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From Aug 2006 - Nov 2013 WeDig provided a live forum for diggers & fans of Vindolanda. It has now been mothballed and will be maintained as a live archive.

Here you will find preserved 7 years of conversation, photos, & knowledge about a site many people love. Vindolanda gets under the skin. (Figuratively and literally as a volunteer excavator!) It's a place you remember, filled with people you remember!

Thanks for 7 great years!

Viewing Single Post From: Mike's Geoblog
Mike McGuire
Member
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Although we can tell quite a lot about a piece of rock by looking at the outside, especially with a hand lens, a way of looking inside it is needed to really learn about it. We need to know what minerals make it up and in what proportions, what sizes and shapes the grains are and how they are related to each other. The most widely used technique for this is to prepare whatís called a thin section.

A polished section of the rock is stuck to a glass slide and ground down to a thickness of just three hundredths of a millimetre. Polarised light is passed through it and examined with a microscope. At this thickness, most minerals are transparent, or at least translucent. The sizes, shapes and relationships of the grains can be seen and many of the minerals can be identified.

But the really clever bit is then to pass the light coming out of the microscope through a polarising filter set at right angles to the original light beam; this is called crossed polars. This arrangement would normally be expected to cut out all the light, but many minerals rotate the light in a way which is characteristic of the mineral and its orientation. As the thin section is rotated between the crossed polars, these mineral grains go black and then become visible again.

The light which passes through the quartz grains which make up most of a sandstone is rather plain shades of grey under crossed polars, but other minerals which may be present have more dramatic effects. Micas glow with brilliant blue and pink colours and feldspars have dark and light bands, sometimes just one of each per crystal, sometimes many parallel bands and sometimes bands at right angles in whatís called a tartan pattern. Iron oxide looks brown in transmitted light but goes black under crossed polars. Sometimes the rock from which the sand was derived contains rare minerals, such as zircon or tourmaline, which can be identified in the sandstone thin sections. Examining the thin section can also enable us to determine what the cement material is which binds the sand grains together, to see how porous the stone is and to see how much clay there is in the pores.

The geologists from the British Geological Survey in Edinburgh who came to Vindolanda a while ago took away samples both from some of the quarry sites and from a number of stones on the site which Andy and his team had selected. The site samples represent a good spread of phases of building, both in the forts and in the vicus. Thin sections were made but the BGS have not yet had time to examine them carefully. So we have arranged that I should borrow them for a few months and I have now had a week to examine them carefully. Although my microscope is not nearly as sophisticated as the ones at the BGS, I can get useful information about the size and shape of the quartz grains and have identified numerous examples of the different types of feldspar and mica and have even found a few tourmalines. The porosities of the samples vary quite a bit, although I donít have an easy way of putting a value to this, and I can see clumps of very tiny clay minerals in many of the pore spaces.

What Iím doing now is trying to see if there are any consistent differences in any of these characteristics between the different quarries and the different phases of building. There does seem to be a reasonable chance of getting some archaeologically useful information from this approach but it will be some time before we can come to any definite conclusions.

My microscope doesnít have a camera attachment so I canít show you any pictures of my own, but there are lots of examples of thin sections on the internet Ė put Ďsandstone thin sectionsí into your search engine and youíll get lots of interesting hits, mostly from university sites. The ones from Oxford (earth.ox.ac.uk) seem to be quite good.

Sadly, I have to be away from the site for much of August but Iíll try to make at least one entry in this blog before then and to put in a couple of good ones at the end of the season.
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