aphanitic


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aphanitic

[‚af·ə′nid·ik]
(petrology)
Referring to the texture of an igneous rock in which the crystalline components are not distinguishable by the unaided eye.
References in periodicals archive ?
4h) is medium grey, and has hornblende phenocrysts ranging from 3 to 5 mm in size in an aphanitic groundmass.
The aphanitic variety displays a typical trachytic texture with flow-aligned feldspars (0.
The matrix is composed of an aphanitic quartz-feldspar mosaic.
However, contacts between coarse and fine-grained to aphanitic felsic rocks are always sharp where exposed, and where intrusive relations can be determined, the coarse-grained phase invariably cuts the fine-grained one.
It is reasonable to conclude that in the case of the San Martino porphyry with its aphanitic matrix, the coarser grain size of the more mafic enclaves suggests that they arrived as solids or near-solids (crystal mushes) rather than crystallizing after emplacement even at the level of the deepest feeder dykes.
The locality consists of broken outcrop and rubble of weakly cleaved, massive, light yellow-grey felsic tuff, aphanitic to sand-size and aphyric, of the Lower Birch Island Member, in a partly slumped half-metre wall along a roadside ditch on the south (uphill) side of the road.
The remainder of the formation comprises intensely recrystallized, black aphanitic siliceous rocks that contain fa int pink to grey siliceous blotches (relict fragments) up to 5 cm in length, which are probably siliceous lapilli.
5-foot thick aphanitic layer of calcarenitic limestone, commonly marly and fossi-liferous; (2) a 233-foot thick aphanitic limestone; and (3) a 110.
The subvolcanic rhyolite porphyry has a red, aphanitic matrix and typically contains small rounded quartz phenocrysts and feldspar laths.