| Terminalogy | Definiton | Thin-section # |
|---|
Crystallinity |
||
|---|---|---|
| Holocrystalline | Consisting entirely of crystals. |
|
Hypocrystalline |
Containing both crystals and glass | |
Holohyaline, Vitric |
Consisting entirely of glass | |
Grain Size |
||
Aphanitic |
Most minerals are too fine-grained to see with the naked eye | BH250-60, -74, -89 |
Phaneritic |
Minerals are coarse enough to see with the naked eye | BH250-11 |
Cryptocrystalline |
Too fine-grained to distinguish the minerals microscopically | |
Fine grained |
Average crystal diameter is less than 1 mm | BH250-49 |
Medium grained |
Average crystal diameter is 1-5 mm | BH250-11, -46 |
Coarse grained |
Average crystal diameter is greater than 5 mm | BH250-50 |
Pegmatitic |
Very coarse grained | PegmatiticBlackHills1, 2 |
Aplitic |
Fine- to medium-grained xenomorphic and equigranular. |
|
Equigranular |
Grains are all approximately the same size | BH250-41 |
| Inequigranula | Grains vary considerably in size | BH250-50 |
Porphyritic Textures |
||
| Porphyritic | Approximately bimodal size distribution. |
|
Megaporphyritic |
Porphyritic texture that can be seen in hand specimen. |
BH250-11, -50 |
Microporphyritic |
Porphyritic texture that is visible only under the microscope | |
| Phyric | An adjective (or suffix) referring to porphyritic texture | |
Phenocryst |
Large crystal set in a fine matrix | BH250-8, -60, -72, -82, -83, -86 |
Microphenocryst |
Microscopic crystals that are still larger than the remainder of the groundmass | BH250-60 |
Megacryst |
An unusually large crystal, either a phenocryst or a xenocryst | BH250-8 |
Poikilitic |
The host phenocryst (oikocryst) contains many inclusions of other minerals | BH250-11, -50, 83 |
Oikocryst |
The host phenocryst in poikilitic texture | BH250-11 |
Groundmass |
The glassy or finer grained element in the porphyritic texture (matrix) |
BH250-15, -60, -80, -82, -83, -85, -88 |
Cumulophyric |
Phenocrysts of the same or different minerals occur in clusters | BH250-85 |
Glomeroporphyritic |
Synonymous with cumulophyric. |
BH250-85 |
Hiatial porphyritic |
There is a pronounced difference in size between the phyric and groundmass phases. |
|
| Seriate | There is a continuous gradation in size | |
| Aphyric | Non-porphyritic (phenocrysts absent) | |
Form of Indvidual Grains |
||
| Euhedral (idiomorphic) | Completely bounded by crystal faces | BH250-41 |
| Subhedral (subidiomorphic) | Crystal faces only partially developed | BH250-11 |
| Anhedral (allotriomorphic) | Crystal faces are entirely absent | |
| Corroded (embayed) | Subhedral or anhedral forms produced by partial melting (resorption) of phenocrysts by the melt | |
| Crystal habits | equaint, prismatic, columnar, accicular, fibrous, tabular, platy, micaceous, lath-shaped, etc. | |
Forms of Grains in the Rock |
||
| Panidiomorphic | Most of the grains are euhedral. | BH250-41 |
| Hypidiomorphic | Consisting predominantly of subhedral grains | BH250-11 |
| Allotriomorphic | Most of the grains are anhedral | |
| Sutured | Characterized by articulation along highly irregular inter-penetrating boundaries. Common in recrystallized de-formed rocks |
BH250-27, BH250-13, BH250-21 |
| Mosaic | A texture of polygonal equigranular crystals |
|
Intergrowths |
||
| Host (Oikocryst) | The large mineral that includes others in poikilitic texture | BH250-11,-50, -86 |
| Guest | The included mineral in poikilitic texture | BH250-86 |
| Poikilitic | One large crystal contains several small discrete crystals of another mineral. Refers to growth phenomena, not exsolution or replacement. |
BH250-11, -50 |
| Graphic | An intergrowth in which the rock shows angular wedge-like forms. Usually occurs with quartz in microcline | BH250-92 |
| Micrographic | Graphic texture visible only under the microscope | |
| Granophyric | A texture of certain porphyritic rocks in which the groundmass minerals (usually quartz and alkali feldspar) penetrate each other as feathery irregular intergrowths. Resembles micrographic texture, but is more irregula |
|
| Exsoiution lamellae | Lamellar bands of a phase exsolved from a host phase | BH250-9 BH250-92, BH250-81 |
| Perthitic | Irregular veins, patches, lenses, etc., of sodic plagioclase in an alkali feldspar host. Usually results from exsolution | BH250-81 BH250-92 |
| Antiperthitic | Exsolution lamellae of alkali feldspar in a plagioclase host. Usually much thinner than perthite | BH250-81 |
| Symplectite | A replacement texture in which a mineral is replaced by an intergrowth of one or more minerals. | |
| Myrmekite | A secondary texture consisting of irregular "wormy" blebs or rods of quartz in a plagioclase host adjacent to alkali feldspar grains |
BH250-13 BH250-49 |
| Spherulitic | A radial intergrowth of fibrous minerals. Commonly alkali feldspar and quartz in devitrified silicic volcanics | BH250-85 |
| Axiolitic | Similar to spherulitic, but the fibers occur in a layer and are oriented normal to its walls | BH250-85 |
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 | BH250-11, -50 |
Subophitic |
The plagioclase laths are larger and only partially enclosed by the pyroxene | BH250-11, -50 |
Nesophitic |
Plagioclase is larger yet and the pyroxenes are interstitial | BH250-11, -50 |
Intergranular |
Small discrete grains of pyroxene, olivine, etc., fill the interstices in a random network of larger plagioclase laths |
|
Intersertal |
Glass, cryptocrysialline material, or alteration products occupy the spaces between plagioclase laths | BH250-11 |
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 |
|
| Orthocumulat | Cumulate texture with other minerals occupying the interstitial areas | |
| Adcumulate | Cumulate texture in which the early cumulate minerals grow to fill the pore space | |
Mesocumulate |
Intermediate between ortho- and adcumulate | |
Replacement Textures |
||
| Pseudomorph | One or more minerals replace another, retaining the form of the original mineral | BH250-50, -60 |
| Symplectite | A replacement texture in which a mineral is replaced by an intergrowth of one or more minerais | BH250-48 |
| Uralitization | Replacement of pyroxene by amphibole | |
| Saussuritization | Replacement of plagioclase by epidote | |
| Biotitization | Replacement of pyroxene, amphibole or granet by biotite | |
| Chloritization | Replacement of any mafic mineral or garnets by chlorite | BH250-49 |
| Seritization | Replacement of feldspar or feldspathoids by fine white micas. Miscellaneous Terms |
BH250-49 |
Miscellaneous Terms |
||
| Interstitial | One mineral fills the interstices between earlier crystallized grains | BH250-11 |
| Crystallites | Minute, inchoate crystals in earliest stages of formation. They are isotropic and cannot be identified under the microscope |
|
| Microlites | Tiny needle- or lath-like crystals of which at least some properties are microscopically detenninable | |
| Felty | Consisting of random microlites | |
| Pilotaxitic | Like felty | |
| Trachytic | Consisting of (feldspar) microlites aligned due to flow | |
| Embayed | Having embayments due to reaction with the melt (resorption) | |
| Skeletal | Crystals which grew as, or have been corroded to, a skeletal framework with a high proportion of internal voids | |
| Sieve | Crystals are filled with channelways (appearing as holes) due to resorption | |
| Epitactic | Oriented nucleation of one mineral on another of a different kind | |
| Rapakivi | Overgrowths of plagioclase on alkali feldspar | |
| Vesicular | Contains gas bubbles | BH250-60, -82, -84 |
| 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 |
See photo |
| Vesicular pipes | Cylindrical bodies highly charged with vesicles | |
| Amygdaloida | Vesicles completely or partially filled with secondary minerals | BH250-85 |
| Lithophysae | Large ovoid structures representing gas bubbles in devitrified rhyolitic glass | |
| Flow | A parallel structure developed as the result of flow | |
| Foliation | Planar parallelism | BH250-26, BH250-52, BH250-66, BH250-20, BH250-21, BH250-22, BH250-26 BH250-66, BH250-67 |
| Banding | Alternating planar layers | |
| Lineation | Linear parallelism | |
| Xenolith | An inclusion of country rock | BH250-112 |
| Xenocryst | A single-crystal foreign inclusion | BH250-112 |
| Perlitic | Concentric fracture pattern resulting from contraction of some volcanic glasses upon cooling | BH250-92, BH250-81 |
| Pyroclastic | Comprised of fragments | BH250-19 |
| Ocelli | Ovoid blobs created by liquid immiscibility, mingled magmas, or filled vesicles | |
| Orbieules | 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 | BH250-87 |
Pyroclastic Terms |
||
| Pele's tears | Glassy lapilli | |
| Fiamme | Compressed pumice fragments in a tuff | |
