The Non-Silicates
As their name suggests, the non-silicate minerals lack the silica-oxygen molecule as a constituent part of their structure. Chemically, however, the different groups of non-silicates are defined rather loosely defined, and over the years mineralogists have differed over how to classify them. Many are defined by the anions (negatively charged atoms or molecules) that play the role of silica in their structure. Thus carbonates are characterised by the presence of a carbonate ion (CO3 - one carbon ion lined to three oxygens), while the sulphates have a sulphate ion (SO4 - one sulphur ion attached to four oxygens). As will be seen below, however, this is not always the case, and some groups are specified according to other criteria.
Their composition and structure is also variable. While some, such as the native metals, are about as simple as you can get (they basically comprise the metal - e.g. gold, silver, lead) alone and unadorned others are far more complex. One reason is that they contain a a wider range of elements, which are often subject to extensive substitution by other elements - usually by ions of a similar size. In this way, pseudomorphs can form, occupying much the same crystal space - and thus with a more-or-less identical form as the original mineral - but of different composition. Below, we summarise the composition, character and properties of the main non-silicate groups.
Native elements
The native element group is the simplest to deal with. Minerals in this class comprise a single (more-or-less) element, in its native (or pure) state. About thirty-five elements are known to occur in nature in native form, variously defined as metals (e.g. gold, silver, platinum, copper), semi-metals (e.g. arsenic, bismuth, antimony) and non-metals (e.g. diamond, graphite, sulphur). Some of these are unstable and do not survive well. Amongst the longest lasting are those that are relatively non-reactive, including the precious metals The non-metal carbon, likewise, is non-reactive, and may occur in its native state as graphite or diamonds. On the other hand, reactive metals such as iron, lead, and aluminium are far more likely to combine with other elements, especially as a result of oxidation, so are rarely found in their native state.
The physical properties of native elements are variable but sometimes diagnostic. Many have a high atomic weight (often in excess of 50) and/or a densely packed atomic structure, which gives them high density and sets them apart from other mineral, and often from each other. The native metals also display a characteristic metallic lustre (when not tarnished) and are opaque. Many, on the other hand, are soft, with values on the Moh's hardness scale of less than 3; likewise several are malleable. Sulphur is recognisable by its odour and colour;.
Their composition and structure is also variable. While some, such as the native metals, are about as simple as you can get (they basically comprise the metal - e.g. gold, silver, lead) alone and unadorned others are far more complex. One reason is that they contain a a wider range of elements, which are often subject to extensive substitution by other elements - usually by ions of a similar size. In this way, pseudomorphs can form, occupying much the same crystal space - and thus with a more-or-less identical form as the original mineral - but of different composition. Below, we summarise the composition, character and properties of the main non-silicate groups.
Native elements
The native element group is the simplest to deal with. Minerals in this class comprise a single (more-or-less) element, in its native (or pure) state. About thirty-five elements are known to occur in nature in native form, variously defined as metals (e.g. gold, silver, platinum, copper), semi-metals (e.g. arsenic, bismuth, antimony) and non-metals (e.g. diamond, graphite, sulphur). Some of these are unstable and do not survive well. Amongst the longest lasting are those that are relatively non-reactive, including the precious metals The non-metal carbon, likewise, is non-reactive, and may occur in its native state as graphite or diamonds. On the other hand, reactive metals such as iron, lead, and aluminium are far more likely to combine with other elements, especially as a result of oxidation, so are rarely found in their native state.
The physical properties of native elements are variable but sometimes diagnostic. Many have a high atomic weight (often in excess of 50) and/or a densely packed atomic structure, which gives them high density and sets them apart from other mineral, and often from each other. The native metals also display a characteristic metallic lustre (when not tarnished) and are opaque. Many, on the other hand, are soft, with values on the Moh's hardness scale of less than 3; likewise several are malleable. Sulphur is recognisable by its odour and colour;.
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Oxides Oxides are composed of oxygen and one or more other elements - commonly aluminium, iron, manganese and magnesium, but also others, some of which readily substitute for each other. They are also formed in many different environments, including igneous (especially granitic), metamorphic, hydrothermal and in a range of sedimentary contexts (e.g. as part of weathering processes). Notwithstanding their varied chemistry and origin, many oxides display broadly similar properties. Ionic bonding is most common among oxide minerals, and this helps to make them relatively hard (ca 5-7 on Moh's scale), though the presence of a hydroxyls in their composition (as a consequence of the presence of water in their formational environment) reduces hardness. They also tend to be opaque or translucent rather than transparent and have a tendency to adopt colours of brown, black or red. |
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Carbonates
Carbonates are a group of minerals, all containing the carbonate molecule (the oxide of carbon, CO3) in association with a cation (a positively charged metal). About 80 carbonate minerals are known, of which the most common are based on calcium. These include calcite (photo right, top) and aragonite (both with the formula CaCO3) but differing in structure, and dolomite which includes also magnesium, giving the formula CaMg(CO3). Other relatively common carbonate minerals are built upon other metals for example siderite (an iron carbonate), rhodochrosite (manganese), strontianite (strontium) witherite (barium) and cerussite (lead). Yet others contain hydrated carbonates (i..e. containing water) or bicarbonates (which contain a linked hydrogen ion to form H-CO3). These form mainly in evaporites and low temperature hydrothermal environments. A further type, also arising from hydrothermal alteration, generally contains rare-earth elements. These include minerals such as bastnäsite and malachite, and azurite (both illustrated in photo right, middle) The properties of these minerals varies, but there are some commonalities. Most have a vitreous lustre, soft (Moh's hardness less than 4) and have perfect cleavage on one face (but sometimes with additional, less perfect cleavages in other directions). |
Calcite
Azurite (blue) and malachite (green)
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Sulphides
Sulphides are minerals that contain sulphur in an unoxygenated form - either as sulphide (S) or disulphide (S2) - as the major ion. It is a large group and important both geologically and economically because it contains many of the metal ores. Most of these ores are structurally simple and share common characteristics such as a metallic lustre, high density (specific gravity >4) and opacity (exceptions are cinnabar, realgar and orpiment). Hardness is also often low (3 or less) though some, such as galena, reach hardnesses of 7.5 or more. The streak is often a diagnostic property because it often doesn't match the colour of the hand specimen. Properties of some of the common sulphides are given below.
Sulphides are minerals that contain sulphur in an unoxygenated form - either as sulphide (S) or disulphide (S2) - as the major ion. It is a large group and important both geologically and economically because it contains many of the metal ores. Most of these ores are structurally simple and share common characteristics such as a metallic lustre, high density (specific gravity >4) and opacity (exceptions are cinnabar, realgar and orpiment). Hardness is also often low (3 or less) though some, such as galena, reach hardnesses of 7.5 or more. The streak is often a diagnostic property because it often doesn't match the colour of the hand specimen. Properties of some of the common sulphides are given below.
Mineral |
Colour |
Streak |
Lustre |
Hardness |
SG |
Chalcopyrite |
Brass yellow |
Greenish-black |
Metallic |
2.5-3.0 |
4.1-4.3 |
Cinnabar |
Bright red, brownish to lead grey |
Red-brown to scarlet |
Adamantine to metallic |
2.0-2.5 |
8.1 |
Galena |
Lead grey |
Dark grey |
Metallic |
2.5-3.0 |
7.6 |
Marcasite |
White-bronze yellow |
Grey |
Metallic |
6.0-6.5 |
4,9 |
Orpiment |
Lemon-, golden- & brownish yellow |
Light lemon yellow |
Resinous (pearly on cleavage faces) |
1.5-2.0 |
3.5 |
Pyrite |
Pale brass yellow |
Greenish-black |
Metallic (may glisten) |
6.0-6.5 |
5.0 |
Pyrrhotite |
Bronze yellow, brown |
Dark greyish black |
Metallic |
3.5-4.5 |
4.6-4.8 |
Sphalerite |
Brown, black, yellow et al. |
Pale yellow-brown |
Resinous to adamantine |
3.5-4.0 |
3.9-4.1 |
Stibnite |
Lead-steel grey; tarnishes black |
Lead grey |
Metallic |
2.0 |
4.6 |
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Sulphates
Sulphates are minerals that contain the sulphate ion (SO4, a molecule composed of one sulphur atom and four oxygens) as thee dominant ion. It is an abundant group, with some 200 members, but is also often extended to include about about fifty others that can be separately described as chromates, molybdenates and tungstates. In all the true sulphates the sulphate molecule is built, like the silicates, as a tetrahedron with the sulphur at the centre. Unlike the silicates, however, this does not link into more complex structures but remains as isolated sulphate tetrahedra. Sulphates occur in a variety of environments including evaporites and in volcanic deposits formed by hot gases or water. Many also developas result of oxidation of pre-existing sulphide minerals as a result of weathering. Most sulphate minerals are pale in colour, though some grade into darker blues and reddish browns. Almost all are soft, with Moh's hardness <3.5, and many have a vitreous lustre. The mineral habit varies considerably both within and between minerals, but many are found in delicate shapes, including fibres, hairs and complex tabular forms. Common sulphates include gypsum, barite, jarosite, cellestite and angelsite. |
Barite, in the form of 'desert rose'
Celestite (formed on a cave roof)
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Halides
Halides have a halide - fluoride (F), chloride (Cl), bromide (Br) or iodide (I) - as the dominant anion in their composition. About 80 halide minerals are known, but most are relatively. Many of which form are water soluble and form mainly in evaporite deposits, often as a surface crust. A few, such as fluorite, however, are insoluble in water and occur in other environments, including as sublimates (i.e. solids formed directly from gases) from fumeroles. The two most common halides are halite, a chloride of sodium, NaCl (aka rock salt) and fluoride, a chloride of fluoride together with calcium, giving the formula CaFCl2. Most have a relatively simple structure, incorporating a large anion and characterised by ionic bonding, as a result of which they have high symmetry and a tendency to simple crystal shapes. Complexes of halides also occur, usually involving aluminium; oxyhydroxy halides (ones that contain both oxygen and hydrogen as well as the chloride) also occur, often as weathering products of sulphides. Atacamite, a copper halide, is an example. Though rare, one of its two reported occurrences in New Zealand is at Champion Mine in the Aniseed Valley. Most halides are soft (Moh's hardness <3.5) and have a vitreous or adamantine lustre, and have a tendency to form cubic, tabular or prismatic crystals, though some also occur as granular or earthy masses. The majority are transparent in crystal form, and although body colours vary most have a white streak (an exception is atacamite which produces an apple green streak). |
Halite crystals
Atacamite
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Phosphates
The phosphate group are of minerals are all characterised by the presence of phosphate (PO4) as the dominant anion. Over 200 have been described, all with the same basic structure of a phosphate tetrahedron of four oxygens with a phosphorus atom at its centre. Invariably, these occur as isolated tetrahedra (i.e. they do not link into more complex structures such as rings or chains). While the structure is relatively uniform, however, their chemistry varies for the phosphate radical combined with more than 30 elements, including iron, calcium, manganese and aluminium, either singly or in combination. Ready substitution also takes place between many of these elements with the consequence that they produce minerals showing a wide range properties. The phosphates can be divided into three subgroups: primary phosphates that precipitate directly from water; secondary, formed by the alteration of primary phosphates; and rock phosphates that form from organic material, normally underwater. Apatite is one of the most common primary phosphates and often occurs in pegmatites. It is chemically complex and includes calcium, fluorite and chlorine as well as hydroxy, and contains a number of different mineral species of which fluorapatite is the most common (image, top right). Monazite (cerium-lanthanum phosphate), a rare earth, is found in the same settings. Many of the secondary phosphates develop through oxidation of metal ores, generally in acid conditions, and often involving lead, copper or zinc. Oxidation of iron and manganese may also produce phosphate minerals, amongst which vivianite is one off the most common. Rock phosphates are of limited relevance geologically, but has value as a fertiliser. |
Fluorapatite, the most common form of apatite
Monazite
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