Structure is macroscopically observed arrangement of building elements of a magmatic rock. Important are spatial order and size of mineral grains, compactness, presence of cavities and degree of their filling. In contrast, structure is microscopic arrangement of the building elements. Borderlines between the structure and the texture are not strict, because some textures are discernible by naked eyes. Several basic structural types can be distinguished:
Compact (massive) structure – mass of rock is continuously filled, and is devoid of discernible cavities and empty spaces. The structure is typical of plutonic rocks.
Porous (vesicular) structure (name derived from Latin vesicula – small blister) – rock contains empty or secondary filled cavities. If empty bubbles originated in the rock by a release of fluid phase from magma during decompression and rapid cooling, this sub-type of porous texture is called vesicular. The vesicular structure is typical of outer margins of lava flows.
Almond-like (amygdular) structure (derived from Greek amygdalé – almond) originates by infilling of free vesicular spaces by secondary minerals (e.g. opal, chalcedone, agate, zeolite, carbonate) precipitating from low-temperature hydrothermal solutions.
Miarolitic structure was derived from miarolo – local name of granites in the region of Beneva, Italy). Miarolitic rocks are those containing cavities, several mm to cm in size, with euhedral quartz, feldspars or other secondary minerals.
Cavernous structure – rock contains cavities (caverns) originating by preferential leaching or weathering certain minerals.
Confining structure – fully crystallized magmatic rock contain crystal witout preferred orientation. Structure is diagnostic of plutonic rocks crystallized at depth without any significant pressure change.
Parallel structure is the reversal of the confining texture. Minerals exhibit pronounced preferential orientation along planes (plane-parallel) or along one direction (line-parallel texture).
Layered (laminar) structure is typical of effusive rocks. Laminarity is caused by flow of partially solidified lava containing large amount of crystallized minerals. Flow direction is marked by orientation of elongated minerals or cavities. Minerals are oriented along the flow direction by the longest dimension.
Orbicular (spherical) structure is characterized by concentric (spherical or ellipsoidal) arrangement of minerals around crystallization centers, thus giving rise to orbiculs with alternating bright and dark bands. It occurs in plutonic rocks, particularly in granites. Orbiculs originate by rhythmic crystallization, or by concentric nucleation around a mineral core. This type of structure is rare, such as conditions for origin of orbiculs in nature.
Spherulitic structure – rock contains spherules – rounded or oval mineral aggregates with fibrous, radially concentric arrangement. The spherules are easily discernible under polarization microscope in crossed polars, and this is the reason why this type of texture is sometimes affiliated to structures. Spherules originate mainly in effusive rocks during rapid crystallization around centres or during devitrification. The structure is more typical of acid rocks, where the spherules are composed of fibrous quartz. Varioles are a variety of spherules, showing spherical forms, but they are composed of lathy plagioclases concentrically arranged around tiny pyroxene – augite. Variolitic structure (from Latin word variola – smallpox) is very rarely observed in shield mafic rocks.
Brecciaed structure – angular fragments of magmatic rocks are cemented in magmatic matrix.