site is most commonly called the "Hunting Hill quarry" or, less often, the "Bardon Stone quarry" or the "Rockville Crushed Stone quarry" by collectors. It is located near Hunting Hill on the western city limits of Rockville, Montgomery County, Maryland, off of Shady Grove Road, where it covers more than half a square mile.
The quarried rock is a dense serpentinite with mineralized dikes of rodingite, the latter producing outstanding crystallized specimens of grossular, clinozoisite, diopside and clinochlore. In addition, unusual minerals such as desaultelsite, pokrovskite, mcguinessite and the first occurrence of coalingite in crystals have been documented. To date, 60 mineral species have been verified from the quarry, and the list grows with continued collecting and study. Despite its mineralogical significance, there is no recent publicly available technical work on the mineralogy, geology, and petrology of Hunting Hill. The locality remains largely unfamiliar to collectors outside the Washington DC-Baltimore region.
GEOLOGICAL SETTING
The Piedmont Uplands along the Maryland-Pennsylvania border contain Precambrian-age rocks, including gneisses, quartzites, schists and marbles, which are locally intruded by ultramafic peridotite (dunite) and pyroxenite. These were subsequently partially or wholly altered to serpentinites. Some of the serpentinites...