Magnesite is native magnesium carbonate occurring in white compact or granular masses, and also in rhombohedral crystals. It has been used as an ore of metallic magnesium but the primary source of magnesium is sea water. Is a source of magnesia for industrial chemicals. Commonly found in veins and masses derived from the alteration of serpentine through the action of waters containing carbonic acid. Magnesite has the formulae MgCo3 and a relative hardness of 4. Research Magnesite
Magnetite is an oxide of iron occurring in isometric crystals, also massive, of a black colour and metallic lustre. It is readily attracted by a magnet and sometimes possesses polarity, being then called loadstone. It is an important iron ore and is also called magnetic iron. Magnetite has the formulae Fe3O4 and a relative hardness of 7. Research Magnetite
Malachite is a native hydrous carbonate of copper, usually occurring in green mammillary masses with concentric fibrous structure. It is a widely distributed copper ore. Found in the oxidized portions of copper veins and is often associated with cuprite, native copper, iron oxides, and sulphides of copper and iron. It often occurs in copper veins that are found in limestone. Green malachite, or malachite proper, admits of a high polish, and is sometimes used for ornamental work. Blue malachite, or azurite, is a related species of a deep blue colour.
Malachite has the formulae Ci2(CO3)(OH)2 and a relative hardness of 4. Research Malachite
Manganite is a strongly magnetic ore of manganese seldom found in commercial quantities. It is often found in veins associated with granitic igneous rocks. It alters to pyrolusite. Barite and calcite are frequent associates. Research Manganite
Marcasite is one of the sulphides of iron. It often shows a white colour on a fresh surface. It is found in iron metallic veins and frequently with lead and zinc ores, and is also found in sedimentary rocks. Marcasite most frequently occurs as a replacement deposit in limestone and often in concretions imbedded in clays, marls, and shales. It has a relative hardness of 7. Research Marcasite
Margarite is a metamorphic mineral related to the micas, but low in silica and yielding brittle folia with pearly lustre. It is associated with staurolite and tourmaline and has a relative hardness of 5. Research Margarite
Massive refers to a mineral that does not show any definite external crystal form or consists of poorly defined masses of small crystals. Research Massive
Meerschaum (sepiolite) is a white or yellowish earthy mineral, consisting of hydrous magnesium silicate. It was first found on the sae shore where, resembling petrified sea-froth was given its name - which in German means sea-froth. Meerschaum lathers like soap when first dug, and was used as a soap by the Tartars. It is highly porous, and may float on water. It is very soft, with a relative hardness of 2 to 2.5 and is easily turned into pipes and cigar-holders. Before use the meerschaum is impregnated with oil, tallow or wax. Research Meerschaum
Mellite (honey stone) is a honey coloured mineral, found in brown coal, and partly the result of vegetable decomposition. It is a mellitate of alumina. Research Mellite
Mendipite is a rare white, grey or colourless mineral with a high density but low relative hardness of 2.5 Mendipite is a halide of lead formed around volcanic vents and in hydrothermal veins. It is most commonly found in association with calcite, cerussite, malachite, manganite, pyrolusite and pyromorphite. Most deposits take the form of columnar or fibrous masses, often with radiating external shapes. Research Mendipite
Mesolite is a zeolitic mineral, greyish white or yellowish in colour, occurring in delicate groups of crystals, and also fibrous massive. It is a hydrous silicate of alumina, lime, and soda. Research Mesolite
Metamorphic rock is rock that has been altered by intense heat, pressure, or both. It may originally have been sedimentary or igneous rock, or even different metamorphic rock. The changes that take place during metamorphism can affect both the structure of the rock and its composition. Very often the rock is re-crystallized. Contact metamorphism is a localized form of metamorphism that is produced by the heat of an igneous intrusion. Limestone may then be altered into marble, and clay into a hard, tough rock (a hornfels). The zone affected in this way is called a metamorphic aureole. Hydrothermal metamorphism, or metasomatism, is produced by hot aqueous fluids emanating from igneous intrusions. China clay is produced in this way from granite. In dynamic metamorphism, or cataclasis, rocks are broken down mechanically by shearing and crushing; mylonite, a fine-grained banded rock, is a typical product. Regional metamorphism takes place on a large scale. The rocks are subjected to heat, deformation, and the action of hot fluids
that may affect their chemical composition. In the lowest grades of regional metamorphism, slates and phyllites (the latter with better- developed crystals than slate) are formed. More intense regional metamorphism results in the development of schists, rocks with a characteristic wavy foliation. At the highest grades, gneisses are formed: coarsely crystalline rocks with alternate light and dark bands. The normal sequence, from lower to higher metamorphic grade, is known as prograde metamorphism. The process can be reversed if, for example, rocks of a high grade are subsequently maintained for a long time at a lower temperature than was reached during the first metamorphism. Alteration from a higher to a lower grade is termed retrograde metamorphism. Research Metamorphic Rock
Metaquartzite is a metamorphosed sandstone, in which the grains of silica have fused together to form a hard, massive rock. Metaquartzite contains more than 90 percent quartz, which gives the rock a pale, sugary appearance. Metaquartzite is a fine to medium-grained rock formed at great heat under relatively low pressure. Research Metaquartzite
Mica refers to a group of more than thirty silicate minerals having perfect cleavage in one direction and which easily split into thin, elastic, sheets.
Micas are closely associated with clay minerals such as gibbsite and kaolinite.
Micas are highly resistant to electricity and heat and are widely used in industry as insulators, and are also added to pint and wallpapers to give them a silky lustre. Research Mica
Microcline is a mineral with the formulae KAlSi3O8 and a relative hardness of 7. It has the same composition as orthoclase but is distinguished by triclinic twinning (usually requiring a microscope to detect). If a feldspar is a deep green it is microcline - and sometimes called 'amazon stone' or amazonite after the Amazon river. It is recognisable from plagioclase by a lack of striations. Research Microcline
Migmatite is a regional metamorphic rock composed of two pre-existent formations that have combined but may remain distinguishable. A medium to coarse-grained rock, migmatite forms in distinct bands of different colours - mainly green or blacks. Research Migmatite
Milarite is a rare mineral containing a large number of metals first identified in 1873 and named after Val Milar in Switzerland. Milarite is a hydrated silicate of potassium, calcium, aluminium and beryllium and is a member of the osumilite group. Crystals of milarite are hexagonal and often have six-sided, prismatic shapes, often very small and coloured green or yellow. Milarite is brittle and has no cleavage and has a relative hardness of 5.5 to 6. Research Milarite
Millerite has the formulae NiS and a relative hardness of 4. It is the richest ore of nickel but too scattered to be commercially important. Forms at low temperatures often in cavities and as an altered form of other nickel minerals, or as a crystal inclusion in other minerals. Research Millerite
Mimetite is an arsenochloride of lead and a minor ore of lead and has the formulae Pb5(AsO4)3Cl and a relative hardness of 4. It is a relatively rare mineral which occurs in the oxidized portions of lead bearing veins. Mimetite occurs in yellow, orange, brown, green and white colours and is related to and resembles pyromorphite. Milarite was identified in 1832 and belongs to the apatite group. Research Mimetite
The majority of minerals can, under certain conditions, occur in regularly shaped crystals. Crystals vary greatly in shape and size. Some are minute, visible only through a microscope; others may measure several metres. The shape of a crystal can be as thin as a needle (acicular), or columnar, tabular, fibrous or lamellar. A closer look at crystallized minerals shows that their crystals are structurally nearly always the same. It is then possible to conclude that a mineral has a definite crystal structure, which is not accidental but is determined by certain laws. The science which pursues the study of these natural laws and explains and describes crystal structures of different minerals is called crystallography. Natural scientists have studied crystal structure since ancient times, but crystallography only gained recognition as a true science in the 17th and 18th centuries. Crystallography, as we know it today, is based upon the findings of many other scientific fields, especially mineralogy, chemistry,
physics and mathematics. The basic difference between crystalline and non- crystalline matter does not lie only in the regularity of arrangement of the external faces. After all, crystalline grains in rock minerals often have irregular shapes and yet they are crystals. The basic difference is in their internal structure, the arrangement of the molecules, atoms and ions. These tiny particles are chaotically arranged in gases, liquids and non- crystalline solids. In crystals, however, they have a regular, repeating pattern. Some minerals crystallize in a definite characteristic structure. But the structure of many crystallized minerals varies. Calcite crystals, for example, occur in various forms: high or low rhombohedra, columnar, acicular or tabular forms. These shapes share one characteristic, however; they are symmetrical, and their symmetry is in line with the group symmetry of the smallest particles of the substance. According to this symmetry, crystals are divided into seven major groups, called crystal systems: 1)
triclinic, 2) monoclinic, 3) orthorhombic, 4) tetragonal, 5) trigonal, 6) hexagonal, 7) cubic. Crystals are classified as belonging to a particular system according to their axes of symmetry which are, basically, imaginary lines passing through the centre of a crystal, and also according to their number and kind. All crystals belonging to an individual system must have a certain characteristic form, which corresponds to the relative symmetry. This means, in fact, that each crystal system has its own individual crystal form. The crystal structure of every mineral is determined mainly by its chemical composition and by physical conditions during its development, especially temperature and pressure. Crystals with completely even faces do not often occur, for the various external conditions make such perfect growth of faces impossible. The size and the development of the individual crystal faces are not so vital for classification as the angle between faces, which is the same for every crystal of the same system. Crystallography is
largely based on the study of these interfacial angles. Instruments used for measuring the angles are called goniometers. Distortions of crystal faces, which develop either during the growth of a crystal, or through the effects of weathering, can be an aid in identifying a mineral (the faces of pyrite crystals are, for instance, often striated). The conditions of environment affect the speed of growth of crystals, and also their physical properties. When a cluster of crystals grows in a confined space, they crowd each other and impede each other's development. If a growing crystal becomes entangled with a neighbouring crystal, it cannot continue to grow in the original direction, but can expand in other directions, This is why the grains of rock have an irregular shape. On the other hand, crystals which originate in a soft, yielding environment, such as volcanic tuff, or in certain sediments, are often able to develop perfect faces. Crystals also form in rock crevices, where they are deposited either by solutions or gases. Here the conditions
for development are very different and much less favourable. The growing crystal cannot develop faces upon the rock surface to which they adhere. They can continue in their growth only towards the centre of the cavity. Clusters of crystals which grew from a common base in fairly parallel lines are called druses. Such parallel growth can occur only if the wall of the crevice is fairly even. If the rock crevice is round, the adhering crystals protrude with their free ends towards its centre, and are called geodes. It is common for minerals to occur grouped in druses and geodes. They are frequently found in ore-veins and in cracks and crevices of the most varied types of rocks. Quartz, calcite, fluorite, barites and similar minerals are often grouped in druses. In some large cavities and crevices truly magnificent crystals can develop, as can be seen, for instance, in the ' crystal cellars' in the Alps. The assemblage of crystals in druses and geodes is usually in a fairly regular pattern. But if the crystals grow in
an interlocking, irregular clump, it is called an aggregate. These aggregates are often made up of multitudes of tiny crystals, more often than not with faces imperfectly bounded. Apart from aggregates there are also mineral clusters which are basically made of parts of individual crystals not visibly bounded externally. The minerals which usually appear in this form are those which commonly develop large crystals, such as quartz. For some irregular or round crystal formations in rocks the term nodule is used in mineralogy and petrography. Usually of quartzitic composition, nodules are often found in limestone beds. Chalcedonic and opal varieties also develop nodules, which originate through deposition of marine plankton. The laws governing the growth of crystals can often be applied to whole groups of crystals. Sometimes two or more crystals of the same chemical composition grow together with definite rules, governing their relative positions. Depending on the number of the twinned crystals, we talk of twins, triplets,
etc. The intergrowth of two different minerals, though perhaps of the same chemical composition but with a different internal structure is not often found in nature (pyrite and marcasite, or even some minerals with a completely different chemical composition, such as haematite and rutile, are some examples). Another mineral form is a so-called pseudomorph or mineral mimic which is frequently found. For instance, limonite, which has no crystal form of its own, is sometimes found in the shape of a perfectly bounded pyrite crystal. This unusual happening occurs because the original crystal of pyrite has become decomposed through the effects of water and has been replaced by limonite. In other cases one mineral changes into another mineral of identical chemical composition. For instance aragonite (CaCO,) has an orthorhombic crystal form, and it changes slowly and gradually into rhombohedral calcite, but the original crystal form remains unchanged. Sometimes a pseudomorph is the result of incrustation which is produced by a
powder like coating of one mineral being deposited on the crystals of another. It is rarer to find in nature minerals which have no traces of crystalline structure and which are said to be amorphous (opal and amber are examples). Research Mineral Structure
A mocha stone is a pale translucent agate with a branching pattern resembling mosses or fern fronds. They were commonly used in England for brooches, and came chiefly from Arabia. Research Mocha Stone
Molybdenite is a grey, metallic-looking mineral with the formulae MoS2 and a relative hardness of 2. It is the main ore of molybdenum and resembles graphite but has a higher specific gravity and a slight blue tint. Molybdenite is formed by hydrothermals in sulphide ore bodies, and also in re-crystallised granitic rocks such as marble, granite and pegmatite. It is frequently found in association with chalcopyrite, fluorite, pyrite, quartz, scheelite, and wolframite. Molybdenite was identified in 1778 by the Swedish chemist Carl Scheele. Research Molybdenite
Monazite has the formulae (Ca,La,Nd,Th)PO4 and a relative hardness of 6. It is the chief ore of thorium and cerium. Thorium is a radioactive element. It is concentrated in sand due to its durability and high specific gravity. Associated with other heavy minerals such as magnetite, rutile, and zircon. Research Monazite
Monoclinic refers to a crystal with six faces and three axes of unequal length. Two axes are at right angles to each other and the third is inclined to the plane of the other two. A ream of paper with a long edge sloped at an angle is an example. Research Monoclinic
Montmorillonite is a mineral discovered at Montmorillon in France, and confirmed as a distinct species in 1847. Montmorillonite is a form of clay found in many sedimentary rocks especially bentonite and in some metamorphic rocks. It is formed mainly through the alteration of pre-existent feldspar in rocks that are poor in silica. Montmorillonite is a hydrous hydrated silicate of sodium, calcium, aluminium and magnesium used in the petroleum industry as a drilling mud. Research Montmorillonite
Mottramite is a vanadate mineral usually occurring as a green to black coloured vitreous crust on other rocks and minerals. Mottramite is found in areas where metal-bearing deposits have been oxidised by the action of air and water or just water. Mottramite was identified in 1876 and named after Mottram St Andrew near Manchester, England where it was discovered. A hydrous vanadate of lead, copper and zinc, it is sometimes used as a minor ore of lead and copper. Research Mottramite
Muscovite has the formulae KAl2(AlSi3)O10(OH)2 and a relative hardness of 3. It has extremely perfect cleavage and thin flakes are flexible. Very common. Used for electrical and heat insulation. Research Muscovite
Mylonite is a type of metamorphic rock created when pre-existent rocks are crushed, ground or rolled into a new formation along major fault lines. Mylonite is generated by pressure with very little heat. Mylonite rock is ground to dust and streaked out into bands during its creation. Research Mylonite
Abbreviations
Rocks & Minerals
Research Macrodiagonal
