Friday 19th January 2018
Evening Meeting and AGM - Talk by Bill Bagley - "The Wonders of Calcite"
Eighteen members attended the first meeting of the year despite the wind and rain. The AGM was followed by a very interesting talk by Bill Bagley in which he gave an overview of the mineral calcite. Bill took us through the processes of formation including a short video clip of the rare type of volcano producing carbonatite i.e. Doinyo Lengai volcano in Tanzania. With photos he explained calcite's structure and colour variations. That the most common crystal forms are scalenohedra, and rhombohedra and calcite readily cleaves into rhombohedra. It may also occur as prismatic crystals. The crystals may be tabular, acicular, prismatic and platy. Calcite can occur as crystals, stalactitic, massive, earthy, as aggregates or in geodes. The colour of calcite can vary from colourless through white, yellow, brown, reddish, bluish to black. It is also one type of mineral that shows birefringence.
The talk, along with the numerous impressive specimens that Bill brought from his own collection made for a very interesting evening.
The next meeting will be on Wednesday 14th February when Dr. Geoff Steel will give a talk entitled “The Origin of Atoms”
Wednesday 1st November 2017
The next meeting will be on Wednesday 17th January. This will be the AGM followed by a short talk by Bill Bagley entitled: The Wonders of Calcite.
Thursday 12th October 2017
At the last meeting Tony Thorp gave an illuminating talk on geological thin sections, the history and practicalities.
It was Henry Clifton Sorby (1826-1908) whose work laid the foundations of microscopical petrology which became the cornerstone of geology. A man of independent means, he took an interest in microscopy and following in the footsteps of William Nicol and sir William Brewster he learned to make transparent sections of specimens like teeth and bones for observation under a polarising microscope. Of greater importance was the fact that he made thin sections of rock and was the first person to appreciate it's value. His thin sections were 0.03mm (30μ) in thickness which is now the standard.
Tony went on to explain the use of the polarising microscope and the property of birefringence. In other words, as polarised light passes through a thin section the different minerals both refract and often re-polarise the light, so that light waves leaving a section are no longer aligned. By inserting a second polariser at 900 to the first all lightwaves that have not been re-polarised will be cut off and appear black. But any that were re-polarised will give a strong colour. Hence the patterning seen when observing a section.
Finally advice was give on how to make your own sections at home. I should add that Tony's ingenuity on achieving this was remarkable.
At the next meeting Michele Becker will give the ins and outs of bricks and brickmaking.
Thursday 14th September 2017
At the August meeting David Pannett ( Shropshire Geological Society) explained how many of the landscape features observed in the Severn Valley are the result of glaciation. He explained how glaciations have occurred a number of times in the past 2-2.5 million years. The last of these being the Devensian from about 120,000 to 11,000 years BP.
The ice from these glaciations have given rise to many features to be found in the landscape, including moraines, kettle holes, outwash basins and river terraces. The origin of these features was explained. He took us on a trip up the Nant-y-Llyn valley to Pistyll Rhaeadr explaining the stepped profile of the valley, the corries and the classic hanging valley that is Pistyll Rhaeadr.
The talk was interesting and informative.
The next meeting will be on 21st September when Dr. Sara Metcalf will give a talk entitled: Mesozoic Mammals.
Thursday 31st August 2017
At the last meeting club secretary Bill Bagley gave a very informative talk on fossil wood. The talk commenced by a look at petrified forests throughout the world, from those in the UK at Glasgow, Brymbo and Lulworth Cove to those in the national park Arizona and on the Greek island of Lesvos. Bill then went on to describe the process of petrification which occurs in two stages of permineralisation followed by replacement and petrification. The original plant material may be replaced with silica, calcite or pyrite or another inorganic material like opal.
The talk ended with Bill showing the audience examples of fossil wood for sale on the internet and although fine specimens were a bit outside the pocket of most of us!
The next talk will be on Wednesday 20th September when Tony Thorp will give a talk entitled “Making and Using Thin Sections”