Slate could well be described as the “kitchen sink” of the mineral world. While we may be familiar with the components that dictate color-Chlorite, Hematite and Carbonaceous matter coloring green, red and black slate respectively-there is an endless array of minerals that play a significant role in the commercial production of slate. To name a few: Silica, Titanium dioxide, Calcium carbonate, Alumina, Chlorite, Ferric oxide, Ferrous oxide, Quartz, Mica, Magnesia, Potash, Soda, Pyrite, Sulfur, Carbon. Their significance due to both positive and detrimental contributions to the quality of slate.

The uniform structure of Mica and Chlorite contribute to the hardness but also the best cleave in a slate, while Calcium, Magnesium and Iron carbonates are soft and vulnerable components. Sulfur and sulfides give the possibility of Sulfuric acid formation- a deadly ingredient-while Pyrites send most people screaming into the streets.

Slate would seem to be a potential tinder-box of chemical reactions, since many of the ingredients are normally volatile in each other’s company. However, the conditions for most chemical reactions require a solvent (water), heat and a catalyst (oxygen). So long as slate is buried underground it is quite stable. Above ground, in the presence of heat, light, oxygen and water, it is surprising that it does not disintegrate in an explosion of chemical reactions! Though in terms of geological time, that is exactly what it does. A hundred and fifty years on a roof is pretty quick compared to 500m years in the making.

It is only the low water absorption rate of slate that makes it a stable material. The tiny amount of water it does absorb is not enough to enable these chemical reactions, even with a plentiful supply of heat, water and oxygen (on a roof). Well, maybe that statement should be modified: most reactions do not occur unless the water absorption is sufficient, unless it is a surface phenomenon, in which case water just needs to be present. Three reactions take place that deserve some explanation.

Calcium Carbonate (also known as Calcite or Limestone), is a component of slate. It is a mineral softer than most others in slate and so, undesirable. More importantly it is vulnerable to acids and as mentioned before, slate has the ingredients to generate sulfuric acid. Given “sufficient” water absorption (low grade slate), the sulfuric acid attacks the limestone (acid vs alkali) and produces gypsum and Carbon Monoxide.

H2SO4     +

Sulfuric acid

CaCO3     ->


CaSO4.2H2O    +



Carbon monoxide

Gypsum has the peculiar quality of requiring more room than the net volume of its constituents. As it expands, it leeches out of the slate, taking the structure of the slate with it. Visible are the chalky stains on the edges of the slate (pic).

The ASTM testing in the US does not include Iron, Magnesium and Calcium carbonates, though some quarries will pour Hydrochloric acid on slate samples to check for the level of Carbonates. An effervescence of Hydrogen carbonate (bubbles) will appear.

HCL        +

Hydrochloric acid

+CaCO3     ->


CaCl    +

Calcium Chloride


Hydrogen carbonate

The equivalent of the ASTM in Europe, the CE (Certificat European) does not test for Carbonates but the more stringent NF (Norme Francais) does. Slate is pulverized and the granules examined microscopically for Carbonates. The percentage tolerated is 1.5%.


Weathering Slate have always been accepted in the United States, though are regarded with suspicion in Europe. Europeans are more familiar with their unfading slate and view weathering as essentially a “rusting” process, which it is. Once again the carbonates are the low-achievers (of Iron, Magnesium and Calcium). In the presence of rain water, heat and oxygen they decompose to form Limonite, displacing elements that originally colored the slate. Limonite is a hydrous oxide (like rust), that ranges in color from yellow to a rusty brown, a very familiar sight on a weathering slate roof. Significant about this process is that it is a limited chemical reaction, only affecting the surface of the slate.

Pyrites attract a lot of attention though are rarely discussed, except as a point of contention.

In 1886, Geologist Alexis Anastay Julien presented a paper to the Microscopical Society of New York (which described their profession, not their size), titled The Microscopical Study of Iron Pyrites. It was a benchmark study influencing later research on the subject. Significantly, in the middle of his lecture he asked if it was legitimate to quarry slate and mine coal if there was a significant presence of Pyrites. The question still begs an answer.

One thing we know is that where there is slate there are Pyrites. The two are inseparable. During the process of metamorphosis, while most elements, crystals and minerals line up, soldier fashion, to make a perfect cleave, Pyrites refuse to get in line. They are the Cool-hand-Lukes of the metamorphic process. Easily observed on the face of a slate, they protrude at odd angles, sometimes minute, sometimes a half-inch across. They also occur within the body of the slate.

There are numerous Pyrites (Gold, Silver, Copper, Nickel), but the ones of concern are the Iron Pyrites.

Chemically these are described as Iron Sulfides (FeS2), but analogous to the forms of Carbon (coal, graphite, diamond), Iron Sulfides take on three distinct crystalline forms, Pyrite, Pyrrhotite and Marcasite. Pyrite is the most common, a cubic or octahedral crystal with a brassy yellow color, known colloquially as Fool’s Gold. This is the stable Pyrite.

Pyrrhotite is something of a shape-shifter, but is most commonly in the form of an ortho-rhombic crystal (think of a classroom model of an atom). Pyrrhotite is also stable, unless in the presence of the third form of Pyrite, Marcasite.

In his lecture of 1886 Julien describes Marcasite and Pyrrhotite, with Victorian melodrama, as “comrades, clinging closely…as they emerge from the black mother-slime…” What he was getting at was that where there is one there is the other.

Marcasite is the bad egg, the bad apple in the barrel, an embarrassment to the Pyrite family. It varies in color from silver to a dull tin, is granular in form and will oxidize (rust) in a heart-beat. It is a deadly form of Pyrite, not only staining the surface but destroying the structure of slate, since rust, like Gypsum, takes up more room than its components.

Julien describes the Pyrites in terms of metamorphosis, where Marcasite is the least and Pyrite the most metamorphosed; hence the stability of one and the instability of the other. Geologists Nelson Dale, Oliver Bowles, Cranshaw and Allen et al chimed in at later dates to confirm Julien’s interpretation and Cranshaw established a “Coefficient of Oxidation”, a measurement of the likelihood of rusting.

Pyrites are sporadically a problem in the United States and Canada. The illustrations show a recent Vermont Weathering Green slate with a large display of Pyrites, a one hundred and fifty year old Peach Bottom slate showing the residue of Marcasite and an unidentified eight year old slate roof in Southern Maryland.

In certain regions of Spain Pyrites have been an issue, though once again French testing has led the way. The NF (Norme Francais) testing regimen is to immerse sample slate in cold water for five hours and then heat them to 100 deg Celsius for five hours. This is repeated twenty times with each sample. The result is T-1 for no evidence of rusting, T-2 for surface rusting and T-3 for possible structural damage. No doubt the roof in Southern Maryland would have a T-3 rating or less.

The seriousness of slate “weathering” seems to be in the eye of the beholder, but Limestone and Pyrites are part of the slate industry. It is not if but how they are dealt with that is significant.

The first line of defense has to be testing.

ASTM testing in the States does not cover Carbonates or Pyrites. The CE (Certificat European) covers Pyrites but not Carbonates and as in the States is performed by individual laboratories. The NF testing (Norme Francais) covers both Pyrites and Carbonates and it includes traceability of product, with legal ramifications.  This is a voluntary test. If a slate producer wants the highest rating for his slate, he invites the LNE-the French national testing agency- to randomly inspect and take samples from his quarry. These are subjected to far more stringent tests than either ASTM or the CE. The goal is to get the best rating and of course France, as the largest importer of Spanish slate, protects its own interests. Each test is a 10,000 Euro event, compared to a $900 ASTM test. The second line of defense is Quality Control. Ultimately, the decision to put a low-grade slate in a pallet (or not) is a business decision made by the quarrier and lastly, whether we are buying from a quarry , broker or any third party, our level of  trust and confidence in them is going to reflect in the quality of the product.. We deal in a natural (somewhat unpredictable) product, in a volatile market. Hopefully we can hold a steady course with all that is within our control.

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