Slides from your rocks


Making Slides from customer supplied rock.

Suitable rocks for making into thin sections.

I have been asked on several occasions if I will cut sections using samples supplied. This poses a few difficulties, in theory it should be no different from making slides in the normal way, but I find that what has become second nature to me when collecting, only picking up rocks that are suitable, requires explanation. Nearly any rock sample can be made into a thin section, but some take much work and time, and basically that makes it uneconomical.

Set out below is a list of what makes a good slide, but bear in mind that only about half of what I collect I find to be any good, and I am supposed to know what I am doing! As the princess discovered, you have to kiss a lot of frogs before you find a prince. What appear on the Geosec Shop are mostly princes, the frogs are in the waste bin.

If you have some special rock the most satisfactory way is for you to supply me a suitable sample, I will make one slide for you for free from the sample, and sell the rest on ebay in the normal manner. If the sample is unsuitable, I can return it, but it would be at your cost. Please send a list of samples before dispatching anything as I can often advise suitability. Also please send details of source, rock type if known, age and anything else I can use in the item description.

What makes a good sample?

  1. It must be big enough. Fist size is good, bigger is quite acceptable. The first stage is to cut a 20 to 24mm cube from the sample, so ideally it needs to be double that size in every dimension. I know it is going to cost more to ship, but trying to make sections out of small samples is expensive. Only exception is really rare stuff, but that would have to be dealt with on a piece by piece basis. It is difficult to overstress this, fist size is really the smallest acceptable, bigger is better.
  2. The right shape. Important for schists and rocks with layers or foliation. I usually cut these at right angles to the layering, so need to cut the cube with one side parallel to the layers. Again make sure it is thick enough. Schist naturally forms flat slabs, min thickness would be about 30mm as it is sometimes difficult to get thicker pieces. Please check that it is not going to cleave too readily along the layering.
  3. Not crumbly or porous. Often a problem with volcanic rocks. Can be overcome but makes more work. The rocks I offer from Stromboli and Montserrat are all impregnated with resin, otherwise they just fall to bits.
  4. Not cracked. If you can see a crack in the sample it may not be suitable. If you have used a hammer to break the rock you may have induced cracks that I will find out about later. Best to collect from scree, beaches, or prise bits out .
  5. Avoid rocks containing any quantity of oxidised/metallic element (unless that is not the reason for wanting the section). May look pretty in sample, but will be opaque in section.

Some examples

Most Plutonic rocks are good, (although Granite can sometimes be uninteresting) Most Volcanic rocks and dyke rocks, although Rhyolite can be just glass, some sedimentary rocks, but not chalk. Metamorphic, varies, schist can be good but difficult often splits along layers, granulites and gneiss ok, phyllites and slates, usually too small to be interesting. Hornfels ok, quartzite and cryptocrystalline quartz, uninteresting and usually very hard.

Finally please let me know when you have dispatched a parcel, so I know to expect it, and let you know if it does not arrive.

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