Pyrite Framboids on Slate

Hello slate people

Today I will talk about something very special I found in slates some years ago, pyrite framboids. Pyrite, as you all know, is an iron and sulphur mineral, usually considered as harmful for roofing slate. Pyrite is very common in black and green slates. Usually, you can see pyrite in the surface of the slate as a bright yellow-golden mineral. Sometimes it can rust, leading to the infamous oxidations that have been a pain in the ass for some quarries. However, pyrite has many shapes. Framboids are a very special type of pyrite. The name framboid comes from the French framboise, raspberry. Framboids are microscopic, their size is about 50 microns (the thickness of a human hair is between 40 and 120 microns), and you need a microscope to see them. Their internal arrangement is really fascinating, they are formed by tetrahedral crystals, clustered in a spherical shape. Every time I´m doing a petrographical analysis in a slate and see framboids, I spend more time than desirable looking at them. Why are framboids so special? Well, from a geological point of view there is a lot of information we can take from them. Framboids talk about the past environment, the formation conditions, and in some rocks, they can host metals of economic interest, such as gold (not in slates, sorry). At this point, some of you are probably thinking something like “perfect, but can these framboids affect the quality of my roofing slate?” the answer is no, they are not dangerous for the roofing slates. However, sometimes the slate uniformly “tan” after some years. This tan is a homogeneous and light oxidation, a slight patina that may appear on the slate surface. I think this patina is due to the oxidation of framboids, and occur only in slates with a high amount of them. This is not really an issue, since it is a progressive, homogeneous color change, and no one really notices it.

As you can see, roofing slates may look monotonous and dull, but in fact these are a very special group of rocks which have a lot of interesting things to discuss.

Have fun!

Do you want to see some framboids? Just type “framboid” in google images and you will see lot of electronic microscope framboid images. The attached image corresponds to a framboid found in a green slate from Vermont, seen at a normal optical microscope.

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