Granoblastic is an adjective describing an
anhedral phaneritic
A phanerite is an igneous rock whose microstructure is made up of crystals large enough to be distinguished with the unaided human eye. In contrast, the crystals in an aphanitic rock are too fine-grained to be identifiable. Phaneritic texture fo ...
equi-granular
metamorphic rock
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...
texture. Granoblastic texture is typical of
quartzite
Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...
,
marble
Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or Dolomite (mineral), dolomite. Marble is typically not Foliation (geology), foliated (layered), although there are exceptions. In geology, the ...
,
charnockite
Charnockite () is any orthopyroxene-bearing quartz-feldspar rock formed at high temperature and pressure, commonly found in granulite facies metamorphic regions, ''sensu stricto'' as an endmember of the charnockite series.
Charnockite series
The ...
s and other
non-foliated metamorphic rocks without
porphyroblast
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained matrix. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape.
The most common porphyrobl ...
s. Characteristics defining granoblastic texture include: grains visible to the unaided eye, sutured boundaries and approximately equidimensional grains. The grain boundaries intersect at 120° triple junctions under ideal conditions. Variation from the ideal results from stress produced foliation during crystallization resulting in
schistose
Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes o ...
textures.
A rock that has a granoblastic texture can be termed a ''
granofels Granofels is a term referring to medium- to coarse-grained metamorphic rocks which have a ''granoblastic'' texture (having poorly formed, equi-granular crystal grains visible to the naked eye) and lack distinct foliation or lineation. The granofels ...
''.
References
Dictionary of Mining, Mineral, and Related Terms
Metamorphic rocks
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