Glossary

Albite – the silicate mineral (Tectosilicate), representing the Na-rich end-member (NaAlSi3O8) of the plagioclase solid solution series. The Ca-rich end-member is anorthite (CaAl2Si2O8). Plagioclase feldspars are the most common minerals of magmatic rocks.

Alkali feldspars –  the silicate minerals (Tectosilicate), consisting in mixtures of sodium aluminosilicate (albite, NaAlSi3O8) and potassium aluminosilicate (KAlSi3O8). Very common in many magmatic rocks.

Allanite – the silicate mineral (Epidote group),  discovered in 1810 by the Scottish mineralist Tomas Allan from which it takes its name. The mineral, economically very important for the extraction of Rare Earth chemical elements (REE), occurs mainly in metamorphosed clay-rich sediments and felsic rocks. It has the general formula A2M3Si3O12[OH], where the A sites can contain large cations, such as Ca2+, Sr2+, REE and the M sites admit Al3+, Fe3+, Mn3+, Fe2+, or Mg2+ among others.

Amphibole – a group of silicate minerals (Inosilicate) very common in magmatic and metamorphic rocks. Of general formula W0-1X2Y5Z8O22(OH,F)2, they display wide compositional and morphological variability and represent the most chemically complex group of minerals in nature.

Anhedral – a crystal lacking any of its characteristic faces. A mineral delimited by regular geometric surfaces is called, instead, euhedral crystal, while if delimited only partially by regular faces is called subhedral.  A rock with an anhedral texture is composed of mineral grains that have no well-formed crystal faces.

Anorthite – the silicate mineral (Tectosilicate), representing the Ca-rich end-member (CaAl2Si2O8)  of the plagioclase solid solution series.  The Na-rich end-member is albite (NaAlSi3O8). Plagioclase feldspars are the most common minerals of magmatic rocks.

Apatite – a generic term used to refer to a mineral composed of calcium phosphate with general formula Ca5(PO4)2[F, OH, Cl]. It is commonly found as an accessory mineral in magmatic rocks but can also be found in sedimentary and metamorphic rocks.

Aragonite – the metastable polymorphic modification of calcium carbonate (CaCO3); it turns into calcite at temperatures between 380 and 470 °C. 

Argillite –  a clastic sedimentary rock, formed by diagenesis of clay sediments. It consists of very fine clasts, the diameter of which is less than 0.062 mm; can be characterized by the presence of fissility (the ability or tendency of a rock to split along flat planes of weakness).

Bastite texture – a texture that occurs in serpentinites, where pyroxene crystals were completely replaced by aggregates of serpentine minerals retaining the prismatic shape of the original minerals.

Biotite – the silicate mineral (Phyllosilicate)  within the Mica group, with the approximate chemical formula K(Mg,Fe2+)3[AlSi3O10(OH,F)2. It is primarily a solid- solution series between the Fe-end member Annite and the Mg-endmember Phlogopite. It is commonly found in many magmatic rocks such as granite, rhyolite, dacite.

Bioturbation – the process by which organisms rework existing sediments by burrowing through them.

Breccia – the clastic sedimentary rock formed by the lithification of a gravel; the clasts, of very variable composition, are larger than 2 mm and are characterized by sharp edges. The cement generally consists of calcite, dolomite, silica, and clay minerals.

Biomicrite – a limestone composed of skeletal grains and micritic matrix as groundmass (Folk, 1959).

Calcite – the mineral consisting of calcium carbonate of which it represents the most stable form. Other polymorphic varieties of CaCO3 consist of aragonite and vaterite. The term calcite is due to Plinio il Vecchio  and comes from the latin Calx which means limestone.

Cement – an authigenic phase precipitated into primary pore space. The composition is mainly calcitic; in some rocks, cement made of quartz or clay may also be observed.

Clay Minerals – the silicate minerals (Phyllosilicate) classified into four main groups according to crystallographic structure. They are very common minerals in sedimentary rocks and can form through processes of feldspars alteration. They play an important economic role due to their use in construction and preparation of ceramics.

Chlorite – a group of silicate minerals (Phyllosilicate) with general chemical formula (MgFeAl)₈(SiAl)₈O₂₀(OH)₁₆. It is stable in a wide range of pressure and temperature; chlorite is found in many rocks both metamorphic and magmatic and sedimentary; In intrusive magmatic rocks it is generally formed by alteration of ferromagnesian minerals.

Convoluted lamination – it represents a particular case of rolling in sediments and constitutes a plastic deformation structure due to instability of an already laminated sediment saturated with water as a result of different mechanisms.

Dolomite – the mineral consisting of double calcium carbonate and magnesium with chemical formula CaMg(CO3)2.

Feldspars –  a group of rock-forming Tectosilicate minerals (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) making up about 41% of the Earth’s continental crust by weight. Feldspars are very common in both intrusive and effusive rocks; they are also found in metamorphic and sedimentary rocks.

Felsic – the term indicates silicate minerals and igneous rocks enriched in the lighter elements such as silicon, oxygen, aluminum, sodium and potassium.  Most minerals (quartz, orthoclase feldspar, and plagioclase feldspar) are light-colored; the are most common felsic rock is granite.  

Fe oxides – all chemical compounds in which iron, in different oxidation states, and oxygen are present, are generically indicated in this way; they are present as accessory minerals in many rocks.

Foraminifera – both benthic and planktonic protozoa; these are predominantly single-celled eukaryotes, either free-living or parasitic. Most foraminifera live in a marine environment but some species have been found in brackish and freshwater.

Gypsum – the highly insoluble  mineral consisting of calcium sulfate bihydrate (MgSO4 * 2H2O). It is the most abundant sulfate and is commonly found in evaporitic deposits in association with carbonates and clay.

Garnet – the silicate minerals (Nesosilicate) having the general formula X3Y2(SiO4)3. The name comes from the latin Granatum which means grain, possibly related to the pomegranate. The most common garnets are pyrope, almandine, spessartine and grossular.

Heteroblastic texture – a typical texture of metamorphic rocks characterized by dimensional variability of crystals. 

Holocrystalline texture – a typical texture of igneous rock  composed entirely of crystals (which may or may not be visible without magnification). It indicates that cooling was sufficiently slow to allow complete crystallization to occur.

Homeoblastic texture – a typical texture of metamorphic rock in which the main mineral constituents are approximately of equal size.

Hornblende – a complex series of silicate minerals (Inosilicates) belonging to the Amphibole group and forming solid solutions of pure terms. The general formula is (Ca,Na)2–3(Mg,Fe,Al)5(Al,Si)8O22(OH,F). It is a constituent of igneous and metamorphic rocks such as granite, syenite, diorite, gabbro, gneiss, shale.

Limestone – a fine-grained sedimentary rock of chemical or biogenic origin, mainly composed of calcium carbonate in the form of calcite or aragonite. The rock may contain small quantities of clay minerals, feldspars, pyrite, quartz, iron carbonates.

Lithic arenite – a clastic sedimentary rock of the sandstone group. It contains less than 75% of quartz and has a content of lithic fragments of more than 25% of the total rock; lithic clasts are more abundant than feldspathic clasts (Gilbert, 1954;  Dott, 1964).

Lithic arkose –  a clastic sedimentary rock of the sandstone group. It contains less than 75% quartz and has a content of feldspathic fragments of more than 25% of the total rock; feldspathic clasts are more abundant than lithic clasts (Gilbert, 1954;  Dott, 1964).

Magnesite – the mineral consisting of magnesium carbonate (MgCO3). It is often found in veins in ultramafic rock alteration products; it is also formed by carbonation processes on olivine in the presence of water and carbon dioxide.

Magnetite – the mineral of chemical formula Fe3O4; small grains of magnetite occur in almost all igneous and metamorphic rocks. Magnetite is black or brownish-black with a metallic luster, has a Mohs hardness of 5–6 and leaves a black streak.

Mafic – the term indicates a silicate mineral or igneous rock rich in Mg and Fe.  Most mafic minerals (olivine, pyroxene, amphibole, biotite) are dark in color; common mafic rocks include basalt and gabbro.

Magmatic rocks – the rocks formed by cooling of a, generally, silicate mass (magma) that is formed within the Earth’s surface by melting the crustal or mantle rocks. Suspended crystals, fragments of unmelted rock and dissolved volatiles may be transported in the magma; this migrates either at depth or to Earth’s surface giving rise to intrusive and effusive rocks, respectively. 

Marly limestone –  the soft carbonate rock composed of a mixture of fine-grained calcite (CaCO3) and subordinate amount of clay. For intermediate compositions between calcium carbonate, both of chemical and biogenic origin, and clay the rock is called marl; if the clay content prevails over CaCO3 it is referred to as limestone marl.

Mesh texture – a texture that develops in serpentinites, where the needle shaped serpentine minerals occur in aggregates interwoven like a mesh.

Metamorphic rocks – the rocks formed by high temperatures and pressures that cause changes in the mineralogy, texture, or chemical composition of any kind of preexisting rock while maintaining its solid form.

Matrix – the mechanically deposited fine particles  that occupy space between larger grains; matrix contain particle of variable composition (clay minerals, micas, feldspars, quartz).

Mica – a group of silicate minerals (Phyllosilicate) characterized by a layered structure and a perfect cleavage; they are easily divided into foils or scales very thin, elastic and with pearly shine (hence the name, from the latin mico “shine”). Very common micas are muscovite, biotite and phlogopite.  Biotite is the most widespread mica in several geological environments; frequently found in rocks such as dacite, rhyolite, granite.

Micrite – the microcrystalline calcite, ranging in diameter from 0.06 to 2 mm, that have been deposited mechanically rather than from solution.

Muscovite – the most common mineral of the group of micas (Phyllosilicate). The name muscovite comes from an English term used in the past to indicate the material, used in Russia in the Middle Ages,as a substitute for iron. Muscovite is commonly found in Al-rich igneous and metamorphic rocks.  

Olivine – a group of silicate minerals (Nesosilicates) consisting of a solid solution, in varying proportions, between the pure iron term, the fayalite (FeSiO4) and the pure magnesium term, forsterite (MgSiO4); they are commonly found in mafic and ultramafic magmatic rocks.

Orthopyroxene – a group of silicate minerals (Inosilicates) of the pyroxene group; they form solid solutions between the term rich in magnesium, the enstatite (MgSiO3) and the term rich in iron, the ferrosilite (FeSiO3).

Peridotite – the coarse-grained, dark greenish gray, ultramafic intrusive igneous rock composed primarily of olivine with smaller amounts of pyroxene and other minerals such as spinel or garnet; the dominant rock in Earth’s mantle and the source rock of basaltic magmas.

Phaneritic texture – the typical texture of magmatic rocks in which crystals are visible to the naked eye. 

Phyllosilicates – a class of silicates that, because of their structure, generally have a laminated appearance with well-defined cleavage. Phyllosilicates represent a very large class of minerals including two main subclasses: clay minerals and micas. Very common micas are muscovite, biotite and phlogopite. Among the clay minerals very common are kaolinite and talc.

Plagioclase – any member of the feldspar silicate minerals (Tectosilicates), with general chemical formula (Na,Ca)(Si,Al)4O8; they form solid solution between the pure term in Na (albite) and the pure term in Ca (anorthite).  The plagioclase feldspars are the most common minerals of magmatic rocks.

Porphyry – a textural term used to indicate an igneous rock consisting of large-grained crystals such as feldspar or quartz dispersed in a fine- or very fine- grained silicate rich groundmass. The larger crystals are called phenocrysts. In its non-geologic, traditional use, the term porphyry refers to the purple-red form of this stone.

Protolith – the original, unmetamorphosed rock from which a given metamorphic rock is formed. Often the original structure of the rock is preserved, mainly in low metamorphic grade conditions, but as the pressure and temperature increase the original structure is erased.

Pyrite –  the most common mineral (FeS) of the sulfur group; it is found in many geological environments both in igneous, sedimentary deposits and metamorphic rocks. Because of the bright yellow color is often confused by inexperienced gold seekers and for this known as “stupids’ gold”.

Pyroxene – a series of silicate minerals (Inosilicates) of variable composition, among which calcium-, magnesium-, and iron-rich varieties predominate. They crystallize in both the orthorhombic and monoclinic crystal systems. They are found in several igneous and metamorphic  rocks. The origin of the name derives from the Greek πυρ,”fire” and ξένος “foreign”, with the meaning of stone stranger to the fire, since in the past it was mistakenly believed that they were only accidentally found in the magmatic rock.                                                            

Quartz – the second most abundant mineral (SiO2) in the earth’s crust, after the feldspars; it is commonly found in many magmatic rocks (rhyolites, granites, dacites), metamorphic and sedimentary rocks. In nature there are numerous polymorphs of SiO2 (e.g. tridymite, cristobalite, stishovite, coesite).

Sedimentary rocks – the rocks formed at the Earth’s surface or at small depths through a series of processes responsible for the lithification of sediments; the latter consist of solid materials, both mineral and organic, of different origin, accumulated and distributed to the surface of the lithosphere by different agents and resulting from the physical, chemical and biochemical degradation of the pre-existing rocks.

Sand – the clastic material derived  from the degradation and erosion of pre-existing rocks. The particles range in diameter from 0.02 to 2 mm. The clastic sedimentary rock composed mainly of sand-sized mineral particles or rock fragments are called sandstone.

Sandstone – the clastic sedimentary rock that is formed by the lithification of a sand (particle diameter from 0.02 to 2 mm). The main minerals of which it is composed are quartz, feldspars, micas, calcite).

Serpentine (minerals of the group of) – a group of silicate minerals (Phyllosilicate) of general formula Mg3Si2O5(OH)4 resulting from hydration processes of mafic and ultramafic rocks; serpentinization is a metamorphic process of low temperature that tends to transform the original minerals, such as the pyroxene and the olivine, into hydrous minerals. There are three important mineral polymorphs of serpentine: antigorite, chrysotile and lizardite.

Silicates – the most important class of minerals characterized by the presence of the silicon-oxygen (SiO4)4– tetrahedron. The silicates can be divided into groups according to structural configuration: nesosilicates, sorosilicates, cyclosilicates, inosilicates, phyllosilicates and tectosilicates. They are found in all types of rocks, both magmatic, sedimentary and metamorphic; approximately 25 percent of all known minerals and 40 percent of the most common ones are silicates.

Siltite –  the  clastic sedimentary rock that is formed by the lithification of a very fine grain sediment (sizes between 1/16 and 1/256 mm) that takes the name of silt. The clasts derive from the degradation and erosion of pre-existing rocks.

Slate – a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock.

Spinel – the mineral with chemical formula MgAl2O4 is the magnesium/aluminum member of the larger spinel group of minerals. It is characterized by a compact cubic crystalline structure.  The group of spinels is commonly divided into three series (spinel series, magnetite series, chromite series); the spinel is the most common term.

Talc – the silicate mineral (Phyllosilicate) with chemical formula Mg3Si4O10(OH)2; it is  formed by alteration of magnesium-rich minerals, such as pyroxene, olivine, amphibole, and dolomite in the presence of water and CO2.The name probably comes from an Arabic word meaning “pure” and is due to Agricola, perhaps alluding to the bright white color of its powder. Talc is the softest of minerals and in powdered form is widely used as baby powder. 

Texture – a term that describes the size, shape and orientation of the grains constituting a rock, as well as the relationship between these grains.

Titanite – the silicate mineral (Nesosilicate) of calcium and titanium (CaTiSiO5), also called sphene. It is commonly found as an accessory mineral of many magmatic rocks, both effusive and intrusive, and in metamorphic rocks-

Wackestone – the limestone rock containing more than 10% granules and the amount of matrix is less than 90% (Dunham, 1962).

Zircon – the silicate mineral (Nesosilicate) with chemical formula ZrSiO4. It is a very common accessory mineral in most magmatic rocks and is commonly found inside other minerals such as biotite.