The Beauty of Minerals

Last week I had the good fortune to share an office with Dr Noel Worley, president of the Yorkshire Geological Society, as he cast an expert eye over the Burgess Collection.

Noel and his expert eyes.

Noel and his expert eyes.

The collection includes the research papers, notes and 1966 thesis of A.S. Burgess: ‘The Geology of the Millers Dale Area with Special Reference to the Igneous Rocks’. In addition, the collection also includes 156 thin section microscope slides prepared from field samples by his father, H.W. Burgess. According to Noel, this is one of the most complete collections of Derbyshire Carboniferous igneous sections that exists anywhere.

Some of slides, prepared by H.W. Burgess from his son's field chippings.

Some of the mineral slides, prepared by H.W. Burgess from his son’s field chippings.

Preparing rocks and minerals to a thickness of 0.03mm allows them to be studied through a petrological microscope, such as the one Noel used to check and identify the samples. Having little background in geology or mineralogy, I was unfamiliar with this piece of equipment, and suitably amazed by its capabilities. This got me thinking about how Collections in the Landscape could also help explain the science behind the collections.

Highly amydaloidal basalt with centripetal succession of gret semctite and green chlorite (Crossed Polars)

Highly amydaloidal basalt with centripetal succession of grey semctite and green chlorite (Crossed Polars)

Stained Glass? No! This photomicrograph shows the vivid, striking images produced when viewing the interference colours of a section. As well as being visually stunning, viewing samples in this way can reveal a wealth of information including the composition of different minerals.A petrological microscope uses polarised light and a rotating stage to examine mineral sections. It uses two polaroid sheets, one of which, the analyser, can be moved in and out. Polarisation restricts the electromagnetic vibrations of light to single plane of movement. Confused? Here’s an amazingly simple example even I understood!

Noel's Petrological Microscope

When examined under plane polarised light (analyser out), mineral sections are seen in their natural colour and features such as crystal shape, transparency and fracture can be observed. The limestone section below is seen in this manner. Forgive the quality, it was taken down the microscope on my phone and is pretty terrible compared to Noel’s experienced snaps.

Limestone section viewed with the analyser out (plane polarised light)

Olivine Basalt section viewed with the analyser out (plane polarised light)

When the analyser is included (analyser in) the sections are seen in a cross polarised light. Instead of the natural colours we see the interference colours. As the rotating stage is turned, colours move between a maximum colour and extinction (where they show no colour i.e. black). The nature of these points allows certain minerals to be identified.

The picture below is the same sample as above, but with interference colours. Again, apologies for my awkward camera work!

Limestone section viewed with the analyser in (Cross polarised light)

Olivine Basalt section viewed with the analyser in (Cross polarised light)

I’m fascinated by the aesthetic beauty of these mineral samples, as well as the incredible amounts of information that be gleaned from them. I’d like to think Collections in the Landscape will be able to feature methods of scientific enquiry and give our collections better, richer scientific context.

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