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eral along such planes is termed cleavage the quality of cleavage can be described based on how cleanly and easily the mineral breaks common descriptors in order of decreasing quality are perfect good distinct and poor in particularly transparent minerals or in thin section cleavage can be seen as a series of parallel lines marking the planar surfaces when viewed from the side cleavage is not a universal property among minerals for example quartz consisting of extensively interconnected silica tetrahedra does not have a crystallographic weakness which would allow it to cleave in contrast micas which have perfect basal cleavage consist of sheets of silica tetrahedra which are very weakly held together 84 85 as cleavage is a function of crystallography there are a variety of cleavage types cleavage occurs typically in either one two three four or six directions basal cleavage in one direction is a distinctive property of the micas two directional cleavage is described as prismatic and occurs in minerals such as the amphiboles and pyroxenes minerals such as galena or halite have cubic or isometric cleavage in three directions at 90 when three directions of cleavage are present but not at 90 such as in calcite or rhodochrosite it is termed rhombohedral cleavage octahedral cleavage four directions is present in fluorite and diamond and sphalerite has six directional dodecahedral cleavage 84 85 minerals with many cleavages might not break equally well in all of the directions for example calcite has good cleavage in three directions but gypsum has perfect cleavage in one direction and poor cleavage in two other directions angles between cleavage planes vary between minerals for example as the amphiboles are double chain silicates and the pyroxenes are single chain silicates the angle between their cleavage planes is different the pyroxenes cleave in two directions at approximately 90 whereas the amphiboles distinctively cleave in two directions separated by approximately 120 and 60 the cleavage angles can be measured with a contact goniometer which is similar to a protractor 84 85 parting sometimes called false cleavage is similar in appearance to cleavage but is instead produced by structural defects in the mineral as opposed to systematic weakness parting varies from crystal to crystal of a mineral whereas all crystals of a given mineral will cleave if the atomic structure allows for that property in general parting is caused by some stress applied to a crystal the sources of the stresses include deformation e g an increase in pressure exsolution or twinning minerals that often display parting include the pyroxenes hematite magnetite and corundum 84 86 when a mineral is broken in a direction that does not correspond to a plane of cleavage it is termed to have been fractured there are several types of uneven fracture the classic example is conchoidal fracture like that of quartz rounded surfaces are created which are marked by smooth curved lines this type of fracture occurs only in very homogeneous minerals other types of fracture are fibrous splintery and hackly the latter describes a break along a rough jagged surface an example of this property is found in native copper 87 tenacity is related to both cleavage and fracture whereas fracture and cleavage describes the surfaces that are created when a mineral is broken tenacity describes how resistant a mineral is to such breaking minerals can be described as brittle ductile malleable sectile flexible or elastic 88 specific gravity galena pbs is a mineral with a high specific gravity specific gravity numerically describes the density of a mineral the dimensions of density are mass divided by volume with units kg m 3 or g cm 3 specific gravity is defined as the density of the mineral divided by the density of water at 4 c and thus is a dimensionless quantity identical in all unit systems 89 it can be measured as the quotient of the mass of the sample and difference between the weight of the sample in air and its corresponding weight in water among most minerals this property is not diagnostic rock forming minerals typically silicates or occasionally carbonates have a specific gravity of 2 5 3 5 90 high specific gravity is a diagnostic property of a mineral a variation in chemistry and consequently mineral class correlates to a change in specific gravity among more common minerals oxides and sulfides tend to have a higher specific gravity as they include elements with higher atomic mass a generalization is that minerals with metallic or adamantine lustre tend to have higher specific gravities than those having a non metallic to dull lustre for example hematite fe 2 o 3 has a specific gravity of 5 26 91 while galena pbs has a specific gravity of 7 2 7 6 92 which is a result of their high iron and lead content respectively a very high specific gravity is characteristic of native metals for example kamacite an iron nickel alloy common in iron meteorites has a specific gravity of 7 9 93 and gold has an observed specific gravity between 15 and 19 3 90 94 other properties carnotite yellow is a radioactive uranium bearing mineral other properties can be used to diagnose minerals these are less general and apply to specific minerals dropping dilute acid often 10 hcl onto a mineral aids in distinguishing carbonates from other mineral classes the acid reacts with the carbonate co 3 2 group which causes the affected area to effervesce giving off carbon dioxide gas this test can be further expanded to test the mineral in its original crystal form or powdered form an example of this test is done when distinguishing calcite from dolomite especially within the rocks limestone and dolomite respectively calcite immediately effervesces in acid whereas acid must be applied to powdered dolomite often to a scratched surface in a rock for it to effervesce 95 zeolite minerals will not effervesce in acid instead they become frosted after 5 10 minutes and if left in acid for a day they dissolve or become a silica gel 96 magnetism is a very conspicuous property of a few minerals among common minerals magnetite exhibits this property strongly and magnetism is also present albeit not as strongly in pyrrhotite and ilmenite 95 some minerals exhibit electrical properties for example quartz is piezoelectric but electrical properties are rarely used as diagnostic criteria for minerals because of incomplete data and natural variation 97 minerals can also be tested for taste or smell halite nacl is table salt its potassium bearing counterpart sylvite has a pronounced bitter taste sulfides have a characteristic smell especially as samples are fractured reacting or powdered 95 radioactivity is a rare property found in minerals containing radioactive elements the radioactive elements could be a defining constituent such as uranium in uraninite autunite and carnotite or present as trace impurities as in zircon the decay of a radioactive element damages the mineral crystal structure rendering it locally amorphous metamict state the optical result termed a radioactive halo or pleochroic halo is observable with various techniques such as thin section petrography 95 classification earliest classifications in 315 bce theophrastus presented his classification of minerals in his treatise on stones his classification was influenced by the ideas of his teachers plato and aristotle theophrastus classified minerals as stones earths or metals 98 georgius agricola s classification of minerals in his book de natura fossilium published in 1546 divided minerals into three types of substance simple stones earths metals and congealed juices compound intimately mixed and composite separable 98 linnaeus an early classification of minerals was given by carl linnaeus in his seminal 1735 book systema naturae he divided the natural world into three kingdoms plants animals and minerals and classified each with the same hierarchy 99 in descending order these were phylum class order family tribe genus and species however while his system was justified by charles darwin s theory of species formation and has been largely adopted and expanded by biologists in the following centuries who still use his greek and latin based binomial naming scheme it had little success among mineralogists although each distinct mineral is still formally referred to as a mineral species modern classification see also abundance of elements in earth s crust minerals are classified by variety species series and group in order of increasing generality the basic level of definition is that of mineral species each of which is distinguished from the others by unique chemical and physical properties for example quartz is defined by its formula sio 2 and a specific crystalline structure that distinguishes it from other minerals with the same chemical formula termed polymorphs when there exists a range of composition between two minerals species a mineral series is defined for example the biotite series is represented by variable amounts of the endmembers phlogopite siderophyllite annite and eastonite in contrast a mineral group is a grouping of mineral species with some common chemical properties that share a crystal structure the pyroxene group has a common formula of xy si al 2 o 6 where x and y are both cations with x typically bigger than y the pyroxenes are single chain silicates that crystallize in either the orthorhombic or monoclinic crystal systems finally a mineral variety is a specific type of mineral species that differs by some physical characteristic such as colour or crystal habit an example is amethyst which is a purple variety of quartz 18 two common classifications dana and strunz are used for minerals both rely on composition specifically with regard to important chemical groups and structure james dwight dana a leading geologist of his time first published his system of mineralogy in 1837 as of august 2026 update it is in its eighth edition the dana classification assigns a four part number to a mineral species its class number is based on important compositional groups the type gives the ratio of cations to anions in the mineral and the last two numbers group minerals by structural similarity within a given type or class the less commonly used strunz classification named for german mineralogist karl hugo strunz is based on the dana system but combines both chemical and structural criteria the latter with regard to distribution of chemical bonds 100 as the composition of the earth s crust is dominated by silicon and oxygen silicates are by far the most important class of minerals in terms of rock formation and diversity however non silicate minerals are of great economic importance especially as ores 101 102 non silicate minerals are subdivided into several other classes by their dominant chemistry which includes native elements sulfides halides oxides and hydroxides carbonates and nitrates borates sulfates phosphates and organic compounds most non silicate mineral species are rare constituting in total 8 of the earth s crust although some are relatively common such as calcite pyrite magnetite and hematite there are two major structural styles observed in non silicates close packing and silicate like linked tetrahedra close packed structures are a way to densely pack atoms while minimizing interstitial space hexagonal close packing involves stacking layers where every other layer is the same ababab whereas cubic close packing involves stacking groups of three layers abcabcabc analogues to linked silica tetrahedra include so 4 4 sulfate po 4 4 phosphate aso 4 4 arsenate and vo 4 4 vanadate structures the non silicates have great economic importance as they concentrate elements more than the silicate minerals do 103 the largest grouping of minerals by far are the silicates most rocks are composed of greater than 95 silicate minerals and over 90 of the earth s crust is composed of these minerals 104 the two main constituents of silicates are silicon and oxygen which are the two most abundant elements in the earth s crust other common elements in silicate minerals correspond to other common elements in the earth s crust such as aluminium magnesium iron calcium sodium and potassium 105 some important rock forming silicates include the feldspars quartz olivines pyroxenes amphiboles garnets and micas silicates main article silicate minerals aegirine an iron sodium clinopyroxene is part of the inosilicate subclass the base unit of a silicate mineral is the sio 4 4 tetrahedron in the vast majority of cases silicon is in four fold or tetrahedral coordination with oxygen in very high pressure situations silicon will be in six fold or octahedral coordination such as in the perovskite structure or the quartz polymorph stishovite sio 2 in the latter case the mineral no longer has a silicate structure but that of rutile tio 2 and its associated group which are simple oxides these silica tetrahedra are then polymerized to some degree to create various structures such as one dimensional chains two dimensional sheets and three dimensional frameworks the basic silicate mineral where no polymerization of the tetrahedra has occurred requires other elements to balance out the base 4 charge in other silicate structures different combinations of elements are required to balance out the resultant negative charge it is common for the si 4 to be substituted by al 3 because of similarity in ionic radius and charge in those cases the alo 4 5 tetrahedra form the same structures as do the unsubstituted tetrahedra but their charge balancing requirements are different 106 the degree of polymerization can be described by both the structure formed and how many tetrahedral corners or coordinating oxygens are shared for aluminium and silicon in tetrahedral sites 107 108 orthosilicates or nesosilicates have no linking of polyhedra thus tetrahedra share no corners disilicates or sorosilicates have two tetrahedra sharing one oxygen atom inosilicates are chain silicates single chain silicates have two shared corners whereas double chain silicates have two or three shared corners phyllosilicates have a sheet structure which requires three shared oxygens in the case of double chain silicates some tetrahedra must share two corners instead of three as otherwise a sheet structure would result framework silicates or tectosilicates have tetrahedra that share all four corners ring silicates or cyclosilicates only need tetrahedra to share two corners to form the cyclical structure 108 the silicate subclasses are described below in order of decreasing polymerization tectosilicates natrolite is a mineral series in the zeolite group this sample has a very prominent acicular crystal habit tectosilicates also known as framework silicates have the highest degree of polymerization with all corners of a tetrahedra shared the silicon oxygen ratio becomes 1 2 examples are quartz the feldspars feldspathoi...