Striking gold

Nagyágite SEM

For the last six weeks, Oxford Earth Sciences undergraduate Charis Horn has been identifying mystery minerals in the Museum’s collections, and she’s struck gold! More precisely, she’s found rare gold-bearing crystals of nagyágite and sylvanite on a specimen which for centuries had been mis-labelled as the common lead mineral galena.

Nagyágite is composed of gold, lead, sulphur, antimony and tellurium and forms metallic grey crystals. Sylvanite is made up of gold and tellurium with a little silver, and is pale, silvery yellow. These minerals and their surrounding rock matrix indicate that the specimen is from the gold mines of Săcărâmb, in Romania, formerly known as Nagyág –  the place where nagyágite was first discovered.

Charis on SEM

Charis using the scanning electron microscope

Charis is one of a number of interns funded by the Higher Education Funding Council (HEFCE) through the University’s Internship Programme to learn more about academic research.

Rocks and minerals have always fascinated me, which is the reason I chose to study Earth Sciences at university. I find it amazing that the history of this planet can be read in the geology beneath our feet.

Many of Charis’s samples were put aside for a bit of extra work many years ago because the minerals on the specimen were potentially more interesting than the labels might suggest. Some can be identified by looking at physical characteristics such as colour and crystal shape, or by testing for properties like hardness and magnetism.

Others are much more challenging, and Charis has been using an analytical scanning electron microscope (SEM) to see close-up images of minute crystals to find out what chemical elements they are made of. There’s an SEM image of the nagyágite crystals, seen in white, at the top of the post.

Another of her discoveries has been a rare lead-bearing silicate mineral called hancockite, which was found on a specimen supposed to be from ‘Glen Coe, Argyllshire’. The orange-red crystals are less than a millimetre long, and hancockite is known from only one place in the world: Franklin mine, in New Jersey, USA.

It seems our specimen comes from Franklin mine too, and is the first sample of hancockite in the Museum’s collections. So both specimens must have had their labels muddled up in past centuries.

I have really enjoyed learning more about mineralogy during my time here at the Museum. Realising that something is far more exciting than it first seemed is definitely a great way to end this internship!

Monica Price – Head of Earth Collections

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