Common mafic rocks include basalt, diabase and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar. Chemically, mafic rocks are enriched in iron, magnesium and calcium and typically dark in color. In contrast the felsic rocks are typically light in color and enriched in aluminium and silicon along with potassium and sodium. The mafic rocks also typically have a higher density than felsic rocks. The term roughly corresponds to the older basic rock class. Mafic lava, before cooling, has a low viscosity, in comparison with felsic lava, due to the lower silica content in mafic magma.
Finer grained equivalents of gabbro are called diabase (also known as dolerite), although the term microgabbro is often used when extra descriptiveness is desired. Gabbro may be extremely coarse grained to pegmatitic, and some pyroxene-plagioclase cumulates are essentially coarse grained gabbro, some may exhibit acicular crystal habits. Gabbro is usually equigranular in texture, although it may be porphyritic at times, especially when plagioclase oikocrysts have grown earlier than the groundmass minerals.
olivine basaltchrysoliteolivine group
That magma crystallizes to mafic rocks such as gabbro and basalt. Ultramafic rocks such as peridotite and dunite can be residues left after extraction of magmas, and typically they are more enriched in olivine after extraction of partial melts. Olivine and high pressure structural variants constitute over 50% of the Earth's upper mantle, and olivine is one of the Earth's most common minerals by volume. The metamorphism of impure dolomite or other sedimentary rocks with high magnesium and low silica content also produces Mg-rich olivine, or forsterite.
Basalt. Basanite. Dacite. Felsite. Phonolite. Rhyolite. Trachyte.
Augite is an essential mineral in mafic igneous rocks; for example, gabbro and basalt and common in ultramafic rocks. It also occurs in relatively high-temperature metamorphic rocks such as mafic granulite and metamorphosed iron formations. It commonly occurs in association with orthoclase, sanidine, labradorite, olivine, leucite, amphiboles and other pyroxenes. Occasional specimens have a shiny appearance that give rise to the mineral's name, which is from the Greek augites, meaning "brightness", although ordinary specimens have a dull (dark green, brown or black) luster. It was named by Abraham Gottlob Werner in 1792. * Fassaite * Deer, W. A., Howie, R. A., and Zussman, J. (1992).
Pyroxene and feldspar are the major minerals in basalt and gabbro. The chain silicate structure of the pyroxenes offers much flexibility in the incorporation of various cations and the names of the pyroxene minerals are primarily defined by their chemical composition. Pyroxene minerals are named according to the chemical species occupying the X (or M2) site, the Y (or M1) site, and the tetrahedral T site. Cations in Y (M1) site are closely bound to 6 oxygens in octahedral coordination. Cations in the X (M2) site can be coordinated with 6 to 8 oxygen atoms, depending on the cation size.
It occurs as clear, white to gray, blocky to lath shaped grains in common mafic igneous rocks such as basalt and gabbro, as well as in anorthosites. The geological type area for labradorite is Paul's Island near the town of Nain in Labrador, Canada. It has also been reported in Norway, Finland and various other locations worldwide. Labradorite occurs in mafic igneous rocks and is the feldspar variety most common in basalt and gabbro. The uncommon anorthosite bodies are composed almost entirely of labradorite. It also is found in metamorphic amphibolites and as a detrital component of some sediments.
For instance, a series of basalt flows are assumed to be related to one another. A composition from which they could reasonably be produced by fractional crystallization is termed a parental melt. Fractional crystallization models would be produced to test the hypothesis that they share a common parental melt. At high degrees of partial melting of the mantle, komatiite and picrite are produced. Magma develops within the mantle or crust where the temperature and pressure conditions favor the molten state. After its formation, magma buoyantly rises toward the Earth's surface.
For instance, ulvospinel in igneous rocks such as basalt and gabbro commonly oxidizes during subsolidus cooling to produce regular intergrowths of magnetite and ilmenite. The process can determine what magnetic record is inherited by the rock. * * Vernon, Ron H., 2004, A Practical Guide to Rock Microstructure, Oxford University Press, Oxford.
Little KarooGreat KarooKlein Karoo
Since this massive extrusion of lava, Southern Africa has undergone a prolonged period of erosion, exposing the older, softer rocks, except where they were protected by a cap of dolerite. The genera present include: The lava outpourings that ended the Karoo deposition of rocks, not only covered the African surface, and other parts of Gondwana with a 1.6 km thick layer basaltic lava, but it also forced its way, under high pressure, between the horizontal layers of sedimentary rocks belonging to the Ecca and Beaufort groups, to solidify into dolerite sills. The long vertical fissures through which the lava welled up solidified into dikes which resemble the Great Wall of China from the air.
oceanicoceanic plateocean crust
Layer 2 could be divided into two parts: layer 2A – 0.5 km thick uppermost volcanic layer of glassy to finely crystalline basalt usually in the form of pillow basalt, and layer 2B – 1.5 km thick layer composed of diabase dikes. Layer 3 is formed by slow cooling of magma beneath the surface and consists of coarse grained gabbros and cumulate ultramafic rocks. It constitutes over two-thirds of oceanic crust volume with almost 5 km thickness. Mohorovičić discontinuity. Plate tectonics.
Hudson PalisadesNew Jersey PalisadesPalisades
The basalt cliffs are the margin of a diabase sill, formed about 200 million years ago, at the close of the Triassic period by the intrusion of molten magma upward into sandstone. The molten material cooled and solidified before reaching the surface. Water erosion of the softer sandstone left behind the columnar structure of harder rock that exists today. The cliffs are about 300 ft (100 m) thick in sections and originally may have reached 1,000 ft (300 m).
A third mode of occurrence of tachylite is as the margins and thin offshoots of dikes or sills of basalt and diabase. They are sometimes only a fraction of an inch in thickness, resembling a thin layer of pitch or tar on the edge of a crystalline diabase dike, but veins several inches thick are sometimes found. In these situations tachylite is rarely vesicular, but it often shows very pronounced fluxion banding accentuated by the presence of rows of spherulites that are visible as dark brown rounded spots. The spherulites have a distinct radiate structure and sometimes exhibit zones of varying color.
Dikes can vary in texture and their composition can range from diabase or basaltic to granitic or rhyolitic, but on a global perspective the basaltic composition prevails, manifesting ascent of vast volumes of mantle-derived magmas through fractured lithosphere throughout Earth history. Pegmatite dikes comprise extremely coarse crystalline granitic rocks - often associated with late-stage granite intrusions or metamorphic segregations. Aplite dikes are fine-grained or sugary-textured intrusives of granitic composition. The term "feeder dike" is used for a dike that acted as a conduit for magma. Magma flowed along, then out of the dike, then formed another feature.
It has been proposed that the Watchung basalt flows of the Watchung Mountains are extrusive eruptions of the same magma that created the Palisades Sill. Magnetic and gravity measurements have indicated the presence of a large subsurface dike between the Palisades intrusion and the Ladentown basalt, an extrusive body of Watchung basalt north of Suffern, New York. More recently, the various Watchung flows have been correlated to geochemically distinct layers within the Palisades sill, bolstering the theory that eruptions of the Palisades magma were responsible for the episodic flood basalts of the Newark Basin.
HudsonNorth RiverHudson River Watershed
Along the river, the Palisades are of metamorphic basalt, or diabases, the Highlands are primarily granite and gneiss with intrusions, and from Beacon to Albany, shales and limestones, or mainly sedimentary rock. The Narrows were most likely formed about 6,000 years ago at the end of the last ice age. Previously, Staten Island and Long Island were connected, preventing the Hudson River from terminating via the Narrows. At that time, the Hudson River emptied into the Atlantic Ocean through a more westerly course through parts of present-day northern New Jersey, along the eastern side of the Watchung Mountains to Bound Brook, New Jersey and then on into the Atlantic Ocean via Raritan Bay.
Ulvöspinel was first described by Fredrik Mogensen (1904-1978) from a dolerite layered intrusion in the Ulvö Islands, Ångermanland, Sweden in 1943. The locality is an iron, titanium and vanadium mining area that has been active since the 17th century. It is common in titaniferous magnetite iron ore deposits. It also occurs in kimberlites, in some reduced iron-bearing basalts and is common in lunar basalts.
blue stonebluestonesPennsylvania Bluestone
In Tasmania, the name bluestone is given to dolerite (diabase), which is a dominant stone variety in the landscape, and used in a variety of building roles. Timaru bluestone is an attractive building material, used both historically and to the present. It is a grey basalt similar to Victorian bluestone, quarried near Timaru in the South Island. Bluestone from near Kokonga in Central Otago is also widely used, and is the main construction material (often with facing of Oamaru stone, a local compact limestone) in many of the notable historic buildings in the southern South Island, most of which were constructed during the financial boom following the Central Otago gold rush.
The hill contains a slender, chilled margin of fine-grained basalt with most of the mass of the intrusion being made up of picrite and dolerite. Volcanic rocks on the hill are predominantly mafic. Minerals, and as well as volcanic material, found on the hill are follows: analcime, apophyllite, basanite, basalt, trachyte, aragonite, barite, calcite, chabazite, chlorite, heulandite, ilmenite, laumontite, leucitite, limburgite, montmorillonite, natrolite, nephelinite, olivine, pectolite, pyrite, phillipsite, prehnite, quartz, rhyolite, siderite, trachybasalt, trachyandesite, tuff and high level intrusives; rare volcaniclastic sediments.
Thulean PlateauBritish Tertiary Volcanic ProvinceBrito-Arctic province
Giant's Causeway – Polygonal basalt columns, which seen from above are large hexagonal pavements. Canna and Sanday – Basalt lava field with great thicknesses of boulder conglomerate, examples of periods of erosion by fast flowing rivers in between the lava flows. Rathlin Island – Paleogene and Neogene lava flows. Fingal's Cave on the Isle of Staffa – Polygonal basalt columns eroded to form a cave. Ailsa Craig – Volcanic plug. Cleveland Dyke, North Yorkshire – Dyke swarm related to the Mull intrusive complex. The dike complexes of the NAIP contain many examples of dolerite dike swarms found throughout the British Isles. Igneous rock. Geologic province. Mantle plume. Hotspot. Oceanic plateau.
plagioclase feldsparcleavelanditesodic plagioclase
Labradorite is the characteristic feldspar of the more basic rock types such as diorite, gabbro, andesite, or basalt and is usually associated with one of the pyroxenes or amphiboles. Labradorite frequently shows an iridescent display of colors due to light refracting within the lamellae of the crystal. It is named after Labrador, where it is a constituent of the intrusive igneous rock anorthosite which is composed almost entirely of plagioclase. A variety of labradorite known as spectrolite is found in Finland. Andesine is a characteristic mineral of rocks such as diorite which contain a moderate amount of silica and related volcanics such as andesite.
mineralsmineral depositsaccessory mineral
A mineral is, broadly speaking, a solid chemical compound that occurs naturally in pure form. Minerals are most commonly associated with rocks due to the presence of minerals within rocks. These rocks may consist of one type of mineral, or may be an aggregate of two or more different types of minerals, spacially segregated into distinct phases. Compounds that occur only in living beings are usually excluded, but some minerals are often biogenic (such as calcite) or are organic compounds in the sense of chemistry (such as mellite). Moreover, living beings often synthesize inorganic minerals (such as hydroxylapatite) that also occur in rocks.
Feldspars (KAlSi 3 O 8 – NaAlSi 3 O 8 – CaAl 2 Si 2 O 8 ) are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight.
titanomagnetitebiomagnetiteFe 3 O 4
Magnetite is a rock mineral and one of the main iron ores, with the chemical formula Fe 3 O 4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. It is the most magnetic of all the naturally-occurring minerals on Earth. Naturally-magnetized pieces of magnetite, called lodestone, will attract small pieces of iron, which is how ancient peoples first discovered the property of magnetism. Today it is mined as iron ore.
Ilmenite, also known as manaccanite, is a titanium-iron oxide mineral with the idealized formula. It is a weakly magnetic black or steel-gray solid. From a commercial perspective, ilmenite is the most important ore of titanium. Ilmenite is the main source of titanium dioxide, which is used in paints, printing inks, fabrics, plastics, paper, sunscreen, food and cosmetics.