Thursday, July 18, 2013


I recently acquired a couple of nifty little specimens that continued to pique my interest in serpentine and hydromagnesite (see Blog posting June 14, 2013).  The first specimen was “collected” from a recent estate auction and originally was taken from the Hunting Hill Quarry near Rockville, Maryland, opened in the 1950’s as a site to obtain crushed stone for road building.  Although I have never visited the site my contacts in the area tell me the quarry is a popular collecting site for rockhounders in the local Washington, DC, area.  Parker (2005) noted that at least 60 mineral species have been documented from the quarry including such rare types as desaultelsite [Mg6Mn2(OH)16[CO3]-4H2O],  pokrovskite [Mg2(CO3)(OH)2], tochilinite [Fe5-6(Mg,Fe)5S6(OH)10],  mcguinessite [(Mg,Cu)2(CO3)(OH2)], and coalaingite [Mg10Fe2(OH24[CO3]-2H2O)].  Of course, I am unfamiliar with all of these!
The Hunting Hill Quarry lies within the Piedmont Physiographic Province of the Appalachian Mountain Chain.  The history of the Piedmont Uplands is extremely complex and I have a great deal of respect for the geologists, especially the early ones, who tramped through the woods and shrubs fitting the pieces together.  In summary, the area now termed the Maryland Piedmont was originally sedimentary rocks deposited in marine waters perhaps beginning ~1.6 Ga (Precambrian: Proterozoic).  These rocks were then deformed and altered (lots of gneiss, quartzite, marble, peridotite, and pyroxenite; Parker, 2005) during continental collisions termed the Grenville Orogeny (~1.2-1.0 Ga).  The result of these collisions was the formation of a supercontinent termed Rodinia.  We often thing of the late Paleozoic supercontinent called Pangaea but here was an earlier event creating a super land mass.  The plates of the Earth are in constant motion so Rodinia begin to break apart in the latest Precambrian (~1.0-.54 Ga) and eroding sediments accumulated along what was then the eastern coast of Laurentia (ancestral North America).   These sedimentary rocks were again altered during the Ordovician by collisional events termed the Taconic Orogeny.  In addition, collisional events in the Devonian (Acadian Orogeny) and late Paleozoic (Alleghenian Orogeny) left their imprint on the rocks and contributed to the amazingly complex geology of the region. 

Serpentine with small amounts of talc? (T), hydromagnesite HM), aragonite (A), and magnesite (M).  Specimen width~5.5 cm.

The Hunting Hill Quarry is located in the Hunting Hill pluton of Ordovician age (igneous event associated with the Taconic Orogeny--intruded into pre-existing sediments/sedimentary rocks, the ones associated with the erosion of Rodinia).  Today the rock is mostly a serpentinite (various serpentine group minerals)---see Blog posting June 14, 2013) that originally was a dunite (plutonic rock more than 90% olivine) and which oxidation (termed serpentinization---common along plate boundaries) converted to minerals of the serpentine group.  As noted in a previous blog posting, it is very difficult, at least for me, to differentiate various minerals of the group.
OK, wow. 

The specimen in my collection has the massive dark green serpentine associated with lighter green talc (I think) [Mg3Si4O10(OH)2], coated with a white hydromagnesite [(CO3)4(OH)2-4H2O] with an aragonite [CaCO3] spray heavily covered with magnesite [MgCO3] (rather than calcite I believe).  There may also be a few other strange minerals that I lack the skills to identify. 

Specimen of green serpentine with molybdenite and dirty marble.  Width of specimen ~6 cm.

The second specimen of serpentine was found while I was rummaging around the cases at Ackley’s Rock Shop here in Colorado Springs.  All of a sudden the rock in question sort of attracted me since I have been looking for other examples of serpentine, and in addition, the specimen contained crystals of molybdenite.  This specimen has been a tough one to track down much information about the geology.  I know that it came from the old Royal Green Marble Quarry, near Phillipsburg, Warren County, New Jersey.  Warren is adjacent to Sussex County to the north where the famous collecting localities of Ogdensburg and Franklin are located.  It appears they might share some of the same rocks.  As best that I can determine the quarry is in the New Jersey Highlands, a physiographic province trending northeast-southwest in western New Jersey containing Precambrian igneous and metamorphic rocks—mostly granite, gneiss, and some marble.  They are the oldest rocks in New Jersey, perhaps late Precambrian  in age (Proterozoic:  ~1.3--.8 Ga).  MinDat noted the quarry was established in 1880 to produce marble and dimension stone and operated until 1941.  Mineralization is the Precambrian marble. Correlation with the Franklin Marble cannot be made with precision. However, as currently interpreted all the marble along the northwestern margin of the Reading Prong highlands represents a shelf sequence along the margin of a back arc basin. Therefore, the marble in the Easton quadrangle is, at least, broadly correlative with the Franklin Marble.  The USGS geological quadrangle GQ 594 maps the area as Precambrian “dolomite, and lesser calcite marble, largely altered to serpentine, tremolite, and talc” (Drake, 1967).

Crystal of molybdenite enclosed in serpentine.  Width of photomicrograph ~1.2 cm.


Photomicrograph of platy and micaceous serpentine group mineral (??lizardite).  Width of photo ~1.2 cm.

The specimen I purchased for a couple of dollars has massive serpentine (??lizardite) that is light green in color, micaceous and platy, with scattered crystals of molybdenite.  A stringer of dirty white marble is attached.
So, in my curious mind serpentine is becoming really quite interesting.  I only wish that my knowledge of metamorphic petrology was sufficient to better understand the situation.

ADDENDUM:  I recently picked up a couple of more specimens from the Royal Green Quarry.  They include "serpentine" of a different sort, much less micaceous than the specimen above.  In addition the specimen contains fibrous ?tremolite.
Fibrous tremolite? [{Ca2}{Mg5}(Si8O22)(OH)2], an amphibole, with pyrite and molybdenite, Royal Green Quarry, Warren County, New Jersey.  


Parker, F. J., 2005, The Minerals of the Hunting Hill Quarry, Rockville, Maryland: The Mineralogical Record,

Drake, A.A. Jr., 1967, The Geologic Map of The Easton Quadrangle, New Jersey-Pennsylvania: U. S. Geological Survey Quadrangle map GQ-594, scale 1:24,000.