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Metamorphic Rocks
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Many metamorphic rocks are bimodal, meaning they contain mineral crystals of two sizes. Such rocks, termed porphyroblastic rocks, contain large crystals that have grown within a sea of finer-grained mineral material. The large crystals, called porphyroblasts, may be anhedral but are often subhedral or euhedral. The finer material is the groundmass. Some of the most spectacular porphyroblastic rocks are mica-rich schists that contain chloritoid, kyanite, garnet, or staurolite as porphyroblasts. Figure 10.6 shows a schist from southern Switzerland, near Italy, that contains porphyroblasts of kyanite (blue) and staurolite (brown). The largest of the blue kyanite blades are about 4 cm long. In this rock, a silvery mica called paragonite constitutes most of the groundmass. Figure 10.7 shows garnet in an outcrop on Syros, one of the western Greek islands. The largest of the porphyroblasts are about 2 cm across. The groundmass in this rock is mostly quartz and feldspar with lesser amounts of biotite. Garnet is a very common porphyroblast in rocks that metamorphose at medium to high temperature, and garnet crystals are commonly euhedral to subhedral, like the crystals shown here.
Metamorphic Rocks
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
The crystalline shape of a metamorphic mineral partly determines the ease of its growth during metamorphism; thus micas and chlorites, with a single cleavage, grow as thin plates oriented perpendicular to the maximum stress, and amphiboles such as hornblende grow in prismatic forms with length at right angles to the maximum stress. Some minerals of high crystallization strength, e.g. garnet and andalusite, grow to a relatively large size in metamorphic rocks and are then called porphyroblasts, i.e. conspicuous crystals in contrast to those which make up the rest of the rock. Feldspars and quartz have low and nearly equal strengths of crystallization, and metamorphic rocks composed of quartz and feldspar show typically a granular texture (granulites).
Optical mineralogy
Published in W.S. MacKenzie, A.E. Adams, K.H. Brodie, Rocks and Minerals in Thin Section, 2017
W.S. MacKenzie, A.E. Adams, K.H. Brodie
In an igneous rock, large crystals in a matrix or groundmass of much smaller crystals are described as phenocrysts (Figure 3). In a metamorphic rock similar large crystals which have grown in a mass of smaller crystals are termed porphyroblasts (Figure 1, Figure 4). If the porphyroblast contains abundant small inclusions, it is termed a poikil- oblast. In some rocks it is not certain whether the large crystals grew from an igneous magma or in a later stage metamorphic event. In these cases it is perhaps better to describe the crystals as megacrysts. In some deformed metamorphic rocks, large relict grains are preserved in a finer grained matrix and these are termed porphyroclasts.
Hydrothermal alteration, lithogeochemical marker units and vectors towards mineralisation at the Svärdsjö Zn-Pb-Cu deposit, Bergslagen, Sweden
Published in GFF, 2022
Anton Fahlvik, Tobias C. Kampmann, Nils F. Jansson
The Svärdsjö deposit has previously been classified as a Falun-type deposit (Levi et al. 1980), one of two types into which deposits in the BLU have historically been subdivided (Magnusson 1948; Sundblad 1994). The Falun-type largely corresponds to the stratabound volcanic-associated limestone-skarn-hosted (SVALS) type as defined by Allen et al. (1996). The SVALS-type deposits are characteristically Zn-Pb-Cu-Ag-Au deposits in dolomite marble and magnesian skarn after former limestone interbeds. Notable examples include the Garpenberg, Falun, Sala and Stollberg deposits. These deposits are commonly developed as systems of stratabound lenses, ranging in style from massive to vein networks and sulphide-cemented breccias with clasts of altered wall rock. Adjacent felsic metavolcanic rocks are commonly hydrothermally altered and metamorphosed to associations rich in quartz and mica (muscovite, chlorite, biotite). Metamorphic porphyroblasts are common, depending on bulk composition and metamorphic grade constituting garnet, amphibole, cordierite and/or aluminosilicates. The porphyroblasts commonly visually accentuate the chemical zonation patterns in the hydrothermally altered zones. Alteration is commonly most intense and widespread in the stratigraphic footwall, which locally hosts stringer-type mineralisation.
Geochemistry and origin of a Mesozoic ophiolite: the Pounamu Ultramafics, Westland, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2018
Alan F Cooper, Richard C Price, Anthony Reay
The biotite rock is a massive to poorly foliated quartzofeldspathic unit, typically several metres thick, that overlies the pelagic meta-chert and marble sequence. It differs from the structurally overlying quartzofeldspathic Alpine Schist in that it lacks bedding and is very pale coloured as a consequence of the lack of graphite. It contains distinctive coarse porphyroblasts of biotite and more rarely, garnet.