Saturday, 15 June 2013

How to make a "Thin Section"

This post is about how thin section microscope slides are made, but before detailing the techniques used at Geosec it should be noted that there are many different techniques, and no one technique is correct, you just use the method that suits you. It is the finished item that is important, not how you get there.

I have several friends that make thin sections and we have an informal information exchange. We all use different methods, glues, machinery etc, and all produce good slides. I was never taught how to make slides, but found by trial and error a method that suits my requirements. My machines have been constructed in my workshop, and though crude by commercial standards they work well enough, and cost a fraction of the "standard" ones. All my slides are finished by hand, so any inaccuracy that comes from the machines is eliminated anyway.

So, Starting from a piece of rough rock, a small block 2cm x 2cm x 3cm is sawn by hand on a diamond saw.
The top surface need to be absolutely flat, so is lapped on a glass plate with fine silicon carbide grit.

Glue is applied and a microscope slide placed on the block. Any air bubbles in the glue are carefully worked out to the edge.

Geosec has moved from the traditional epoxy resin glues to UV setting ones, it is now necessary to put the slide/block under a hi-intensity UVlamp. The glue sets in a few seconds and is now ready to cut.

The slide/block is sawn as close to the glass as possible using a thin bladed diamond saw. The section is now 1mm to 0.5mm (500 microns) thick.

The slide is now mounted on to the first grinding machine. The slide is retained by vacuum in a slide holder and is held against a rotating diamond flat wheel. 

The arm reciprocates to ensure even cutting. This machine reduces the thickness to 200 microns.

The slide is transfered to the second grinding, or flat lap machine. 

 Using a finer grade diamond disc this can cut to almost the finished thickness, but there is always some score marks left by the cutting process, so the slides are removed when they are about 50 microns. The slide is now ready for hand finishing.  

The final stages are done by hand on a glass plate with silicon carbide grit, and repeated checking of the thickness by judging interference colours with the microscope.

Saturday, 8 June 2013

Pictures (experimental)

Here are three pictures that I have been tinkering with to see the limits of what can be achieved with the present camera set up, and to see whether a further investment in additional lenses is necessary.  As before these are zoomable images, click on the  icon, bottom left of the picture to get a larger view which you can zoom in and drag. ( they look good fullscreen, usually available on most browsers by pressing the F11 key)

First image is a volcanic rock from southern Germany known as Limbergite, really an exotic basalt. The bright crystal is titanium rich pyroxene showing differing types of zoning, and also amygdules filled with a radiating zeolite mineral. XPL view. The length of the big pyroxene is 9mm long

Next is from nearer home, from a basalt dyke on the Isle of Skye. Again some radial zeolites, and colourful pyroxenes, curved and branching. A very rarely seen texture. XPL view

Last another volcanic rock, this time from La Palma, the Cumbre Veija volcano. Well known to watchers of disaster documentaries as the potential source of a landslip that may cause a tsunami that could, in turn, devastate the east coast of USA. (Possibly). The rock is known as a phonolite and is silica undersaturated. This gives rise to a rare mineral called Hauyne (pronunced "how-een"), the pale blue mineral with good shapes, and the darkest colour concentrated towards outer edge. Also visible are brown amphibole and green pyroxene, probably aegirine, the whole in a matrix of feldspar laths, grains of magnetite and volcanic glass. PPL view as the hauyne is isotropic and would appear black in XPL.

The experimental nature of these is that these are taken at the highest magnification I can currently achieve with the current camera/lens setup. (no microscope). The first is a single shot, the bottom two are several shots stitched together with varying degrees of success.

Sunday, 2 June 2013



A short bit of science.(skip this bit if you want)

The basic difference between a rock and a mineral is that the former is made up of one or more of the latter. Rocks can be infinity variable as the proportions of the minerals vary. Minerals are much more precisely defined, and have a set chemical formula AND crystal structure. Different crystal structure produces a mineral that may be quite different, in colour, shape or other properties, even if chemically identical. These different forms are known as polymorphs
Different polymorphs are formed when the temperature and/or pressure at the time the crystal was formed differs from the 'normal' range.
When a new mineral is discovered the location is given the accolade of Type Locality and is registered in the official texts along with the Mineral Name. Any new discovery of the mineral, anywhere in the world and even if of better quality, is still referenced back to the original name and type locality.
Enough! back to Kilchoan.

Kilchoan is the type locality of not only Kilchoanite, but of two other minerals, Rustumite and Dellaite. see: Mindat for details if you are interested. Not bad when you consider there are only 30 type localities in the whole of Scotland and 110 in the UK (Cornwall has 35)

Kilchoanite is a polymorph of another mineral called Rankinite, itself quite rare, but not as rare as Kilchoanite. All three Kilchoan TL minerals are Calc-silicates, that is they contain Calcium, Silicon and Oxygen. The other two have a small amount of water in their structure, Kilchoanite does not. There are many other calc-silicates, some are of great commercial importance as they form in concrete, so this group has been much studied. Rustumite and Dellaite were discovered in laboratory test of man made substances, but were not know to occur in nature until they were discovered here.

So what is special about Kilchoan? and where can you see Kilchoanite?

The second question is not easy to answer, I have been looking for some time now, and have sent samples to be tested, but with only negative results so far. The site is reported to be on the north side of Glas Bheinn, the hill/ridge to the north of the Kilchoan House Hotel.

Glas Bheinn north aspect.

The Kilchoanite started life as a limestone, uplifted and heated by the intrusion that now forms Glas Bheinn. It would not have been at the surface at the time, but subject to considerable heat and pressure. The Glas Bheinn rocks are associated with centre 1, the earliest phase of the Ardnamurchan 'volcano'. Much later centre 3 was intruded nearby, and the limestone (or whatever it was at the time),  was subjected to another bout of heating and pressure, probably at somewhat different levels. This double metamorphism in differing conditions are what caused the unusual minerals to be formed.

So where is it? I thought I had found the location recently

But I was wrong! So I need to revisit the site and check again. How do I know I was wrong? That will be the subject of another blog entry.

To be continued.....

Thursday, 30 May 2013

Ardnamurchan Volcano.

Visitors arriving in Ardnamurchan by car, after 25 miles of twisting single track road through beautiful woodland emerge at a viewpoint at Camus nan Geall. This is the first view of the volcanic rocks of Ardnamurchan.
View Larger Map
Ahead can be seen Ben Hiant. This is sometimes mistaken for the volcano, it is in fact a series of rock layers starting from pre-cambrian metamorphic rocks, sedimentary layers and topped with basalts and a dolerite lateral intrusion (think sill) emplaced into rocks now eroded so that it is now the top of Ben Hiant. This last interpretation is the result of research only recently published. 

Volcanoes in this area, or at least the centres of them, tend to be lower lying areas of ground. Loch Ba on Mull, Loch Coruisk on Skye and Glencoe are all volcanic centres which are now valleys. Depressions originating from caldera collapses allowed glacial valleys to form and resulted in the glacial landscape you see today. Ben Nevis is an exception, also a volcano and site of a caldera collapse. 

The Ardnamurchan volcanic centre is again low ground, surrounded by the world renowned  'Great Eucrite'. The center itself is a small intrusion rising from boggy ground  (waterproof boots required). This is best reached from the Kilchoan to Sanna road, 1/2 mile before Achnaha. There is a quarry set slightly above the road on the right if you are heading towards Sanna. There is a short length of fence here. There is parking room for about 4 to 5 cars in the quarry. A track that leads to the abandoned village of Glendrian leaves the quarry on the Sanna side, and it is worth following this. There is a deer fence across your route, and an access gate is provided on this track. Just through the gate leave the track and bear left, you should be able to see where others have walked. The centre is the small rise about 200 meters in front of you.


It is an easy short walk and the view from the center is well worth it, especially in clear fine weather. As well as a magnificent view of the crags of the Great Eucrite surrounding you on all sides, the Isles of Eigg Rum and Skye can be seen, and also the Outer Hebrides on a clear day.

Saturday, 25 May 2013


 After several years of living not too far from Staffa I finally made it to the island. Previous visits having been thwarted by the weather, or lack of proper planning on my part. I must say it was worth the wait. Pictures do not do justice to the atmosphere of the place, and the allotted hour ashore was much too short. Below are some zoomable images, click on the  icon, bottom left of the picture to get a larger view which you can zoom in and drag.

This is the well known view of the entrance to Fingals Cave. The columns are larger than I expected, and a matt intense black, not quite the usual basalt black, and contain a few zeolites. Access to the inside of the cave is along the right wall.

Just before landing from the boat you pass a remarkable set of columns, known as Am Buachaille, or the herdsman. Aligned from vertical to horizontal with a groove in the centre, it challenges interpretation using the usual thermal gradients etc. The clear water shows a deep channel of water between, possibly a collapsed cave at one time?

Finally a view from the top of the island looking north. The headland shows two layers of basalt columns with a more normal flow between, the whole emplaced on a bed of volcanic ash.

Thursday, 23 May 2013

New Online Store -

As mentioned in a previous blog Geosec's new store is now LIVE! It can be found at
Selling slides started as a bit of an experiment in 2004, and ebay was an easy way to get started. Ebay has changed quite a bit in the nine years since, becoming increasingly restricting and expensive for sellers. Another 'Ebay Improvement' is scheduled for August, it seems to be an improvement to allow for increased seller fees in my (admittedly somewhat jaundiced) view. Time has come for a change. has taken a while to set up, as with any new software it takes a while to get to know, which boxes need to be filled and which can be safely ignored. I hope it is fully functional, time will tell. If anyone finds a fault please let me know, and I will try to fix it. 

On other matters Geosec has been assisting to host a geology tour group from Germany. Visits to Iona, Staffa, Mull, as well as Ardnamurchan. I am always happy to show off our outstanding geology, this area can compete with anywhere in the world, and beat most. The group seemed suitably impressed and the weather joined in by providing a sample of every type normally enjoyed here, often in very quick sucession.

Saturday, 18 May 2013

Iona Silverstone

The island of Iona is about 1km off the south west tip of Mull. it is world famous for its religious significance as one of the earliest centres of Christianity in Britain. Among geologists it is also known for being a source of Marble from a quarry on the southern end of the island. Less well known is another outcrop on the western side. This is different from the southern marble as it contains phlogopite mica. This gives the stone from here a "sparkle" and is known as Silverstone.
The outcrop is small and difficult to find. The marble does not occur in large slabs so this area has been little worked. It is quite distinct from any other marble I know. It contains Serpentine (from forsterite), diopside, brucite, and phlogopite in a calcite matrix.
Directios to the site can be provided for anyone interested in visiting it, please contact me via my contact page on It is a fine walk in good weather, from the ferry you need to allow 4 hours for the round trip, as there is plenty of interesting geology (lewisian) and to distract you, and beautiful views to photograph.

Wednesday, 8 May 2013

A wider audience....

To view Thin Section slides in the usual way requires a special microscope, equipped with polarising filters and other refinements not normally found on standard microscopes. These microscopes are not cheap, neither are the thin section slides themselves. Over the past few years Geosec has been trying to find ways to spread the interest in and enjoyment of viewing our slides, to people who do not have access to a microscope and slides.

Photographs are the usual means, but are limited in how much they can convey. There is no movement or change of colour, you cannot 'explore' the slide by moving around it and trying different focus and other settings.

At Geosec we are currently experimenting with video, zoomable pictures and other ideas as much as time permits when not actually making the next batch of slides for sale. An interactive software program, the Geoscope, was tried, but ran into problems with the web browser programs that the program ran in, being automatically updated and stopping it working. 

Ammonite from Madagascar 

So, still work in progress....

Monday, 6 May 2013

Last postings and camera information

As mentioned before there will now be a temporary lull in postings. I will be spending some of the time in publishing on the Geosec Slides website more high resolution images of thin sections.

I now have more than 100 of these waiting for processing, and intend to add descriptions of the minerals, to the best of my abilities. The aim is to aid you, the buyers and anyone interested in minaralogy, to understand what you see. The usual name for these images are photomicrographs, i.e. photographs taken through a microscope. This is not exactly correct in this case. No microscope is involved, the camera is a Canon EOS 60D and the lens a Canon 60mm macro (1x). Polarizing lenses (linear not circular) are mounted either side of the slide.

The advantage of this is the wide field of view, about 20mm, which enables most of the specimen to be photographed. Using a microscope with the lowest power objective usually available, 4x, gives a field of view of about 6mm. This is fine for closeups of individual crystals, but does not give an impression of the specimen as a whole.

Sunday, 5 May 2013

Vesuvius Lava 
GEOSEC. Future plans as of 5th May 2013.

As you may have noticed the number of slides offered for sale has reduced recently. This is only temporary, and there are a number of reasons.

Firstly I am assisting with a geology group who are touring the area, and will be occupied full time with this all next week. 13th to 22nd. Although I will still be here, burglars please note, I will not have time to manufacture anything.

Secondly, much of my machinery is in much need of some maintenance. The machines get used with only minimal servicing for months without break, and some are now urgently requiring attention.

Finally, I am working on a couple of projects, high resolution photographs, movies, and website http://www, development. I am also reviewing how the slides are sold, I have relied much on ebay in the past, and I will still be using it in the future, but there may be alternatives that are more appropriate and can be offered as an alternative. Follow this blog for more information.

Rob Gill.

Saturday, 4 May 2013

Welcome to Geosec's Blog.
This blog will provide current information for Geosec Slides. Thin section geology (petrology) microscope slides produced here in Achnaha. More information about Geosec Slides can be found by clicking HERE