To promote a wider interest in the science of geology through organised lectures, field excursions and social activities.
To provide a link between the amateur, the student, the teacher and the professional geologist.
To foster interest in geological sites within the area with a view to their study and wise conservation.
To establish and maintain good relations with organisations that have common interests.

 

 

 

 
 
 

 

 
 The Deleff Collection of Giant Crystals and minerals is now to be found in the Museum National D'Histoire Naturelle in Paris. After we had spent the morning looking at the treasures of the Ecole des Mines (see here) the displays were a complete contrast. Instead of rows of display cases housing specimens gathered over many many years from all over the world we were walking round and gazing up at the huge crystals.

One area in the east of Brazil was the source and since the 1960's one man was responsible for their preservation. Minas Gerais, an area about the size of France. had long been known as the richest mining state in Brazil; diamonds, precious gemstones and gold were abundant. Victorian explorer Richard Burton turned his attention to the Americas after his African adventures and records a visit he made, with his wife, down a gold mine already at 3000' in the 1860's. *

The land comprises an area of large plateaux incised by deep valleys carved by rivers running off the Serra da Mantiqueira mountains in the south.

Bulgarian adventurer Ilia Deleff was born in 1921. His businessman father had been a great traveller and Ilia was keen to follow in his footsteps. After about ten years of wandering in South America he had managed to join the local miners and became a successful diamond miner in the Amazon. It was about now that hundreds of thousands of miners were turning their attention to the vast deposits of quartz and other industrial minerals needed by the new piezoelectric industries.


Deleff saw that miners were breaking up the huge crystals being discovered even if they were not suitable for industrial purposes. For the next 25 years he spent his time and a large amount of money building up his collection. It was a constant battle to preserve the crystals in situ and then he was faced with the problems of removing them across such difficult terrain. The 1970's saw specimens weighing anything from a few 100 kgs to 2000kgs being discovered.

Blue topaz and beryl, pink beryl and rose quartz, green amazonites, rock crystal and tourmalines all feature in the collection. Further south in the Rio Grande do Sul large amethyst lined geodes were being added to the collection. It became quite common for miners to fight over ownership of the best specimens and money had to be passed over to calm tempers as well as buy the minerals.

The giant crystals are found in pegmatite veins which are silicate rocks and they form from the residues of a granitic magma which are able to move into the surrounding rocks after the parent magma has crystallised and reached a stable level. The host rocks will have been heated by the nearby granite and if these conditions remain constant for a very long period large crystals may grow. A number of veins altogether make up a pegmatite field.

The granites which were the source for the Brazilian minerals and the gem bearing areas of Mozambique, Madagascar and eastern Siberia were emplaced under an already ancient mountain range that extended through that area long before the continents separated. When this eventually happened the Brazilian part of the range began its long move west and, because it was now on the trailing edge of a plate remaining in a tropical regime where rates of weathering are high, erosion has revealed these mountain roots. The ages for these pegmatites have been measured at 490/450MY.

The large geodes with dark violet amethysts come from the Jurassic basalt where the gases trapped by the thickest flows formed cavities and once again slow rates of cooling allowed time for the crystals to form from hot solutions. The colour comes from iron in the liquid. Two geodes in the display were once a whole specimen totalling 700 kgs.

Our visit to Paris was a memorable experience. When examples are quoted to illustrate the vast length of geological time it is usual to mention sites where major unconformities are seen but the sight of these amazing specimens provides an equally valid, if more subtle, way to express the same idea.


* From the Sierras to the Pampas. Frank McLynn. Pub. Century. ISBN 0-7126-3789-3