TIME

June 25, 2020

Duncan Miller

Our individual lives are so short, and geological time so long, that it is difficult to comprehend ‘deep time’, the most awe inspiring aspect of geology. Geologists often seem to work in units of a million years, as though that is the basic unit for the passage of time on Earth. So let’s make some effort to comprehend geological time – after all it is what makes geology tick.

Consider a rare, long-lived human life span of 100 years. There would be 10 000 of those in a million years. A physical example is to image a hypothetically rapid rate of erosion of something substantial like Table Mountain. The summit of Table Mountain is just over one kilometre above sea-level. If each year you were to shave off one millimetre, the size of a large grain of sand, from the top of Table Mountain within a million years it would have been eroded down to sea-level, not down to Tafelberg Road but to Camps Bay beach. (Fortunately it doesn’t erode that quickly.) You could play this game horizontally too. There are a million millimetres in a kilometre. Measure a straight stretch of road a kilometre long, then imagine marking it off in millimetres. It would take you a loooong time. Or take a large sheet of paper, one metre square, and rule lines one millimetre apart to make small squares. When you have a thousand rows of squares down and across, you will have a million small squares. If you have time on your hands, you could start numbering them…

Now that you have got to grips with the enormity of a million years, let’s think of the age of the Earth, which is 4 560 ± 50 million years. (No, we are not going to argue about how geologists know this. It depends on the same well-understood physics that makes nuclear power stations work, so is beyond rational argument.) Cape Town geologist John Rogers likes to illustrate the age of the Earth by stretching out a 4,5 metre measuring tape on the floor. There are a thousand millimetres to a metre; so 4,5 metres are four and a half thousand millimetres. If that represents the age of the Earth, each millimetre on the tape represents one million years – a whole, rapidly eroding Table Mountain in each millimetre. Or you can think of it as 4 500 km marked off in millimetres, with each millimetre representing one year. That’s Cape Town to Windhoek three times, in millimetres representing one year each.


A window into deep time – a petrographic thin section in crossed polarized light of part of the 4 560 million year old Korra-Korrabes chondritic meteorite from Namibia (width of field of view 1 mm)

That piece of Cape Granite in your hand solidified 540 million years ago. That’s an “awefully” long time ago. But in Barberton there are rocks dating to 3 500 million years. And in our museum and university collections there are meteorites that are 4 560 million years old, the oldest things any human being will ever hold in the hand.

 

The World of Tourmaline (in brief)

June 25, 2020

Peter Rosewarne

I’ve borrowed the title of a new coffee table book by Gerhard Wagner for this article as it seems appropriate in that the Tourmaline Group encompasses some 14 species currently and it is found in classic localities around the world. The idea for doing this article came from a comment from Jo that EXCO had raised tourmaline as a possible discussion topic. I also have and have had quite a few tourmaline specimens in my collection over the years and have attempted to limit dis...


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COVID-19 eclipses the Amazonite clouds

May 24, 2020

by Jay and JD Haasbroek

I know for sure it was last year some time, but that’s about all I am prepared to admit to any sense of time or space in these times.

As was usual back then Verna Jooste was visiting round the kitchen table with me, and Jay was busy with stones in her adjacent cutting studio and joining the conversation every so often, with a stone in her hand. Verna, a teaching jeweller and artist, is always interested in the stone in the hand. She comes from a family of diamond c...


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BLUE LACE AGATE FROM YSTERPUTS, SOUTHERN NAMIBIA

May 24, 2020

by Jo Wicht and Duncan Miller

For several decades small mines in southern Namibia have produced an attractive banded agate marketed as lapidary material. The major source has been a mine on Ysterputs farm, producing blue lace agate. It was promoted widely by the late George Swanson who owned the mine, so this material with its wavy blue and white lines is quite familiar. What is less well known is that the blue lace agate from Ysterputs is accompanied by several minerals forming aesthetic, c...


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HOW TO KEEP YOURSELF BUSY WHEN UNDER HOUSE ARREST

April 25, 2020

Duncan Miller

When you have finished the chores, tidied the garage, weeded the garden, washed the windows, painted the house and knitted the dog a winter coat, how do you keep yourself busy around the home during lock-down? You could take advantage of the enforced holiday to photograph and catalogue your mineral collection, even if you are not preparing it for sale.


Bonnie, decked out in MinSoc green (courtesy of Jo Wicht)

Photography of minerals need not involve very expensive equipment. My...


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GROTTOS, ANCIENT AND MODERN

April 25, 2020

Lesley Andrews

A grotto is “a natural or artificial cave used by humans in both modern times and antiquity” (Wikipedia). Grottos are fascinating subjects for a mineralogist – the article below deals with European grottoes, but there are also grottos of a different kind in north Africa and China.

Natural grottos are often found near water and may flood at high tide, such as the famous grottos around the Mediterranean and Adriatic Seas, often accessible by swimming or diving. Inland, grot...


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SOME REFLECTIONS ON STARTING A MINERAL SPECIMEN SALE WEBSITE

March 24, 2020

by Peter Rosewarne

 About a year ago I wrote an article on selling a mineral collection from South Africa, which was featured in the MinChat. One of the methods I listed, unsurprisingly, was setting up my own internet site. I didn’t go that route initially, relying on selling back specimens to dealers such as Hummingbird Minerals, John Betts Fine Minerals, Fabre Minerals and The Mineral Gallery, and some on Club Open Days. However, the former route seems to have run its course in terms of ...


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Richtersveld Revisited

February 24, 2020

Dave Hawes

I was lucky to be able to go on the trip that the club recently organised to the Richtersveld but unfortunately unable to attend the report back a few months later.

As I have been able to visit the area on numerous occasions, for a variety of reasons, since my first visit in the early 1980s I thought that I could share some of my experiences with the club.

While I had visited Namaqualand as a typical tourist to see the flowers, my first serious visit in the early 1980s was to del...


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A Gneiss Change: New trends in Scottish Lapidary

January 18, 2020
by Lesley Andrews

On a recent visit overseas, Richard and I travelled around the Scottish coast, including the Highlands and Islands. I found that nothing had changed weather-wise in the country of my birth – rain in the west, and wind in the east – but that new varieties of ornaments and jewellery are now available country-wide. The factors driving these changes are the increased number of potential buyers (tourist numbers have rocketed; on top of this there are now many on-line orders), ...


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FACETIPS - AN ‘EPIDOTE’ ANECDOTE

November 24, 2019
Duncan Miller

A few years ago, faceting friends of mine in Durban bought some green gem quality material sold as epidote or possibly peridot. It was nice clear green, and some pieces of rough still adhered to a matrix, "dug out of the ground right in front" of the vendor from Moçambique. The cutting and polishing was easy, apparently working like tanzanite. But the surface of the polished stone degraded quite rapidly, developing hazy spots, so samples were sent to me for identification.

The ...


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