Most of them are best described as subhedral. The size of phenocrysts is essentially independent of their abundance relative to the groundmass, and they range in external form from euhedral to anhedral. When this has occurred, these phenocrysts are referred to as xenocrysts, while the aggregates can be termed xenoliths. Based on chemical composition, texture, and other criteria such as isotopic analysis, it has been demonstrated that some phenocrysts and glomerocrysts were not crystallized from the host magma but rather were accidentally torn from the country rock by the magma as it rose to the surface. In some cases, such glomerocrysts are monomineralic, but more commonly they are composed of two or more minerals. When this is observed, the term glomeroporphyritic is used to describe the texture, and the aggregate is referred to as a glomerocryst. Quite commonly in many volcanic rocks, phenocrysts are aggregated. The relatively large crystals in a porphyritic rock ordinarily occur as separate entities, known as phenocrysts, set in a groundmass or matrix of much finer-grained crystalline material or glass. Both of these kinds of texture are common. Rocks that are unevenly grained, or inequigranular, are generally characterized either by a seriate fabric, in which the variation in grain size is gradual and essentially continuous, or by a porphyritic fabric, involving more than one distinct range of grain sizes. Many fine-grained allotriomorphic-granular rocks are more simply termed sugary, saccharoidal, or aplitic. For such rocks the combination terms panidiomorphic-granular, hypidiomorphic-granular, and allotriomorphic-granular are applied according to the occurrence of euhedral, subhedral, and anhedral mineral grains within them. Even-grained, or equigranular, rocks are characterized by essential minerals that all exhibit the same order of grain size, but this implied equality need not be taken too literally. A more general contrast can be drawn between grains of equal (equant) and inequal dimensions. Quite apart from the presence or absence of crystal faces, the shape, or habit, of individual mineral grains is described by such terms as equant, tabular, platy, elongate, fibrous, rodlike, lathlike, needlelike, and irregular. The degree to which mineral grains show external crystal faces can be described as euhedral or panidiomorphic (fully crystal-faced), subhedral or hypidiomorphic (partly faced), or anhedral or allotriomorphic (no external crystal faces). How solid is your knowledge of all things geological? Test your knowledge by taking this quiz. The term geology refers, according to Britannica, the fields of study concerned with the solid Earth.
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