| One
of our new GMS members, Bill Spencer, called me to see if I could help
identify a mineral that he found at a construction site off of Peachtree-Industrial
Blvd. In Gwinnett County. We agreed to meet on a Saturday morning
near the site. He showed me a coffee can full of a beautiful sky blue mineral
lining vugs in what appeared to be a graphite schist. Bill had tentatively
identified the mineral as Chrysocolla. Well I was intrigued as Chrysocolla
would be a rare find for Gwinnett County. So off we went to visit the site.
Geologically, the site is located just inside the Brevard Fault Zone on
the northern edge of the Wolf Creek Formation. Geographically, the site
is just south of the city of Duluth, Georgia. The host rock is thinly bedded,
highly foliated, graphite schist with abundant pyrite. (I believe another
GMS member had reported this site). The light blue mineral was found as
a pod that looked like a secondary fracture filling as it cuts across the
general strike and dip of the host rocks. We collected as much of the mineral
as we could. My thinking was that this blue mineral could indeed
be Chrysocolla or perhaps one of the iron or copper sulphates that are
common in the area. I knew from a previous investigation that Chalcopyrite
is present in the Wolf Creek Formation. If the mineral was confirmed as
Chrysocolla, the Chalcopyrite could be the source for the copper. I brought
back some samples to my lab at home and did some tests. I found that the
mineral was not soluble in water. This eliminated many of the iron or copper
sulphates. The borax bead test was positive for copper (green cold
bead, blue hot bead). Testing with the harness points put the mineral in
the hardness range for Chrysocolla as did its physical appearance, that
of no visible crystal structure and botryoidal habit. The charcoal
block test was interesting! Roasting a bit of the crushed mineral in the
blowpipe oxidizing flame produced a faint white sublimate on the block.
No green color to the flame. Back to that faint white sublimate in a minute.
Moistening the roasted mineral with HCL (hydrochloric acid) and re-heating
produced the characteristic azure blue flame of copper chloride. Another
positive test for copper! Everything was pointing to Chrysocolla except
for that faint white sublimate on the charcoal block. Looking at the mineral
very carefully through the stereo microscope revealed a thin coating of
micro-crystalline gypsum covering the light blue mineral. In fact I found
some small vugs lined with micro crystals of gypsum. I believe it is the
sulfur component in the gypsum that was causing the white sublimate on
the charcoal block. The mineral lokks to indeed be Chrysocolla. Also found
with this mineral were several small pods of Bornite as well as crystals
of pyrite. All-in-all a very enjoyable mineral detective story using
the “old ways”. A check of the mineral literature shows this is only the
second reported occurrence of Chrysocolla in Gwinnett County.
Photographs by Dr. Babulski |
 
BILL SPENCER AND FAMILY |
