Common Rocks

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Common Rocks in Hand Specimen

source most images from Earth Science Australia
descriptions from various resources

35 of the most common rocks with brief descriptions

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rock cycle

amphibolite,

is a coarse-grained metamorphic rock that is composed mainly of green, brown, or black amphibole minerals and plagioclase feldspar. The amphiboles are usually members of the hornblende group

andesite,

is an extrusive igneous, volcanic lava  rock, of intermediate composition, with aphanitic to porphyritic texture. It is the intermediate type between basalt and dacite, and ranges from 57 to 63% silicon dioxide (SiO2)

anorthosite,

is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar (90–100%), and a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.

basalt,

a dark fine-grained extrusive igneous  volcanic lava rock that sometimes displays a columnar structure, typically composed largely of plagioclase with pyroxene and olivine.

breccia,

is a term most often used for clastic sedimentary rocks that are composed of large angular fragments (over two millimeters in diameter). The spaces between the large angular fragments can be filled with a matrix of smaller particles or a mineral cement that binds the rock together.

conglomerate,

is a coarse-grained clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts, e.g., granules, pebbles, cobbles, and boulders, larger than 2 mm (0.079 in) in diameter.

diabase / dolerite,

microgabbro is a mafic, holocrystalline, subvolcanic rock, the medium-grained shallow intrusive equivalent to volcanic basalt or plutonic gabbro. While similar to basalt, it contains crystals which can be seen with a hand lens. This indicates that it cooled a little more slowly than basalt. Typically it is found in volcanic plugs and dykes which channeled the basalt to the surface. It  is composed of plagioclase feldspar (mostly labradorite) crystal laths surrounded by  pyroxene (augite). Ophitic texture (randomly oriented plagioclase laths enclosed by pyroxene grains) is a characteristic feature of diabase

diorite,

is an intrusive igneous rock composed principally of the silicate minerals plagioclase feldspar (typically andesine), biotite, hornblende, and/or pyroxene.

dolomite / dolostone

is a common sedimentary rock primarily composed of the mineral dolomite - an anhydrous calcium magnesium carbonate with a chemical composition of CaMg(CO3)2

gabbro,

is a dark, medium- to coarse-grained intrusive igneous rock composed of calcium plagioclase, pyroxene, and minor olivine, but no quartz. It is the intrusive equivalent of a basalt.

gneiss - biotite,

is a common distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. It is often foliated (composed of layers of sheet-like planar structures). The foliations are characterized by alternating darker and lighter colored bands, called "gneissic banding" Light-gray, medium- to coarse grained, in this case containing interlayered biotite

gneiss - garnet,

is a common distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. It is often foliated (composed of layers of sheet-like planar structures). The foliations are characterized by alternating darker and lighter colored bands, called "gneissic banding" Light-gray, medium- to coarse grained, in this case containing interlayered garnet

granite - biotite,

is a common type of felsic intrusive igneous rock that is granular and phaneritic in texture. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Granite is an igneous rock with between 20% and 60% quartz by volume, and at least 35% of the total feldspar consisting of alkali feldspar, although commonly the term "granite" is used to refer to a wider range of coarse grained igneous rocks containing quartz and feldspar.In this case the main "mafic" or dark mineral is biotite

granite - hornblende,

is a common type of felsic intrusive igneous rock that is granular and phaneritic in texture. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Granite is an igneous rock with between 20% and 60% quartz by volume, and at least 35% of the total feldspar consisting of alkali feldspar, although commonly the term "granite" is used to refer to a wider range of coarse grained igneous rocks containing quartz and feldspar.In this case the main "mafic" or dark mineral is hornblende

greywacke,

 is a sedimentary rock - hard dark "dirty"sandstone with poorly sorted angular grains of quartz, feldspar, and small rock fragments in a compact, clay-fine matrix consisting of at least 15% clay  

lamprophyre - mica,

is an ultrapotassic igneous intrusive rock characterized by the presence of phenocrysts of mica, usually phlogopite or biotite, and/or amphibole, usually hornblende or pargasite, together with smaller amounts clinopyroxene and/or melilite,primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions.

limestone - crystalline,

is a chemically dervied sedimentary rock, composed of the mineral calcite (calcium carbonate) precipitated  directly from  a  solution  of calcite carbonate in lake or ocean water.or by evaporation such as in cave statagtites and stalagmites / hot springs

limestone,

is an organic sedimentary rock primarily made of the mineral calcite derived from mainly of skeletal fragments of marine organisms,  and/or organic calcareous material. It most commonly forms in clear, warm, shallow marine waters.

marble,

is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite

peridotite / dunite

 is an important ultramafic plutonic igneous rock  because the Earth’s mantle is predominantly composed of it. So while we rarely see perodotite,  it is probably by volume the most common rock on earth, just mostly found out of sight, beneath the solid crust of the earth. A dense, coarse-grained igneous rock consisting mostly of the minerals olivine and pyroxene, peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron
Dunite is a type of peridotite that is almost monomineralic (more than 90% of it is olivine).

porphyry - feldspar,

is an intrusive igneous rock that contains larger crystals (phenocrysts) of feldspar  in a fine-grained groundmass. It is formed by "two-stage" cooling. At first the magma cools very slowly permitting the feldspar crystals to grow large then the magma cools quickly forming a fine grained matrix.

pyroxenite,

is an ultramafic plutonic igneous rock consisting essentially of minerals of the pyroxene group, such as augite, diopside, hypersthene, bronzite or enstatite. Pyroxenites occur in discrete inclusions, in layered sills (tabular bodies inserted between other rocks) and lopoliths (laccoliths with basin-shaped bases), in branching veins, in narrow dikes (tabular bodies injected in fissures), and at the edges of silica-poor plutons (intrusive igneous rock bodies).

quartzite,

is a hard, non-foliated metamorphic rock which was originally pure quartz sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts

rhyolite,

is an extrusive igneous lava rock with a very high silica content. It is usually pink or gray in color with grains so small that they are difficult to observe without a hand lens. Rhyolite is made up of quartz, plagioclase, and sanidine, with minor amounts of hornblende and biotite. Distinguished from andesite by its sugary texture and often the presence of quartz "eyes"

sandstone,

is a clastic sedimentary rock composed mainly of sand-sized minerals or rock grains The cement that binds the clasts can vary from clay minerals to calcite, silica or iron oxides.
Sandstone can be further divided according to: clast size - fine (0.06-0.2mm), medium (0.2-0.6mm), coarse (0.6-2mm)

schist - biotite,

is a medium-grade metamorphic rock[ with medium to large, flat, sheet-like grains in a preferred orientation (nearby grains are roughly parallel). It is defined by having more than 50% platy and elongated minerals, in this case biotite mica  finely interleaved with quartz and feldspar. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced.

schist - chlorite,

is a  greenschist - a metamorphic rock that formed under the lowest temperatures and pressures.Chlorite schist is mostly made of chlorite and few other minerals. This chlorite schist sample has silt size chlorite crystals. The platy chlorite is roughly aligned parallell to make weak foliation.Chlorite schist is usually metamorphosed from basalt in a low grade metamorphic condition. Since basalt contains no quartz, chlorite schist does not contain quartz either.

serpentenite - impure,

is a metamorphic rock that forms at tectonic plate boundaries deep within the Earth. It can form for example when ocean water carried down with subducting ocean crust is heated and moved through upper mantle and basal ocean crust rocks, hydrating their magnesium- and iron-rich minerals, like olive and pyroxene, to form magnesium-rich serpentine minerals. almost exclusively made of serpentine minerals which give the serpentinite its characteristic light to dark green color. Serpentine minerals are made of tiny sheets of silica tetrahedrons that are loosely held together. The weak bonds between these sheets gives serpentine its greasy or scaly look, and slippery feel (like a snake skin). Serpentinite often contains many veins, some of which may be filled with the fibrous mineral chrysotile (a form of asbestos). Chrysotile is a serpentine mineral in which the silica sheets are rolled into tiny tubes to form hollow fibers. Loose asbestos fibers cause lung damage if you inhale them.

shale,

is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite.

slate,

is a fine-grained, foliated metamorphic rock that is created by the alteration of shale or mudstone by low-grade regional metamorphism. Slate is composed mainly of clay minerals or micas, depending upon the degree of metamorphism to which it has been subjected.

syenite - hornblende,

is a coarse-grained intrusive intermediate plutonic igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (<5%). . Alkali feldspar (e.g. orthoclase) is the major mineral component of syenite, total feldspar content is > 65%, and quartz is typically lacking. It is found in continental settings, and typically results from partial melting of the lower crust. Syenite is the plutonic equivalent of trachyte.

syenite - nepheline,

is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are also known. Phonolite is the fine-grained extrusive equivalent. typically are formed by low degrees of partial melting in the Earth's mantle. Carbon dioxide may dominate over water in source regions. Magmas of such rocks are formed in a variety of environments, including continental rifts, ocean islands, and supra-subduction positions in subduction zones.

talc - schist,

known for being the softest mineral on earth, talc is a clay mineral composed of hydrated magnesium silicate with the chemical ... from white to grey or green and it has a distinctly greasy feel. Its streak is white. It  is a common metamorphic mineral in metamorphic belts that contain ultramafic rocks, such as soapstone (a high-talc rock), and within whiteschist and blueschist metamorphic terranes. Soapstone is a metamorphic rock composed predominantly of talc, that results from the metamorphism of magnesian minerals such as serpentine, pyroxene, amphibole, and olivine, in the presence of carbon dioxide and water. This is known as "talc carbonation" or "steatization" and produces a suite of rocks known as talc carbonates.

trap - amygdaloidal,

is a basalt, a dark coloured volcanic rock formed from a magma of basic composition erupted on the Earth's surface. Magmas generally contain dissolved gas, which can form bubbles in the magma as the pressure is released on eruption. These bubbles can get trapped in the solidified rock. After some time, groundwater or hot solutions connected with the volcanic activity pass through the porous lava and deposit crystals in the open cavities, which gradually fill up with quartz, calcite (calcium carbonate) or other minerals called zeolites. Filled cavities in lavas are called amygdales, and a rock full of them can be called amygdaloidal. The amygdales are usually white in colour.

tuff

is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is compacted into a solid rock in a process called consolidation.