Wednesday, September 16, 2009

African Rockhounds

Betsy and I have been members of our local rock club since 1977. The Earth Science Club Of Northern Illinois, ESCONI, played a large role in our family’s lives as the older members shared their finds and their knowledge with Becky and Lisa as they grew into young ladies. So it should come as no surprise that we joined a club in Cape Town this year.

The Cape Town Gem and Mineral Club has a narrower scope than ESCONI, focusing on minerals and lapidary. They are quite active, holding monthly sales, swaps, programs and barbecues as well as the occasional field trip. The field trip opportunity was the final straw on our decision to join as it was a “members only” event.

Vredendal is a medium-sized town about a three hour drive north of Cape Town. We were to meet the other members of the club at nine in the morning at the mine. Since we had to drive some narrow roads through mountains to get there, we went up the night before.

You can see our Volvo wagon parked in the middle of the line of cars waiting for the signal to move into the mine.

First, of course, is the mandatory lecture covering safe practices and what we might expect to find in this quarry. Notice the small quartz crystal that the club president is holding. Perhaps that specimen established a level of expectation within
the prospectors?

So we went in to the Vredendal dolomite quarry.

“The Cape Lime Company dolomite quarry is really a series of quarries excavated into the rocky bluffs on either side of a shallow gorge cut by the Wiedo River as it approaches the town of Vredendal. These hillsides have been terraced in various places by the mining activity, exposing the blue-grey dolomite rock, which is extracted and crushed for use in high temperature resistant refractory ceramics.”

“In one particular location there are at least two parallel quartz veins which host all the interesting mineralization. The veins in the “quartz” quarry are about 1m apart, undulate somewhat, and vary in thickness from a few centimeters to pockets up to 0.4m in diameter. The pockets contain the well-crystallized mineral specimens, while the intervening vein material is often very sheared, with the appearance of being crushed. When they have been excavated, it is clear that the pockets are elongated pod-shaped features within the veins. The quartz veins also underwent both shearing and tension, to create the voids, which subsequently formed the crystal pockets. The pockets themselves are filled with orange sandy clay into which crystals project from the walls, with some “floaters” or loose doubly terminated crystals in the clay itself.”

“Excavation of the pockets is hazardous because the rock above the veins is severely fractured by explosives used to break open the face. In some places the vertical quarry walls are several tens of meters high, so the risk of falling rocks is considerable, especially with people working on the face at various levels. It is mandatory that anyone working on the quarry face must wear a hard hat. The pockets may be narrow, but some of them reach at least 2m in length and diggers sometimes can only be identified by their shoes. The soft sandy clay is dug out using a variety of tools, including discarded pieces of iron found around the quarry. Loose crystals are recovered from the clay itself, while crystal clusters must be pried off the pocket walls.”

Some of the clearer quartz crystals include red, golden, or black rutile (TiO2) needles. We found one but needed some bright sunlight and imagination to really see the needle.

There are many other collectible minerals along with the quartz crystals. Crystals of dolomite, calcite, and adularia feldspar (shown) are also found both in the pockets and scattered on the ground as a result of the mining operations.

We also enjoyed the base rock with frequent displays of the dendrites you find in limestone formations. They look so much like actual moss growing on the rock.

We had a great time, made some new friends, got scraped up a bit – altogether terrific.

The quotes in this journal entry are from an article in the March 2006 edition of the South African Lapidary Magazine, used with permission of the editor (Allan Fraser) and the article author (and our good friend, Duncan Miller).


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