Copyright March 2004
Chrissy and I have spent a great deal of time at the Diamond Hill Quartz Mine near Antreville, South Carolina over the past several months. Chester Karwoski, who purchased the property in 2003, brought in some heavy earth-moving equipment last fall and has opened up some new collecting opportunities. On our most recent trip to the mine on February 28, 2004, members of the Rome Georgia Gem and Mineral Society and the Southern Appalachian Mineral Society joined us. Since it is no secret that that I am a geologist by trade, I am often asked geological and mineralogical questions about collecting locales. While digging at Diamond Hill, I was asked to explain how the rocks and crystals formed. Now there's a $64,000 question for you! I answered the question as best I could at the time. Since then, I have researched the literature to obtain a more detailed explanation that I would like to share with you.
Diamond Hill has an amazing variety of different quartz crystals including amethyst, smoky, skeletal, milky, clear, and phantom. The mine has also become known for the rare phosphate mineral, cacoxenite (pictured on left), first discovered at the mine by chance in late 2002. The quartz crystals are generally found in clay filled pockets in a series of large quartz veins that trend in a northeast to southwest direction. The quartz veins were emplaced into a granitoid pluton (Antreville Pluton) as a result of a series of late Paleozoic Era tectonic events. The Antreville Pluton, itself, was intruded during the Taconic orogeny that extended from Late Ordovician to Early Silurian Periods. Heat and pressure from Paleozoic regional metamorphism transformed the rock into granitoid gneiss. Silica-rich hydrothermal fluids entered fractures within the metamorphosed pluton and deposited the quartz veins. These veins must have contained voids that would later become sites for quartz crystals to form during a series of subsequent hydrothermal events. All of these geological processes took place miles beneath the present earth's surface. It took over several hundred million of years of erosion to uncover the rocks. The granitoid gneiss has been deeply weathered resulting in the development of a thick soil zone made up of rock weathered in place. Such in situ weathered rock is called saprolite. Oxidation of iron bearing minerals formed clay that was transported by infiltrating meteoric water into the quartzlined pockets. Much of the quartz and saprolite is heavily stained with red, brown, yellow and black iron and manganese oxides.
The wide variety of quartz crystal varieties that exists at Diamond Hill is nothing short of remarkable when you consider that the actual collecting areas occupy a total of less than 3 acres and that each variety requires its own unique set of conditions to form. The three most sought after varieties of quartz at the mine are skeletal, smoky and amethyst. Skeletal quartz (also known as elestial quartz) exhibits a layered or ribbed pattern. Its appearance gave rise to the term "skeletal" as the crystals resemble what someone with a good imagination would expect the skeleton of a quartz crystal to look like (not that they exist).
The patterns exhibited by skeletal quartz represent internal and external dissolution-growth features resulting from unstable conditions during crystallization. At the time that the crystals were forming, the conditions were such that the crystals would alternate between growing and dissolving. This "two-step forward and one-step backward" process resulted in the unusual step-like appearance. Very large plates and clusters of skeletal quartz can be recovered from quartz veins in the eastern end of the upper pit at Diamond Hill. Like much of the other quartz varieties at Diamond Hill, many of the skeletal quartz crystals exhibit an overgrowth of clear quartz indicating a later phase of crystallization. This later crystallization forms "phantoms" where you can see the tips of skeletal quartz crystals beneath a clear crystal overgrowth that exactly mimics the underlying crystal.
Smoky quartz is generally transparent to translucent quartz that is gray to black in color. The characteristic color of smoky quartz occurs when rock crystal quartz is exposed to natural radiation from radioactive elements or adjacent radioactive rocks over long periods of time. The process by which this occurs is not completely understood but it has been theorized that the color is the result of altered oxidation states of the silica molecules caused by radiation. The granitoid pluton would be the most likely natural source of the radiation. Smoky quartz seems to be most concentrated in the lower or western pit at Diamond Hill. Much of the smoky quartz appears to have grown in phases represented by larger blocky crystals growing on top of smaller crystals. Thin films of silica overgrowths are commonly found on top of the smoky quartz plates. In some cases, it appears that the overgrowths have been partially to completely dissolved, indicating an unstable environment during or after crystallization.
Amethyst is translucent to transparent purple quartz. The purple color is thought to be the result of two factors: 1) small amounts of iron impurities at specific sites in the crystal structure and, 2) exposure to natural radiation in a manner similar to smoky quartz. Again, the process by which this occurs is not fully understood but it is known that exposing amethyst to sunlight for a long period of time will cause the purple color to fade. Therefore, it can be concluded that it isn't the presence of iron alone that causes the purple color or else the mineral would not fade simply due to sun exposure. Amethyst at Diamond Hill generally occurs as larger secondary crystals on top of translucent to transparent smaller quartz crystal druzes in quartz vein pockets in the central portion of the mine. Local rockhounds use the term "jumping bigger" to refer to this phenomenon and as a key to finding amethyst while digging. My experience has shown that amethyst crystals are almost always found pointed downward in a pocket and that a hint of purple color can sometimes be found in smoky quartz crystals.
If you are looking to find a collecting location that offers a rare opportunity to collect a variety of quartz varieties in one place, then the Diamond Hill Quartz Mine is your ticket. More information about Diamond Hill can be obtained on our website at www.mcrocks.com. Proceed to the "Field Trips" page to find several reports including pictures of the mine and minerals. Additional information, including directions to the mine, can be obtained on the Georgia Mineral Society website at ++++ TBD ++++
Mike Streeter works as a geologist for the State of North Carolina and is the author of "A Rockhounding Guide to North Carolina's Blue Ridge Mountains", April 2003. Check out his website at www.mcrocks.com.
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