porphyroblast

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Related to Porphyroblasts: Porphyroblastic Texture

porphyroblast

[pȯr′fir·ə‚blast]
(petrology)
A relatively large crystal formed in a metamorphic rock.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Much attention has focused on passive inclusions in porphyroblasts that are texturally unzoned; these porphyroblasts contain a relatively continuous distribution of passive inclusion trails (see diagram in Fig.
Biotite is not only present as small flakes underlying the planar structure but also as lozenge-shaped crystals which represent early porphyroblasts and their asymmetry is sometimes useful to determine sense of shear.
Like the schists, the gneisses are characterised by the abundance of syn-S2 albite and garnet (Grt2) porphyroblasts. Internal foliation (S1) in Ab porphyroblasts is defined by graphitic inclusion trails and opaque minerals.
Optically continuous calcite layers are present between the garnet growth bands within some of the garnet porphyroblasts. In general, the garnets become more iron-rich and titanium-rich away from the cote until an abrupt change in garnet composition is noted at the transition to the red, outer growth zones.
If the sum of volumes of both newly crystallized minerals in the symplectite (hercynite + quartz) was equal to the volume of the host mineral (kyanite), the resulting symplectite would copy the form of kyanite porphyroblasts; it would be a simple columnar pseudomorph.
The main types include sulphide reef (massive sulphide enclosing clasts of quartz and feldspar), siliceous reef (fine sulphide in a banded cryptocrystalline quartz matrix and sometimes associated with altered feldspar porphyroblasts), mylonitic reef (streaky sulphide in a biotite-chlorite host) and mineralized country rock (fine sulphide in granulite).
The member here consists of massive hornfels, in which the cleavage has been largely annealed, and originally finer-grained pelitic units display strongly pitted surfaces due to weathering of coarse retrogressed cordierite porphyroblasts. A similar range of features is observed at the western limit of exposure in the Bluestone Quarry; these outcrops also display excellent exposures of the basal contact of the unit (described further below).
Garnet porphyroblasts which contain inclusions of quartz and biotite are arranged parallel to dominant crenulation cleavage.