Texture categories

Degree of crystallinity

Grain size

Bubble textures

Crystal shapes

Cumulate textures

Other primary textures

High-T reaction textures

Subsolidus intergrowth textures

Subsolidus reaction textures


Many of the images have two views, most showing paired plane- and cross-polarized light images. Move the cursor over the visible image to see the other view.


Degree of crystallinity


Glassy texture in rhyolite obsidian that has only about 2% crystals, most minute. There are numerous tiny microlites with some flow alignment, and some small spherulites. Iceland.


Plane/cross-polarized light, field width is 3 mm.


I-45


Microlites are small crystals, typically elongate, typically more or less uniform in size, and typically scattered more or less evenly through a glassy sample. Several other samples in this category have microlites, but these are big enough to be easy to see. One could call them microphenocrysts, perhaps, but their abundance and uniform size suggest that eventually they would have grown to produce a non-phenocrystic crystalline rock, had the rhyolite liquid cooled slowly enough. Iceland.


Plane/cross-polarized light, field width is 3 mm.


ICE-22


Microcrystalline texture, showing relatively even grain size with a few larger mafic minerals. Though it looks entirely crystalline, about 25% glass remains in this sample. Narrow rhyolite dike, Montezuma, Colorado.


Plane/cross-polarized light, field width is 3 mm.


34


Grain size


Microcrystalline, <0.2 mm. Basalt with a few small olivine phenocrysts. Iceland.


Plane/cross-polarized light, field width is 6 mm.


I-27A


Fine-grained, 0.2-1 mm. This is an equigranular biotite-hornblende granite, with abundant quartz, plagioclase, and microperthite. Gilford, New Hampshire.


Plane/cross-polarized light, field width is 6 mm.


4.8.84P


Medium-grained, 1-5 mm. This is a biotite granite, with visible quartz, plagioclase, and perthite. Halifax, Nova Scotia, Canada.


Plane/cross-polarized light, field width is 6 mm.


NEGSA03-403-8


Coarse-grained, 5-15 mm. This is a somewhat altered, augite-bearing granitoid from a dike. The igneous complex is mostly granitic in composition, but this piece is a tonalite, possibly a cumulate of some sort. Vinalhaven Island, Maine.


Plane/cross-polarized light, field width is 6 mm.


VH-73


Equigranular biotite granite. Most grains are about the same size. Notice the allanite crystal in the upper-right. Cape Ann, Massachusetts.


Plane/cross-polarized light, field width is 6 mm.


4.7.84G


Inequigranular anorthosite. Ths rock has a continuum of grain sizes, from very small to quite large. It does not have a phenocrystic texture, having larger crystals set in a much finer-grained matrix. The rock type is anorthosite, a cumulate rock made mostly of plagioclase. Cumulate rocks typically have the same minerals about the same size. In this case, grain textures look somewhat annealed, possibly because of deformation and annealing when it was still very hot. Stillwater Complex, Montana.


Plane/cross-polarized light, field width is 6 mm.


STW-37


Phenocrysts of olivine and augite in an otherwise microcrystalline basalt. These phenocrysts are medium-grained, and euhedral to subhedral. Iceland.


Plane/cross-polarized light, field width is 6 mm.


I-5


Porphyritic texture in a trachite. Porphyries are rocks with a large fraction of phenocrysts, typically around 50%. At this phenocryst fraction, grain-on-grain contact would make them extremely viscous. Traditionally the phenocryst matrix in porphyries is relatively fine-grained, suggestive of extrusive rocks or shallow levels of intrusion. Notice the strong zoning in the alkali feldspars, with orthoclase cores, a layer of plagioclase lath inclusions, and a rim of highly twinned alkali feldspar, possibly albitic. The green mafic minerals are aegirine, some of which have nearly colorless augite cores. The complex textures suggest magma mixing. Black Hills, South Dakota.


Cross-polarized light, field width is 6 mm.


Black Hills


Bubble textures


Pumice, made of about 90% gas bubbles. The glass colorless to light-brown, and hard to see with all the air bubbles entrained in the epoxy (bubbles in epoxy have thin, birefringent rims). There are several phenocrysts of brown to green pyroxene, magnetite, and plagioclase. Hekla volcano, Iceland.


Plane/cross-polarized light, field width is 3 mm.


I-50


Scoria. The left piece is more oxidized, and the right one relatively fresh. Both contain crystals of augite and plagioclase in addition to about 60% vesicles. Trachyandesite, Vulsini, Valentano, Italy.


Plane/cross-polarized light, field width is 3 mm.


RMP-16


Volcanic ash, with oddly-shaped, colorless glass shards that are broken pumice vesicle walls. Mixed with them are finer-grained ash, partly altered, and small crystals of biotite and feldspar. Wyoming.


Plane/cross-polarized light, field width is 0.6 mm.


W56


Non-welded tuff, with several different texturally and compositionally different fragments that have different proportion of glass and crystals, set in a fine-grained ash matrix. The fragments have not been compressed, and the rock is only loosly held together. Iceland.


Plane/cross-polarized light, field width is 6 mm.


I-X2


Welded tuff, with pumice fragments, glass shards, vesicles, and rock fragments variously flattened and folded. The rock is somewhat altered and oxidized. Rhyolite, Continental Divide near Estes Park, Colorado.


Plane/cross-polarized light, field width is 3 mm.


CO88-2


Filled vesicles, called amygdules (amygdales in the Old World). Here the basalt originally had phenocrysts of plagioclase and numerous bubbles. Watery fluids passing through the rock altered it somewhat, and filled the vesicles first with quartz, then celadonite, and finally calcite. Vesicle fillings can include a wide array of low-temperature minerals which, besides those shown here, may include zeolites, albite, K-feldspar, epidote, clay, and others. Celadonite, by the way, is a low-temperature dioctahedral mica, approximately K(Mg,Fe2+)(Fe3+,Al)Si4O10(OH)2. Source unknown.


Plane/cross-polarized light, field width is 6 mm.


Altered Basalt


Shapes of crystals


Euhedral olivine phenocryst in basalt. Euhedral crystals are entirely bounded by flat crystal faces. Basalt dike in the Skaergaard Intrusion, Greenland.


Plane/cross-polarized light, field width is 6 mm.


Green 9


Subhedral crystals of plagioclase, olivine, augite, and magnetite in a cumulate gabbro. Subhedral crystals have some flat crystal faces, or shapes that approximate them. In this sample, plagioclase grains are subhedral, but olivine, augite, and magnetite are anhedral. Skaergaard intrusion, Greenland.


Cross-polarized light, field width is 6 mm.


Green 6


Anhedral crystals in a thin hornblende-biotite granite dike. Anhedral crystals have no flat crystal faces and generally don't have the approximate shapes of a euhedral crystal. Skaergaard intrusion, Greenland.


Plane/cross-polarized light, field width is 6 mm.


Green 8


Spherulites are spots where fibrous crystals radially grow from a nucleation point, usually during strong supercooling of the magmatic liquid. The radial growth causes them to characteristally have an extinction cross, typically with the N-S and E-W directions being at or close to extinction. This sample of rhyolite also has strong flow alignment of the microlites. Iceland.


Plane/cross-polarized light, field width is 1.2 mm.


I-46


Radial growth of feldspar needles in obsidian. These are distinct from the brownish spherulites that occur in the same sample. Iceland.


Plane/cross-polarized light, field width is 0.6 mm.


I-46


Feathery texture of pyroxene in the matrix of basalt, in the matrix between glomerocrysts. The feathery texture is a variety of rapid growth between spherulitic and skeletal. Near the center-top there is some radial growth of plagioclase. Iceland.


Plane-polarized light, field width is 0.6 mm


I-59


Skeletal plagioclase phenocrysts in basalt, along with some of olivine, in a microcrystalline matrix. Skeletal growth takes place during strong supercooling, when crystals grow projections into the surrounding liquid. Iceland.


Plane/cross-polarized light, field width is 1.2 mm.


I-30


Embayed crystal of quartz in a dacite porphyry. Embayed crystals are evidence that the mineral was unstable in the magma, and was dissolving away. Dissolution is faster along crystal defects, which can give such crystals odd shapes. Porphyrys are volcanic or fine-grained, hypabyssal rocks with abundant phenocrysts, typically around 50%. Continental Divide, Montezuma, Colorado.


Plane/cross-polarized light, field width is 3 mm.


28-Py


Sieve texture, in microcline in rhyolite obsidian. The dark blobs that make up the sieve appearance are melt inclusions that formed during rapid, probably skeletal crystal growth. Slower growth later closed off the space between skeletal projections, trapping liquid. The microcline has two Carlsbad twins. There are also phenocrysts of magnetite and augite. North of Mýrdalsjökul, Iceland.


Plane/cross-polarized light, field width is 3 mm.


ICE-21


Annealed texture, in this case basalt that was held at high temperature for a long period of time. The original grains have recrystallized into approximately equant, anhedral grains of colorless plagiocase, colorless, high-index olivine, pale brown augite, and black magnetite. Grain intersection angles are typically near 120° angles. Xenolith in the Skaergaard Intrusion, Greenland.


Plane-polarized light, field width is 1.2 mm.


Green 40


Cumulate textures


Orthocumulate texture, with cumulate plagioclase and olivine crystals enclosing quenched basaltic liquid. If this were more slowly-cooled, the trapped liquid pockets would have crystallized into overgrowths on adjacent plagioclase and olivine, plus a few relatively large grains of magnetite, augite, apatite, and other interstitial phases. Iceland.


Plane/cross-polarized light, field width is 6 mm.


I-27B


Mesocumulate texture, in which liquid trapped between cumulate crystals was able to exchange material with the nearby magma, allowing more extensive overgrowths of cumulate minerals and a smaller volume of ultimately trapped liquid. Small grains of olivine, pyroxene, magnetite, and apatite hint indicate the original trapped liquid. Skaergaard Intrusion, Greenland.


Plane/cross-polarized light, field width is 6 mm.


Green 14


Adcumulate texture, where essentially all trapped liquid was able to exchange material with the nearby magma, allowing overgrowths to fill the trapped liquid pockets. This results in a rock almost completley made of the cumulate minerals, and so very simple mineralogy. Here the cumulate minerals were plagioclase and orthopyroxene. Stillwater Complex, Montana.


Plane/cross-polarized light, field width is 6 mm.


STW-33


Other igneous textures


Glomerocryst made of plagioclase and olivine in basalt. Glomerocrysts are clumps of large crystals set in a fine-grained matrix, a variety of phenocrystic texture. Glomerocrysts probably result in nucleation of new crystals taking place on or very near the surfaces of crystals already present. Iceland.


Plane/cross-polarized light, field width is 3 mm.


I-59


Ophitic texture, in which large pyroxene crystals enclose randomly-oriented plagioclase and commonly other minerals. This texture forms where there are few pyroxene nuclei, so those few grow quite large, enclosing the other minerals. This example, from the marginal border series of the intrusion, has augite enclosing plagioclase and olivine. Skaergaard Intrusion, Greenland.


Plane/cross-polarized light, field width is 3 mm.


Green 24


Subophitic texture, in which randomly-oriented plagioclase crystals touch one another, generally enclosing pyroxenes and other minerals. This diabase from a thick lava flow contains abundant interstitial granitoid material, brown, isotropic altered glass, skeletal magnetite, and other interesting features. East Granby, Connecticut.


Plane/cross-polarized light, field width is 3 mm.


NEGSA95-C1-1


Poikilitic texture, in which one large crystal encloses many smaller crystals. It is similar to ophitic textures, but the enclosing mineral need not be pyroxene. Greenville, Maine.


Plane/cross-polarized light, field width is 6 mm.


NEIGC83-C1-8


Chilled margin from a pillow lava rim. The pillow margin quenched to glass, enclosing euhedral phenocrysts. The more slowly-cooled interior first grew spherulites, which increase in abundance inward, coalescing into a solid mass of them on the right side of this image. Farther in, sizable crystals grew in the matrix to give more typical basaltic textures. Iceland.


Plane/cross-polarized light, field width is 6 mm.


ICE-24A


Xenocryst in basalt. In general, it's difficult to tell xenocrysts from phenocrysts if they are the same mineral. In this case, the olivine basalt has xenocrysts of quartz, which could not have crystallized from this magma. The quartz grains reacted with the magma to form a garland of small augite crystals. You can actually get a uniaxial positive interference figure from this grain. Iceland.


Plane-polarized light, field width is 3 mm.


I-12


Xenolith in rhyolite. Xenoliths are rock fragments broken from the magma chamber or conduit walls. They have textures or mineralogy different than the surrounding material, and so are generally easy to spot. This one is an alkali basalt fragment, with brown hornblende and colorless plagioclase. Much of the hornblende is nearly black because of oxidation in air around the time of eruption. Unknown source.


Plane/cross-polarized light, field width is 6 mm.


Rhyolite-2


Flow-aligned plagioclase crystals in an olivine basalt. Flow alignment can occur in any magma type or grain size, representing differential (shear) flow probably close to the time of solidification. Iceland.


Plane/cross-polarized light, field width is 1.2 mm.


I-25


Granophyre in a highly fractionally crystallized gabbro. Granophyre, or 'graphic granite,' is an ordered intergrowth of quartz and alkali feldspar. In some cases these are found in pegmatites and other felsic rocks, but the textures are also found in mafic rocks as the last bit of evolved, felsic liquid crystallizes. Here, the granophyre formed in the last bit of interstitial liquid, among cumulate olivine, plagiocalse, magnetite, apatite, brownish magmatic augite, and, shown here, green augite that is inverted ferrobustamite. Skaergaard Intrusion, Greenland.


Plane/cross-polarized light, field width is 6 mm.


Green 29


Magmatic reaction textures


Hornblende rimming quartz. This shows a quartz-rich xenolith enclosed in a granitic porphyry. At the time the xenolith became trapped, the magma was undersaturated with respect to quartz. The quartz reacted with the magma to grow a rim of hornblende. Later, the magma became quartz-saturated and grew quartz and hornblende as distinct, separated crystals. Laconia, New Hampshire.


Plane/cross-polarized light, field width is 6 mm.


4-8-84K


Hornblende replacing olivine and augite. This is an alkaline olivine gabbro. The primary magmatic minerals include plagioclase, olivine, augite, biotite, and late brown hornblende. Probably very close to solidus temperatures olivine and augite reacted with the hydrous residual liquid to grow hornblende rims on both olivine and augite. Imperfections in the augite, possibly trapped liquid inclusions or exsolution lamellae, also reacted to grow hornblende within augite crystals. Note that the hornblende rims are brown on augite and magnetite, and green on parts of the olivine. The green hornblende is low in titanium, contrasting with the brown high-titanium hornblende. Salem, Massachusetts.


Plane/cross-polarized light, field width is 3 mm.


4.7.84E


Subsolidus unmixing textures


Perthite exsolution texture, with irregular albite exsolution lamellae in microcline host crystals. This represents subsolidus unmixing of an originally homogeous K-Na feldspar solid solution. Vinalhaven Island, Maine.


Plane/cross-polarized light, field width is 6 mm.


VH1


Mesoperthite exsolution texture, in which an intermediate K-Na feldspar has exsolved to approximately equal proportions of albite and K-feldspar. In this case, the rock is a cumulate syenite, with large, slab-shaped cumulate feldspars. Red Hill, Center Harbor, New Hampshire.


Plane/cross-polarized light, field width is 3 mm.


RH-4


Antiperthite exsolution texture, in which K-feldspar has unmixed from a plagioclase host. In this case the rock is a somewhat deformed diorite, and the exsolved K-feldspar lamellae have annealed somewhat into angular shapes. Union, Connecticut.


Cross-polarized light, field width is 6 mm.


WL-H36A


Pyroxene exsolution textures. This rock is a gabbro which originally had crystals of homogeneous pigeonite and augite. The augite, represented by the pinkish-yellow grain to the right, has pigeonite exsolution lamellae. The original pigeonite, represented by the large gray grain to the left, exsolved augite lamellae as it inverted to orthopyroxene. Remember that orthopyroxene has much lower birefringence than augite. Unknown source.


Plane/cross-polarized light, field width is 3 mm.


Un5


Subsolidus reaction textures


Myrmekite. Myrmekite is a wormy intergrowth of quartz and feldspar, generally forming as a subsolidus reaction product between feldspar and migrating fluids. Continental Divide near Estes Park, Colorado.


Cross-polarized light, field width is 1.2 mm.


CO88-1


Chlorite replacing garnet. On the left is a magmatic garnet in a peraluminous granite. It was fractured, and aqueous fluid caused it to react to form chlorite. The chlorite is pale-green to pale-brown in plane light, with low first-order anomalous Berlin blue interference colors. Kinsman Granite, New Hampshire.


Plane/cross-polarized light, field width is 3 mm.


NHM-21


TiO2 replacing ilmenite in a highly altered gabbro. The original gabbro probably was made of plagioclase, augite, magnetite, and ilmenite, among other things. Alteration without deformeation changed this assemblage to one largely made of actinolite and hornblende, chlorite, and sericite. As ilmenite broke down, the TiO2 in its structure precipitated in-place to partially pseudomorph the original ilmenite. Such fine-grained TiO2 is sometimes called leucoxene. The TiO2 polymorph is commonly anatase. Northwestern New Hampshire.


Plane/cross-polarized light, field width is 0.6 mm.


NNH-7