Slate Geology
How slate and phyllite form — natural rocks used for roofs, facades and architectural stone.
Rock Classification
All rocks on earth fall into three major categories based on their origin and formation process.
Granite, basalt, pumice
Coal, limestone, sandstone, shale
Marble, slate, phyllite, quartzite
Formation Timeline
Slate and phyllite both begin as sedimentary deposits, but their journeys diverge under heat and pressure over millions of years.
Sediments
Fine clay particles and minerals are deposited in layers in ancient sea beds.
Shale
Sediments compact under pressure into shale — a soft, layered sedimentary rock.
Slate
Under increased heat and pressure, shale transforms into slate through metamorphism. Ideal for roofing.
Phyllite
Further metamorphism produces phyllite — harder, denser and stronger than slate, with a distinctive satin sheen.
What is Phyllite?
Phyllite, compared to conventional slate, is harder, denser and stronger. It formed over several hundred million years longer than slate — becoming harder, denser and stronger at each stage of formation. Phyllite also has a distinctive satin-like sheen and a greenish tint that distinguishes it from slate.
Phyllite is a rare rock worldwide. Only a handful of quarries produce phyllite with the splitting properties required for roofing and cladding. SSQ Riverstone® is one of the few commercially available phyllites worldwide — sourced from the San Luis quarry in La Repressa, Argentina, with geological reserves dating 560 million years to the pre-Cambrian era.
Riverstone® achieves a flexural strength of over 60 MPa — significantly exceeding all standard slate requirements — and carries a full 100-year written warranty.
Mineral Inclusions & Pyrite Classification
Slate is composed of fine grains of quartz and mica. The presence of certain minerals — particularly pyrite (iron sulphide) — affects long-term performance and is classified under European standards.
Discolouration possible, but structural integrity unaffected. All SSQ products are T1 certified.
Discolouration likely over time. Surface staining may occur but core remains stable.
Holes may form due to pyrite oxidation. Not recommended for roofing applications.
The less iron and calcite present in slate, the more resistant the products are to moisture and low temperatures. SSQ sources only from quarries meeting A1-T1-S1 classification — the highest European standard for fire resistance, pyrite content and flexural strength.
Want to know more about slate products? Explore the SSQ catalogue or contact us.