Ottrelite slate

Slate is a low-grade metamorphic rock generally formed by the metamorphosis of mudstone, shale, or sometimes volcanic ash, under relatively low pressure and temperature conditions (prehnite-pumpellyite facies). With increasing metamorphic grade increases in muscovite grain-size transforms slate into phyllite. Slate is characterized by fine foliation along which it breaks to leave smooth, flat surfaces (often referred to as "slaty cleavage"). Sometimes relict (original) bedding is visible on foliation planes.

Slate is is mainly composed of quartz and muscovite or illite. Some minerals like biotite, chlorite, hematite, and pyrite are also usually present in slate. Apatite, graphite, kaolinite, magnetite, tourmaline, and feldspar are sometimes present as well, although less frequently. Slates may be black, blue, purple, red, green, or gray. Dark slates usually owe their colour to carbonaceous material or to finely divided iron sulfide. Reddish and purple varieties owe their colour to the presence of hematite (iron oxide), and green varieties owe theirs to the presence of much chlorite, a green micaceous clay mineral.

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Black pseudohexagonal ottrelite crystals (dark). Ottre, Belgium. From RRUFF.

Black pseudohexagonal ottrelite crystal. Ottre, Belgium. From RRUFF.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Photo
Chloritoid (Ottrelite) porphyroblasts in a fine-grained slate. Otterly, Ardennes (Belgium). PPL image, 2x (Field of view = 7mm)	Chloritoid (Ottrelite) porphyroblasts in a fine-grained slate. Otterly, Ardennes (Belgium). PPL image, 2x (Field of view = 7mm)	Chloritoid (Ottrelite) porphyroblasts in a fine-grained slate. Otterly, Ardennes (Belgium). PPL image, 2x (Field of view = 7mm)
Chloritoid (Ottrelite) porphyroblasts in a fine-grained slate. Otterly, Ardennes (Belgium). PPL image, 2x (Field of view = 7mm)	Chloritoid (Ottrelite) porphyroblasts in a fine-grained slate. Otterly, Ardennes (Belgium). PPL image, 2x (Field of view = 7mm)	Chloritoid (Ottrelite) porphyroblasts in a fine-grained slate. Otterly, Ardennes (Belgium). XPL image, 2x (Field of view = 7mm)
Chloritoid (Ottrelite) porphyroblasts in a fine-grained slate. Otterly, Ardennes (Belgium). PPL image, 2x (Field of view = 7mm)	Chloritoid (Ottrelite) porphyroblasts in a fine-grained slate. Otterly, Ardennes (Belgium). PPL image, 2x (Field of view = 7mm)	Chloritoid (Ottrelite) porphyroblasts in a fine-grained slate. Otterly, Ardennes (Belgium). XPL image, 2x (Field of view = 7mm)
Chloritoid (Ottrelite) porphyroblasts in a fine-grained slate. Otterly, Ardennes (Belgium). PPL image, 2x (Field of view = 7mm)	Chloritoid (Ottrelite) porphyroblasts in a fine-grained slate. Otterly, Ardennes (Belgium). XPL image, 2x (Field of view = 7mm)	Chloritoid (Ottrelite) porphyroblasts in a fine-grained slate. Otterly, Ardennes (Belgium). PPL image, 2x (Field of view = 7mm)
Chloritoid (Ottrelite) porphyroblasts in a fine-grained slate. Otterly, Ardennes (Belgium). PPL image, 2x (Field of view = 7mm)	Chloritoid (Ottrelite) porphyroblasts with hourglass inclusions in a fine-grained slate. Otterly, Ardennes (Belgium). PPL image, 10x (Field of view = 2mm)	Chloritoid (Ottrelite) porphyroblasts with hourglass inclusions in a fine-grained slate. Otterly, Ardennes (Belgium). PPL image, 10x (Field of view = 2mm)
Chloritoid (Ottrelite) porphyroblasts with hourglass inclusions in a fine-grained slate. Otterly, Ardennes (Belgium). XPL image, 10x (Field of view = 2mm)