Andalusite Slate (intermediate porphyroblastic slate zone)

This sample is a fine-grained, poorly foliated, porphyroblastic metamorphic rock, dominated by porphyroblasts of chiastolite and cordierite in a fine-grained matrix rich in quartz and iron-oxides. Andalusite porphyroblasts are euhedral columnar crystals up to 8 mm in length with square basal sections. Andalusite has been entirely replaced by mesh-textured sericite and kaolinite. Cordierite poikiloblasts are anhedral, rounded crystals up to 2.5 mm in size and contain inclusions of quartz, muscovite and graphite.

This sample represents thermal metamorphism of the skiddaw slate with the growth of andalusite and cordierite occurring at the expense of micas and chlorite. Iron-oxide rims surrounding andalusites formed during growth of these porphyroblasts and suggests the protolith was an iron-rich pelite. Replacement of andalusite and cordierite may have been hydrothermal.

Andalusite Slate (the elongated white crystals). Glenderaterra Valley. Photo by Kevin Walsh.

Andalusite Slate (the elongated white crystals).

Bibliography

• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• Gray.E Bebout; Derek. C Cooper; A. Don Bradley; Seth J. Sadofsky (1999): Nitrogen-isotope record of fluid-rock interactions in the Skiddaw Aureole and granite, English Lake District. American Mineralogist, Volume 84, pages 1495–1505, 1999

Photo
Andalusite crystal in a fine-grained matrix rich in quartz and iron-oxides. Glenderaterra Valley, Cumbria, England. PPL image, 2x (Field of view = 7mm)	Andalusite crystals in a fine-grained matrix rich in quartz and iron-oxides. Glenderaterra Valley, Cumbria, England. PPL image, 2x (Field of view = 7mm)	Andalusite crystals entirely replaced by mesh-textured sericite and kaolinite. Glenderaterra Valley, Cumbria, England. XPL image, 2x (Field of view = 7mm)
Andalusite crystal in a fine-grained matrix rich in quartz and iron-oxides. Glenderaterra Valley, Cumbria, England. PPL image, 2x (Field of view = 7mm)	Andalusite crystals in a fine-grained matrix rich in quartz and iron-oxides. Glenderaterra Valley, Cumbria, England. PPL image, 2x (Field of view = 7mm)	Andalusite crystal in a fine-grained matrix rich in quartz and iron-oxides. Glenderaterra Valley, Cumbria, England. PPL image, 2x (Field of view = 7mm)
Andalusite crystal entirely replaced by mesh-textured sericite and kaolinite. Glenderaterra Valley, Cumbria, England. XPL image, 2x (Field of view = 7mm)	Andalusite crystals and rounded cordierite spot in a fine-grained matrix rich in quartz and iron-oxides. Glenderaterra Valley, Cumbria, England. PPL image, 2x (Field of view = 7mm)	Andalusite crystal and rounded cordierite spost in a fine-grained matrix rich in quartz and iron-oxides. Glenderaterra Valley, Cumbria, England. PPL image, 2x (Field of view = 7mm)
Andalusite crystal and rounded cordierite spost in a fine-grained matrix rich in quartz and iron-oxides. Glenderaterra Valley, Cumbria, England. PPL image, 2x (Field of view = 7mm)	Andalusite crystal in a fine-grained matrix rich in quartz and iron-oxides. Glenderaterra Valley, Cumbria, England. PPL image, 10x (Field of view = 2mm)	Andalusite crystal entirely replaced by mesh-textured sericite and kaolinite. Glenderaterra Valley, Cumbria, England. XPL image, 10x (Field of view = 2mm)
Andalusite crystal in a fine-grained matrix rich in quartz and iron-oxides. Glenderaterra Valley, Cumbria, England. PPL image, 10x (Field of view = 2mm)	Andalusite crystal entirely replaced by mesh-textured sericite and kaolinite. Glenderaterra Valley, Cumbria, England. XPL image, 10x (Field of view = 2mm)	Andalusite crystals in a fine-grained matrix rich in quartz and iron-oxides. Glenderaterra Valley, Cumbria, England. PPL image, 10x (Field of view = 2mm)
Andalusite crystals entirely replaced by mesh-textured sericite and kaolinite. Glenderaterra Valley, Cumbria, England. XPL image, 10x (Field of view = 2mm)	Andalusite crystal in a fine-grained matrix rich in quartz and iron-oxides. Glenderaterra Valley, Cumbria, England. PPL image, 10x (Field of view = 2mm)	Andalusite crystal entirely replaced by mesh-textured sericite and kaolinite. Glenderaterra Valley, Cumbria, England. XPL image, 10x (Field of view = 2mm)
Andalusite crystal entirely replaced by mesh-textured sericite and kaolinite. Glenderaterra Valley, Cumbria, England. XPL image, 10x (Field of view = 2mm)	Andalusite crystal entirely replaced by mesh-textured sericite and kaolinite. Glenderaterra Valley, Cumbria, England. XPL image, 10x (Field of view = 2mm)	Andalusite crystal entirely replaced by mesh-textured sericite and kaolinite. Glenderaterra Valley, Cumbria, England. XPL image, 10x (Field of view = 2mm)
Andalusite crystal in a fine-grained matrix rich in quartz and iron-oxides. Glenderaterra Valley, Cumbria, England. PPL image, 2x (Field of view = 7mm)	Andalusite crystal entirely replaced by mesh-textured sericite and kaolinite. Glenderaterra Valley, Cumbria, England. XPL image, 2x (Field of view = 7mm)	Andalusite crystal in a fine-grained matrix rich in quartz and iron-oxides. Glenderaterra Valley, Cumbria, England. PPL image, 10x (Field of view = 2mm)