Slate is a fine grained metamorphic rock that was formed from shale or mudstone under low grade metamorphic conditions. Its colour varies due to the different mineral content and conditions during its formation, with common colours including grey, blue, plum and green. It's known for its ability to be easily split into thin, durable sheets, making it a popular choice for roofing, flooring and other construction applications. Here's a breakdown of the common slate colours and the minerals responsible for their colour.
1. Grey Slate
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Colour: Light to dark grey
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Cause: Predominantly due to carbonaceous material and fine-grained quartz and mica (muscovite and biotite).
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Notes: Grey is the most common slate colour and is often seen in roofing and architectural tiles.
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Found: Various regions globally, with significant deposits in Wales, Spain, and parts of the USA.
2. Black Slate
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Colour: Deep black or dark grey
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Cause: High organic carbon content and pyrite (iron sulfide), sometimes in tiny quantities.
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Notes: Commonly used in chalkboards and as a decorative stone.
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Found: primarily found in Spain and China
3. Green Slate
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Colour: Pale to deep green
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Cause: Presence of chlorite, epidote, or serpentine minerals, which form during metamorphism.
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Notes: Often used in flooring and roofing; green slates are durable, water-resistant and decorative.
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Found: primarily found in the Lake District of England, specifically within the Borrowdale Volcanic Group of mountains.
4. Purple or Plum Slate
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Colour: Purplish red to deep plum
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Cause: High concentrations of iron oxide minerals, especially hematite.
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Notes: Often used in landscaping, garden paths, decorative borders and rockeries.
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Found: North Wales, specifically at the Penrhyn Quarry in Bethesda.
5. Red Slate
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Colour: Bright red to rust red
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Cause: Rich in hematite or other iron oxide minerals.
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Notes: Less common and often more decorative due to its vivid colour.
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Found: primarily found in Slate Valley, a region along the New York and Vermont border in the United States.
6. Blue Slate
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Colour: Blue-grey or bluish tones
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Cause: The exact mineralogical cause can vary, but it's usually due to a combination of chlorite, graphite, and finely distributed iron minerals.
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Notes: Highly prized for roofing, particularly in the UK as they are relatively cheap and lightweight.
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Found: Primarily found in North Wales, specifically around the Penrhyn Quarry near Bethesda, and also in the Blaenau Ffestiniog area.
7. Brown or Buff Slate
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Colour: Earthy tones like brown, tan, or ochre
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Cause: Oxidation of iron-bearing minerals, especially limonite or goethite.
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Notes: Often used in garden pathways and decorative stone applications.
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Found: Mainly in Devon and Cornwall, but also in the Lake District and North Wales.
Please study this slate and answer these three simple questions;
1. Observe and describe the rock at GZ. Remember to comment on its overall colour and anything else you can see.
2. Which minerals do you think might be present and why?
3. Based on your answers to tasks 1 and 2, suggest where you think the rock might have come from?
4. COMPULSORY. Please take a picture of yourself, your GPS device or a personal item or a photo at GZ with your name visible but please do not inclure the slate and be careful that your picture does not give away any answers.
Please feel free to log a find straight after you have sent me your answers. All answers will be looked at and if there are any issues I will contact you. ANY found logs without a photo and any found logs without answers submitted within a reasonable period will be deleted. This is to ensure fairness to those cachers who do complete the tasks required.