Porphyroblast
In mineralogy and petrology, porphyroblasts are the inclusions of larger crystals of one type of mineral in a fine-grained matrix of other minerals. Porphyroblasts and matrix are collectively referred to as porphyroblastic structure . It is the most important and most common heteroblastic structure.
Porphyroblasts arise in metamorphic processes in both contact and regional metamorphic rocks . The growth of the crystals occurs in the solid state by rearrangement and diffusion of the chemical components of the matrix. They therefore replace parts of the matrix. Porphyroblasts form where there is a strong contrast between the chemical and crystallographic properties of the matrix and the porphyroblasts. This causes these crystals to grow faster than the slow growing crystals of the matrix. Porphyroblasts vary in size between a few millimeters and a few centimeters in diameter.
Frequent representatives of porphyroblastic training are, among others, andalusite , garnet , kyanite ( Disten ), magnetite and staurolite . The matrix mostly consists of various carbonates , chlorites , feldspars , mica and quartz .
literature
- Martin Okrusch, Siegfried Matthes: Mineralogy: An introduction to special mineralogy, petrology and deposit science . 7th edition. Springer Verlag, Berlin / Heidelberg / New York 2005, ISBN 3-540-23812-3 , pp. 13, 313 .