Geology and Selected Minerals of the Diamond Hill
Quartz Mine
Antreville, South Carolina
By Mike Streeter
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 relatively scarce 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 series of late Paleozoic Era tectonic events. The Antreville pluton,
itself, was intruded during the Taconic orogeny that extended from late Ordivician 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
quartz-lined pockets. Much of the quartz and saprolite is heavily stained with
red, brown, yellow and black iron and manganese oxides.
The number of different 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 quartz type requires it 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 types in one place, then the Diamond Hill
Quartz Mine is your ticket. You can obtain more information and pictures of the
Diamond Hill Quartz Mine by clicking on the following links:
January 10, 2004 Diamond Hill Quartz Mine - Again Report
December 24, 2003 Diamond Hill Smokey Quartz Report
November 22, 2003 Diamond Hill Field Trip Report
Georgia Mineral Society - Diamond Hill Mine
CLICK THE LITTLE MINER TO RETURN TO THE FIELD TRIP PAGE
