Definitions and images to illustrate geological terms, links to images and website articles


The texture of a rock is determined by the size and configuration of its constituent minerals and any presence of gas bubbles. Rock fabric refers to the general appearance of a set of crystals that have grown together to produce a distinctive shape or texture. The term "fabric" is best applied to groups of crystals, or to invidividual crystals within groups, which have grown together so that their growing surfaces have encountered each other. Crystal habit is the general appearance of a crystal that results from the nature and prominence of crystal forms, such as faces or sets of faces. The term "habit" is best applied to invidividual crystals that have grown without their growing surfaces encountering any pre-exisiting solid (encounter = fabric). The microstructure of a rock is its set of structural features, such as grain boundaries, grain size and structure, which can be observed in thin-section (under a microscope).

igneous rocks:
aphanitic – all crystals are too small to be visible to the naked eye or with a hand lens, giving the rock a dull appearance; resulting from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments. Eg. basalt
phaneritic – all crystals sufficiently large to be clearly visible to the naked eye; resulting from slow cooling of magma deep underground in plutonic structures. Eg. granite, gabbro
porphyritic – larger phenocrysts embedded in a finer textured matrix; resulting from two-stage cooling of rising magma, first at depth and subsequently at the surface or shallow subsurface. Eg. porphyry
glassy – non-crystalline rocks; resulting from very rapid cooling of lava at or very near the Earth's surface. Eg. obsidian
vesicular – porous rock with vesicles (holes, pores, or cavities); resulting from gas expansion within rapidly cooling ejected lava. Eg. vesicular basalt
fragmental or pyroclastic – irregular grains welded together, sometimes with glassy shards; resulting from pyroclastic volcanic eruptions. Eg. volcanic tuff, ignimbrites

metamorphic rocks:
foliated – in which mineral constituents are oriented in a parallel or subparallel arrangement resulting from imposed pressure during regional metamorphism. From low to high metamorphic grades:
--slaty – parallel orientation of microscopic grains; Eg. slate
--phyllitic – parallel arrangement of platy minerals, usually micas, that are barely visible to the naked eye; Eg. phyllite
=-schistose – subparallel to parallel orientation of platy minerals such as chlorite or micas; Eg. schists
--gneissic – coarsely foliated texture in which minerals have segregated into discontinuous hands, each of which is dominated by one or two minerals; Eg. gneisses
...granuloblastic – typical of granulites, which have even-sized, granular mineral grains with weak preferred orientation. Microscopic structure reveals small, rounded grains forming a closely-fitted mosaic.
non-foliated – in which constituent minerals lack an ordered arrangement, retaining roughly the orientation of grains present in the country rock before thermal metamorphism; Eg. quartzite, marble, metaconglomerates, hornfels, anthracite coal.

Euhedral crystals are distinct, well-formed crystals with sharp, easily-recognized faces (almandine garnet in quartzitic gneiss at left).

Euhdral is opposite to the interlocked grains of anhedral textured rocks that have cooled in the crowded environment of magma chambers. Subhedral crystals are intermediate in character between distinct euhedral crystals and enmeshed anhedral textures.

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