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amem a auras am Table 3-1 Common Igneous Textural Terms Crystallinity Holocrystalline Hypocrystalline Holohyaline, Vitric Grain Size Aphanitic Phaneritic Cryptocrystalline Fine grained Medium grained Coarse grained Pegmatitic Aplític Equigranular Inequigranular Porphyritie Textures Porphyritic Megaporphyritic Microporphyritic Phyric (cphyric) Phenocryst Microphenocryst Megacryst Poikilitic Oikocryst Groundmass Cumulophyric Glomeroporphyritic Hiatial porphyritic Seriate Aphyric Form of Individual Grains Euhedral (idiomorphic) Subhedral (subidiomorphic) Consisting entirely of crystals (default term, not commonly used). Containing both crystal and glass. Consisting entirely of glass. Most minerals are too fine-grained to see with the naked eye. The minerals are coarse enough to see with the naked eye. Too fine-grained to distinguish the minerals microscopically. Average crystal diameter is less than 1 mm. Average crystal diameter is 1-5 mm. Average crystal diameter is greater than 5 mm. Very coarse grained. Fine- to medium-grained xenomorphic and equigranular (looks sugary). Grains are all approximately the same size. Grains vary considerably in size. Approximately bimodal size distribution (usually requires a great difference). Porphyritic texture that can be seen in hand specimen (rarely used). Porphyritic texture that is visible only under the microscope. An adjective (or suffix) referring to porphyritic texture. Large crystal set in a fine matrix. Microscopie crystals that are still larger than the remainder of the groundmass. An unusually large crystal, either a phenocryst or a xenocryst. The host phenoeryst (oikocryst) contains many inclusions of other minerals. The host phenocryst in poikilitic texture. “The glassy or finer grained element in the porphyritic texture (matrix, mesostasis). Phenocrysts of the same or different minerals occur in clusters (Figure 3-1la). Synonymous with cumulophyric (used by some to specify that only one mineral is involved). There is a pronounced difference in size between the phyric and groundmass phases (the default term: rarely used). There is a continuous gradation in size. Non-porphyritic (no phenoerysts). Completely bounded by crystal faces. Crystal faces only partially developed. Anhedral (allotriomorphic) Crystal faces are entirely absent. Crystal habits: equaint, prismatic, columnar, accicular, fibrous, tabular, platy, micaceous, lath-shaped, etc. (see any mineralogy text). Corroded (embayed) Subhedral or anhedral forms produced by partial melting (resorption) of phenoerysts by the melt. Forms of Grains in the Rock as a Whole Panidiomorphic Hypidiomorphic Allotriomorphic Sutured Mosaic Intergrowths Host (Oikocryst) Guest Poikilitic Graphic Micrographic Granophyrie Exsolution lamellae Perthitic Antiperthitic Symplectite Myrmekite Spherulitic Asxiolitic Most of the grains are euhedral (rare). Consisting predominantly of subhedral grains (common in many granitic rocks). Most of the grains are anhedral. Common. Characterized by articulation along highly irregular inter-penetrating boundaries. Common in recrystallized de- formed rocks. A texture of polygonal equigranular crystals. The large mineral that includes others in poikilitic texture. The included mineral in poikilitic texture. One large crystal contains several small discrete crystals of another mineral. Refers to growth phenomena, not exsolution or replacement. An intergrowth in which the guest shows angular wedge-like forms. Usually occurs with quartz in microcline (Figure 3-9). Graphic texture visible only under the microscope. A texture of certain porphyritic rocks in which the groundmass minerals (usually quartz and alkali feldspar) pen- etrate each other as feathery irregular intergrowths. Resembles micrographic texture, but is more irregular. Lamellar bands of a phase exsolved from a host phase (Figure 3-9a). Irregular veins, patches, lenses, etc., of sodic plagioclase in an alkali feldspar host.Usually results from exsolution (Figure 3-18a). Essolution lamellae of alkali feldspar in a plagioclase host. Usually much thinner than perthite. A replacement texture in which a mineral is replaced by an intergrowth of one or more minerals. A secondary texture consisting of irregular “wormy” blebs or rods of quartz in a plagioclase host adjacent to al- Kali feldspar grains (Figure 3-21). A radial intergrowth of fibrous minerals. Commonly alkali feldspar and quartz in devitrified si Similar to spherulitic, but the fibers occur in a layer and are oriented normal to its walls. cie volcanics. (continued) 44 Chapter 3 Table 3-1 Continued Variolitic Spherulitic structures consisting of divergent plagioclase fibers in certain basalts. Coalescent Anhedral textures developed by simultaneous growth of two mineral grains in contact. Textures of Mafic Igneous Rocks Ophitic Large pyroxene grains enclose small random plagioclase laths (Figure 3-8). Subophitic The plagioclase laths are larger and only partially enclosed by the pyroxene. Nesophitic Plagioclase is larger yet and the pyroxenes are interstitial. Intergranular Small discrete grains of pyroxene, olivine, ete., fill the interstices in a random network of larger plagioclase laths (Figure 3-15). Intersertal Glass, cryptocrystalline material, or alteration products occupy the spaces between plagioclase laths. Hyalo-ophitic An intersertal texture in which a larger amount of glass is present and less pyroxene. Hyalopilitic Still more glass is present and plagioclase occurs only as tiny random microlites. Diktytaxitic Texture of certain basalts containing abundant angular interstitial gas cavities between the plagioclase laths. Cumulate Interstitial growth of a mineral between earlier ones which are all in contact and give the distinct impression that they accumulated at the bottom of a magma chamber (Figure 3-14). Orthocumulate Cumulate texture with other minerals occupying the interstitial areas (Figure 3-14b). Adeumulate Cumulate texture in which the early cumulate minerals grow to fill the pore space (Figure 3-14c). Mesocumulate Intermediate between ortho- and adcumulate. Replacement Textures Pseudomorph One or more minerals replace another, retaining the form of the original mineral. Symplectite A replacement texture in which a mineral is replaced by an intergrowth of one or more minerals. Other forms include replacement of rims, cores, or patches of one mineral by another. Common processes include: Uralitization Replacement of pyroxene by amphibole (Figure 3-21a). Saussuritization Replacement of plagioclase by epidote. Biotitization Replacement of pyroxene or amphibole by biotite. Chloritization Replacement of any mafic mineral by chlorite (Figure 3-20b). Seritization Replacement of feldspar or feldspathoids by fine white micas. Miscellaneous Terms Interstitial One mineral fills the interstices between earlier crystallized grains (Figure 3-7). Crystallites Minute, inchoate crystals in carliest stages of formation. They are isotropic and cannot be identified under the mi- croscope. Microlites Tiny needle- or lath-like crystals of which at least some properties are microscopically determinable. Felty Consisting of random microlites (Figure 3-13b). Pilotaxitic Like felty. Trachytic Consisting of (feldspar) microlites aligned due to flow (Figure 3-12a). Embayed Having embayments due to reaction with the melt (resorption) (Figure 3-2). Skeletal Crystals which grew as, or have been corroded to, a skeletal framework with a high proportion of internal voids (Figure 3-4). Sieve Crystals are filled with channelways (appearing as holes) due to resorption (Figure 3-11a). Epitactic Oriented nucleation of one mineral on another of a different kind. Rapakivi Overgrowths of plagioclase on alkali feldspar. Vesicular Contains gas bubbles. Scoriaceous Highly vesicular. Pumiceous Frothy vesicular structure characteristic of pumice. Diktytaxitic Containing vesicles into which microphenocrysts protrude. Miarolitic Gas cavities present in certain plutonic rocks into which euhedral minerals protrude. Pipe vesicles Tube-like elongate vesicles that result from rising gases. Vesicular pipes Cylindrical bodies highly charged with vesicles. Amygdaloidal Vesicles completely or partially filled with secondary minerals. Lithophysae Large ovoid structures representing gas bubbles in devitrified rhyolitic glass. Flow A parallel structure developed as the result of flow. Foliation Planar parallelism. Banding Alternating planar layers. Lincation Linear parallelism. Xenolith An inclusion of country rock. Xenocryst A single-crystal foreign inclusion. Perlitic Concentric fracture pattern resulting from contraction of some volcanie glasses upon cooling. Pyroclastic Comprised of fragments. Ocelli Ovoid blobs created by liquid immiscibility, mingled magmas, or filled vesicles. Orbicules Ovoid masses of radiating crystals, commonly concentrically banded, found in some granites. Spinifex Centimeter-scale sub-parallel to dendritic growth of olivine crystals in some quenched ultramafics. Pyroclastic Terms See Section 2.5. Pyroclastic Glass textures Pele's tears Glassy lapilli. Pele's hair Hair-like strands of glass. Fiamme Compressed pumice fragments in a tuff. 
