basalt
| Basalt is a hard gray or black, mafic igneous volcanic rock that is usually fine-grained due to rapid cooling of lava, though it contain larger crystals in a fine matrix (porphyritic), be vesicular, or be a frothy scoria.
Basalt (B) is TAS classified according to the relationships between the combined alkali content and the silica content. (diagram below right - click to enlarge). QAPF classification relies on the fact that basalt typically contains a preponderance of calcic plagioclase feldspar and pyroxene; olivine can also be a significant constituent. Accessory minerals include iron oxides and iron-titanium oxides, providing basalt with a paleomagnetic signature.
Phaneritic, shallow intrusive igneous rocks with a basaltic composition are generally referred to as dolerite (also called diabase) or gabbro. (image above left - click to enlarge - courtesy USGS - top-down: basaltic lava; lava field; flow-lines in basalt formation; close-up of vesicular basalt with olivine crystals; surface of basalt hand specimen; basalt columns.) |
| Basalts are subdivided according to their geochemistry and mineralogy: |
 links: images - roll-over link for preview (where available); large images of hand-speciments (well worth a visit) show only as a corner on preview; formations: water-sculpted basalt at Fossil Falls in Yosemite : Basalt Fall unterhalb des Hengifoss, basalt columns, Dverghamrar basaltic columns in Iceland, 2 : cliff of basalt columns : Columbia River basalts, Catherine Creek arch in Miocene columnar basalts : flowing curves of basalt entablature in Yellowstone : water-carved basalts, Fossil Falls State Park in California; basalt columns Armenia : basalt field : basalt columns : basalt and sandstone : top surface of basalt : top of columnar basalts : Oblique view of columns : Basalt Rock Columns, 2: Pahoehoe (ropy structures in fluid flow) : Pahoehoe (oxidized) - Temperance River : Pipe vesicles concentrated at flow top, Sugarloaf Cove : pillow structures : paleosol developed on basalt : amethyst geodes in basalt, Ametista do Sul, Rio Grande do Sul, Brazil : basalt dikes that cut across the St. Cloud granite : Layer cake exposures at Temperance River : hand-specimens: 3.7 Ga moon-rock basalt : hand-specimen : hand-specimen vesicular basalt, vesicular basalt with olivine phenocrysts, 2 : hand-specimen diabase : hand-specimen diabase porphyry : hand-specimen diorite : hand-specimen gabbro : hand-specimen scoria : close-up: Amygdules (closeup) thin-sections: thin-section basalt, 2, 3; thin-sections moon basalts |
biotite
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mineral / chemical formula |
properties / significance / occurrence |
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biotite K (Fe, Mg)3 AlSi3 O10 (F, OH)2 |
click image to enlarge - biotite crytals and slice. 
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Phyllosilicate, mica, monoclinal crystal, layered structure of iron magnesium aluminum silicate sheets weakly bonded together by layers of potassium ions; the biotite series has Fe-endmember, annite, Mg-endmember, phlogopite, and ; Al-endmembers including siderophyllite. Biotite is present in most igneous rocks, in pegmatite veins, and in both regional and contact metamorphic rocks where the partitioning of Fe and Mg between biotite and garnet is sensitive to temperature. Employed in K-Ar or Ar-Ar radiodating. |
| biotite : | ||
| composition | K (Fe, Mg)3 AlSi3 O10 (F, OH)2 potassium iron magnesium aluminum silicate hydroxide fluoride | |
| color | brown to black; yellow on weathering | |
| lustre | vitreous to pearly | |
| transparency | translucent to transparent | |
| crystal system | monoclinic; 2/m | |
| crystal habits | tabular to prismatic crystals; prominant pinacoid termination; four prism faces and two pinacoid faces form pseudo-hexagonal crystal "books"; lamellar or granular rock forming masses providing the luster for most schists and gneiss. | |
| cleavage | perfect in one direction producing thin sheets or flakes | |
| fracture | uneven (not readily observed because of tendency to perfect cleavage) | |
| hardness | 2.5 | |
| specific gravity | 2.9 - 3.4 | |
| streak | white | |
| other | pleochroic; present in most igneous rocks, and common in both regional and contact metamorphic rocks | |
| associations | quartz, feldspars, apatite, calcite, hornblende, garnets and schorl. | |
| occurences | numerous, including Bancroft and Sudbury, Ontario; Sicily; Russia | |
| indicators | crystal habit, color, cleavage, elastic sheets and associations | |
| links: crystals: biotite, book, 2, muscovite and biotite wp; rocks: diorite, trachyte, Madagascar; formations: Série métapélitique de l'Agly, Schiste à biotite; thin-section: garnet-mica schist with anomalous red interference color, biotite wp, 2, 3, 4, 5 | ||
image courtesy USGS
◊◊◊ Mineral Index ◊◊◊
blueschist facies
Blueschists are generally considered to be diagnostic of former subduction zones, because they imply relatively high pressure conditions relative to the temperature (compared to normal geothermal gradients). This low temperature, high pressure prograde metamorphic path is exposed in the US westcoast Franciscan facies series. (image of section of Ward Creek blueschist, courtesy of Andrew Alden, geology.about.com, reproduced under educational fair use: wallpapers: lo-res, hi-res) Blueschists appear blue, black, gray, or blue-green in outcrop, and approximate basaltic elemental composition. When blue, these rocks are tinted by sodium-bearing blue amphiboles glaucophane or crossite rather than by calcium-bearing green or black amphiboles actinolite or hornblende, which are developed in the more commonly encountered greenschist facies or amphibolite facies metamorphism. Glaucophane is azure-blue, lavender-blue or bluish-black mineral. Felsic rocks and pelitic sediments subjected to blueschist facies conditions form different mineral assemblages than does metamorphosed basalt.
(blueschist thin-sections at right - top, blueschist facies basalt, Sivrihisar, Turkey; middle, blueschist facies quartz sediment, Sivrihisar, Turkey; bottom, garnet-lawsonite-glaucophane blueschist from Sivrihisar, Turkey)
Ultramafic rocks subjected to blueschist metamorphism contain the minerals serpentine/lizardite +/- talc +/- zoisite. Pelites yeild kyanite + zoisite +/- pargasite or phengite +/- albite +/- quartz +/- talc +/- garnets 
Granites yield kyanite +/- paragonite +/- chlorites +/- albite +/- quartz +/- pargasite or phengite Blueschists terranes are typically found within young orogenic belts in faulted contact with rocks of the higher-temperature greenschist facies (rarely eclogite facies). The rocks are typically metamorphosed oceanic sediments and basaltic oceanic crust. Blueschists and other high-pressure subduction zone rocks are probably exhumed rapidly (and thus preserved) by flow and/or faulting in accretionary wedges or the upper parts of subducted crust. They may also be exhumed partly due to isostatic buoyancy when the metabasaltic rocks are associated with low-density continental crust (marble, metapelite, and other rocks of continental margins). Evidence of many blueschist belts has probably been lost because the hydrous minerals of blueschist facies metamorphism are overprinted by facies resulting from normal geothermal gradients. Surface geothermal gradients of the order of 30°C/km are common in continental crust and in thick sedimentary basins, whereas a steady-state surface gradient of only about 15 to 20°C/km are sufficient for blueschist metamorphism. Overprinting occurs when circulating fluids convert blueschist minerals to greenschist and amphibolite facies mineral assemblages. In dehydrating reactions, the rocks become denser and lose volatiles. In situ eclogitic schist lenses occur in the coherent low-grade epidote-zone Ward Creek metabasite unit of the Central Franciscan belt.[r] In addition to the circum-Pacific Franciscan complex, Japanese chains, and New Caledonia, blueschists are also well exposed in Alpine-Himalayan chains in Greece, Turkey. The islands of Sifnos and Syros in the Cycladic archipelago in Greece and the Tianshan Mountains in NW-China are examples of previous subduction zones that may be relics of subduction channels with outcrops that conform to tectonic melanges. Hydrous reactions occured during retrogressive metamorphism from the eclogite facies back to the blueschist facies. These rocks of the blueschists, some show relics of omphacite and garnets indicating eclogitic facies.[wp] |
links: images: hand-specimens: blueschist, 2, 3, 4, 5, 6, 7, blueschist with eclogite, close-up; close-ups: blueschist with stubby needles of glaucophane (metamorphosed volcanic rock of basic composition, part of the old ocean floor of Tethys); lineated schistose foliation produced by the lining up of elongate amphibole (var. glaucophane) crystals; seen trending lower left and upper right in this blueschist; glaucophane schist, 2, blueschist, 2, 3; eclogite-facies pillow basalt, partly overprinted to blueschist; eclogite surrounded by blueschist (Syros, Greece); subduction zone: fragment of blueschist containing coarse, white lawsonite (CaAl2Si2O8.H2O) in a vug; metamorphosed under blueschist conditions towards the base of the subducted oceanic slab - compare with dull green, tectonically polished, lower greenschist/blueschist metamorphic rocks mainly composed of chlorite and epidote - note vein of glaucophane and phengite (muscovite) in the material; see: blueschist formations: thin-sections: Blueschist in the Dalradian of Achill Island; green chlorite, blue glaucophane, clear white mica flakes in blueschist xp, pl, epidote-clinozoisite wp; webpages: metamorphic facies; field trip CA; Closeup Rock Photos, Geology Wallpaper Index; Pitt |
blueschist formations
Supplement to blueschist facies: Blueschists are generally considered to be diagnostic of former subduction zones, because they imply relatively high pressure conditions relative to the temperature (compared to normal geothermal gradients). This low temperature, high pressure prograde metamorphic path is exposed in the US westcoast Franciscan facies series. Blueschists terranes are typically found within young orogenic belts in faulted contact with rocks of the higher-temperature greenschist facies (rarely eclogite facies). The rocks are typically metamorphosed oceanic sediments and basaltic oceanic crust. In addition to the circum-Pacific Franciscan complex, Japanese chains, and New Caledonia, blueschists are also well exposed in Alpine-Himalayan chains in Greece, Turkey. |
formations: Ring Mountain blueschist (foreground); Rand Mountains blueschist; folded blueschist from near the type locality of glaucophane on Syros, Greece; eclogite clast wrapped by blueschist (10 cm), isoclinal folding in blueschist with limbs attenuated by shearing (40 cm); refolded isoclinal folds in blueschist (20 cm), 2; high pressure blueschist and eclogite of the Maksutovo Complex, Urals, wp; blueschist boulder, Lake Pillsbury; Japan; webpages: The Franciscan Complex |
breccias
 Breccias are clastic, sedimentary rocks comprising a matrix with embedded angular fragments derived from fracture of a previous rock structure.
(images: left, basalt breccia with epidote groundmass; below right, impact breccia composed mainly of gneiss and amphibolite; below left, hydrothermal breccia, Cloghleagh Iron Mine, county Wicklow, Ireland.)  Breccias are varied rocks, depending upon their mode of formation. Classification of breccias relates to their constituents, mode of occurrence, constituent fragment size, types of clasts, and source of clasts. Megabreccias are very extensive breccias and large clasts.
Some breccias (lithified colluvium and debris flow sedimentary breccia) contain fragments that have been deposited as angular fragments or from mass wasting events on land or underwater – avalanches, debris flows, mudslides. Many breccias comprise consolidated talus or scree material, and are made up of accumulations of rock fragments that have fallen down steep hill slopes or cliffs. Breccia are often found above uncomformities, and so are associated with conglomerate, arkose and sandstone. (Conglomerates have rounded fragments, whereas breccias have angular clasts, within consolidated matrix.)  Other breccias are produced by the fragmentation of rocks during faulting (tectonic or monomictic breccia), or during volcanic eruptions (eruption breccia, vent breccia), or collapse breccia such as in karst areas, or form upon meteorite impacts (often in breccia dikes). Monomictic breccias result from rock deformation by shearing and granulation (cataclasis) in the process of tectonism or dislocation metamorphism, while impact breccias have been called "monomictic movement breccias". |
| images: hand-specimens: breccia, 2, 3, 4, chert breccia; breccia in copper, 2; impact breccia, 2; thin-sections : volcanic breccia; fault breccia in the Antietam Formation; Breccia Pernice marble, Breccia Aurora marble; formations: cliffs of volcanic breccias formed by lahars; Wawa xenolithic breccia, Wawa Heterolithic Breccia; Ries impact structure, Iggenhausen quarry, monomictic movement breccia, close-up; Azuara impact structure close-up; grit brecciation and mortar texture, Rubielos de la Cérida impact basin, and heavily brecciated and polished scour surface; strongly brecciated chert nodule, Malmian limestone, Ries impact structure; breccia dikes in Liassic limestones south of Belchite (Deutsche); webpages: Impact Breccias; Impact breccias and breccia dikes in the Azuara structure; Basal suevite breccia; Breccia dikes in the Azuara impact structure; The Barrachina suevite and suevite-like breccias; Impact breccia near Singra; Megabreccias; Monomictic movement breccias |
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