Shale is one of the most common sedimentary rocks. Shale can range in color from black and gray to green, brown, yellow, or red based on its makeup. Red shale forms mainly due to iron oxide minerals, also known as hematite. Hematite is an iron ore that is reddish-brown to blood-red in color. This gives the shale a red shade. In geology, this type of red shale is called hematitic shale.
The iron in hematite-rich shale may come from a few sources. One is from volcanic ash and debris that was deposited with other sediments that later became shale. This volcanic stuff has iron that was oxidized when erupting, making the particles red. Another source is erosion of land with iron oxide minerals. The particles were moved by rivers and streams before building up on the seafloor and in lakes as shale.
Red shale may also form in highly oxygenated places where iron turns into hematite. This can happen at the sediment-water interface or deep within the shale if oxygenated fluids go through it.
Some well-known red shale formations are the Clair Group shale in Scotland, the Queenston Formation shale in Ontario, Canada, and the Longmaxi shale gas formation in southern China. The red color of these shales is mainly from iron oxide minerals.
In some cases, red shale may also contain other red minerals like chert or jasper. These are red sedimentary rocks with mostly silica. Sometimes, red shale may have clay minerals with red pigments too. But the main cause of red coloring in shale is hematite.
So, shale certainly can be red, largely due to hematite. Red shales form under specific conditions that allow iron concentration and oxidation. The red color can range from pale pinkish-orange to deep brick red based on iron content. Some major shale gas and oil deposits are actually red shales. So the color does not affect their potential as hydrocarbon source rocks. When you see red shale, know that its unique color comes from ancient geological processes affecting its iron makeup.
