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The Red Stripe Test

Guy van Rentergem

(Apr 24, 2004)

I won't discuss the survey that was done at Roehampton School Cave on April 3, 2004, here, but I will talk about the RED STRIPE TEST.

In the Roehampton School Cave, we saw these strong bedded layers of brown rock. This rock seemed to be more resistant to erosion than the other limestone. So what was it? As a caver, you don't carry a laboratory with you to analyse rocks, so we have to be inventive.

But first let me tell you something about the hardness of rocks and minerals. What is hardness? It is the resistance of the surface of a mineral or rock to scratching. All right, what can we do with it? Now, depending on the chemical composition, each mineral has a well determined hardness. Early in the nineteenth century, there was that clever man, Mr F. Mohs, from Austria, who made a hardness scale based on a series of ten fairly common minerals. The softest mineral got a number one and the hardest a number ten. So, here we have as an amateur mineralogist a very nice tool to check what mineral or rock we have in our hands.

The minerals of Mohs Hardness Scale

1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Orthoclase
7. Quartz
8. Topas
9. Corundum
10. Diamond

If for instance our unknown species of mineral can scratch Calcite but can't scratch Fluorite then the hardness of this specimen must be between 3 and 4. But we normally don't carry those minerals with us and do we need all those different hardnesses? No, because as a caver you will mostly see rocks based on Calcite or on Quartz. So we need a reference material somewhere between hardness 3 and 7. And what is very abundant throughout the island? The Red Stripe Bottle!! These multifunctional bottles have a hardness of 5.5!!

The different steps of the RED STRIPE TEST!!!

Red Stripe

Step 1. Search for a fresh unweathered surface on your mineral or rock. Best thing to do is to break the rock and work with the fresh fracture. Weathered rock or mineral often produces a soft rind on the surface.


Step 2. Take the Red Stripe bottle and search for an unscratched surface. Hold the bottle firmly in one hand and the mineral or rock in the other hand. Now make a firm sratch with the specimen on the bottle.


Step 3. Check carefully if the specimen has actually scratched the bottle. Rub away the powder trail and feel with your fingernail if there is a groove in the bottle.

Test successful, Red Stripe delicious...

Step 4. Drink the Red Stripe, you deserved it!

Practice a lot ;)

We took some specimens of the brown rock in Roehampton School Cave. When you break this rock, the inside is white. The brown colour is only a very thin patina on the surface. The Red Stripe Test showed us that the mineral was harder than the bottle. Also the typical resinous luster and conchoidal fracture gives a certainty of 99.99 % that this rock is indeed Flint. This mineral is a subvitreous form of quartz (silica) which has a hardness of 6.5.
Limestone which is almost pure microcrystaline Calcite has a hardness of 3, so doing the Red Stripe Test with a piece of limestone would give a powdery line on the bottle which we can wipe off without any scratch.

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