Glossary of Rock and Mineral Terms - M

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macrocrystalline

Having crystals large enough to be seen with an unaided eye.

mafic

Describing dark colored rocks or minerals that are composed particularly of magnesium and iron.

magma

Molten (melted) rock that forms naturally within the Earth. Magma may be either a liquid or a fluid mixture of liquid, crystals, and dissolved gases.

magmatic

Igneous

magmatite

Term describing any igneous rock.

magnesia

1) Industrial material made of magnesium oxide. 
2) Describing a mineral that is composed of magnesium oxide.

magnetic field

The region within which the magnetism of a given substance or particle affects other substances.

magnetic reversal

The process by which the Earth's magnetic north pole and its magnetic south pole reverse their positions over time.

magnet

An object that is surrounded by a magnetic field, causing iron or steel materials to be attracted to it.

magnetic

This term can be used to describe materials that give off magnetic fields, that is  act as a magnet, or materials that are attracted to magnetic fields. To note the distinction, this guide defines "magnetic" as giving off a magnetic field, and "attracted to magnets" as being attracted to a magnetic field.

magnetic field

Area encompassing a magnet or electric current which has the ability to attract or repel certain objects anywhere in the field. Points in the magnetic field closer to the magnet or electrical current are stronger, and points further away are weaker.

magnetism

The property, possessed by certain materials, to attract or repel similar materials. Magnetism is associated with moving electricity.

malleable

Ability of a certain minerals that are able to be pounded into thin sheets.

mammilary

Aggregate describing smooth, rounded, agglomerations. Rounded agglomerations of mammilary aggregates are larger than reniform agglomerations and considerably larger than botryoidal agglomerations.

mantle

The middle layer of the Earth, lying just below the crust and consisting of relatively dense rocks. The mantle is divided into two sections, the upper mantle and the lower mantle; the lower mantle has greater density than the upper mantle. See also core and crust.

marble

A coarse-grained, nonfoliated metamorphic rock derived from limestone or dolostone.

marine magnetic anomaly

An irregularity in magnetic strength along the ocean floor that reflects sea-floor spreading during periods of magnetic reversal.

massive

Term used to describe a rock or mineral that has no particular shape, either because it is non-crystalline;  it is composed of tiny, unorganized crystals;  it is a shapeless fragment of a crystal. A massive sulfide deposit  is an unusually large deposit of sulfide minerals.

matrix

A material that has an embedded crystal inside or emerging from it.

meandering stream

A stream that traverses relatively flat land in fairly evenly spaced loops and separated from each other by narrow strips of floodplain.

mechanical exfoliation

A form of mechanical weathering in which successive layers of a large plutonic rock break loose and fall when the erosion of overlying material permits the rock to expand upward. The thin slabs of rock that break off fall parallel to the exposed surface of the rock, creating the long, broad steps that can be found on many mountains.

mechanical weathering

The process by which a rock or mineral is broken down into smaller fragments without altering its chemical makeup; weathering that affects only physical characteristics. See also chemical weathering.

mélange

A body of rock that forms along the inner wall of an ocean trench and is made up of fragments of lithosphere and oceanic sediment that have undergone metamorphism.

melting point

The temperature it takes a certain material to change from a solid to liquid state.

Mercalli intensity scale

A scale designed to measure the degree of intensity of earthquakes, ranging from I for the lowest intensity to XII for the highest. The classifications are based on human perceptions.

mesothermal vein

Vein created due to intense heat (in range from around 390° to 570° F [200° to 300° C] ).

Mesozoic Era

The intermediate era of the Phanerozoic Eon, following the Paleozoic Era and preceding the Cenozoic Era, and marked by the dominance of marine and terrestrial reptiles, and the appearance of birds, mammals, and flowering plants.

metal

Any of a category of electropositive (positively charged) elements or combinations of them in the form of minerals that exhibit a metallic luster, malleability, ductility, and conductivity.

metallic

Having the attributes of a metal. Sometimes can be used to describe a mineral with a metallic luster even though it is not a true metal.

metallic elements

The metallic elements are minerals belonging to the native elements group. They consist of minerals that are pure elements or metallic alloys. They are all true metals, that is they have a metallic luster, are malleable and ductile, and are good conductors of electricity.

metallic luster

Exhibiting the luster of a metal, which is opaque and reflective. Some minerals exhibit a metallic luster even though they are not true metals.

metallic bonding

The act or process by which two or more atoms of electron-donating elements pack so closely together that some of their electrons begin to wander among the nuclei rather than orbiting the nucleus of a single atom. Metallic bonding is responsible for the distinctive properties of metals.

metallurgy

The science and procedures involved in extracting metals from ore, refining the metals, blending them into alloys, and fashioning useful objects from them. Three forms of metallurgy are hydrometallurgy, electrometallurgy, and pyrometallurgy.

metamic

Crystalline minerals that lose their crystal structure due to radioactive destruction.

metamorphic differentiation

The process by which minerals from a chemically uniform rock separate from each other during metamorphism and form individual layers within a new metamorphic rock.

metamorphic grade

A measure used to identify the degree to which a metamorphic rock has changed from its parent rock. A metamorphic grade provides some indication of the circumstances under which the metamorphism took place.

metamorphic index mineral

One of a set of minerals found in metamorphic rocks and used as indicators of the temperature and pressure conditions at which the metamorphism occurred. A metamorphic index mineral is stable only within a narrow range of temperatures and pressures, and the metamorphism that produces it must take place within that range.

metamorphic rock

A rock that has undergone chemical or structural changes. Heat, pressure, or a chemical reaction may cause such changes.

metamorphism

The process by which conditions within the Earth, below the zone of diagenesis, alter the mineral content, chemical composition, and structure of solid rock without melting it. Igneous, sedimentary, and metamorphic rocks may all undergo metamorphism.

metasilicate chains

Synonym of inosilicates: Group of silicate minerals that have their tetrahedrons form single or multiple chains, with two oxygen atoms of each tetrahedron part of its neighboring tetrahedron forming long, thin, chains.

metasilicate rings

Synonym of cyclosilicates: Group of silicate minerals that have their tetrahedrons linked into rings. Each silicon atom is bound by two oxygen atoms that are part of another tetrahedron. Each ring consists of three, four, or six linked tetrahedrons.

metasilicates

This term either refers to the metasilicate chains (inosilicates), the metasilicate rings (cyclosilicates), are both the metasilicate chains and rings.

metasomatism

The chemical alteration of rocks or minerals by interaction with liquids. The alteration must only take place if the rock or mineral was not in a molten state in order for it to be metasomatism.

metastable

Describing the condition of radioactive minerals which cannot withstand their composition and begin to decay. Also refers to minerals that crystallize at a certain temperature and can only retain the initial crystal structure at certain temperatures. Synonym of unstable.

meteor

Mass of extraterrestrial rock that gets pulled in to the earth's gravitational pull. Some meteors can be seen streaking across the atmosphere before they burn up or land on earth. These are known as "shooting stars". Meteors that don't burn up and land on the earth are known as meteorites.

meteoroid

Solid mass suspended in outer space that revolves around the sun. Meteoroids are smaller than asteroids.

meteorite

Meteor that did not fully burn up in the atmosphere and landed on the earth.

meteoric water

The precipitation of condensed water from clouds as rain, snow, sleet, or hail.

mica

Individual member of the mica group. May also refer to any mineral with perfect basal cleavage that can be "peeled".

mica group

Group of related minerals that contain aluminum silicate. The percentage of its component elements may vary, which causes many people not to recognize the micas as a series. The micas are soft and have perfect basal cleavage, and can be seen when a mica is "peeled". The mica group is in the phyllosilicates subdivision of the silicates group.

micaceous

Aggregate of compact, flat, parallel, flexible, and peelable sheets, or describing minerals that occur in such aggregates.

microcontinent

A section of continental lithosphere that has broken off from a larger, distant continent, as by rifting.

microcrystalline

Composed of tiny crystals that cannot be seen with an unaided eye. Microcrystalline minerals appear amorphous, since no apparent crystal shape can be detected.

micromount

A mineral specimen that is not more 1/10 of an inch (15 -27 mm.) in size.

migmatite

A rock that incorporates both metamorphic and igneous materials.

mine

1) Deposit in which minerals or ore is or was industrially extracted.
2) To exploit a mineral deposit.

miner

Individual who exploits mineral deposits.

mineral

Any naturally occurring, three dimensional, inorganic substance, with a chemical structure that can be exact, or can vary within limits. Elements that occur naturally are also listed as minerals.

mineral group

A scientifically recognized selection of minerals similar in structure. All groups have a specific group name, and individual minerals may also be categorized. A mineral in the group may have the same name as group name. An example is the Olivine Group, which contains the minerals Forsterite, Olivine (also known as Chrysolite), and Fayalite.

mineraloid

A naturally occurring, usually inorganic, solid consisting of either a single element or a compound, and having a definite chemical composition but lacking a systemic internal arrangement of atoms. Opal and Obsidian are two examples.

mineralogy

The study of minerals. Person who works in this discipline is a mineralogist.

mining

The process of extracting minerals or metal ore out of a mine or mineral deposit.

mixed crystal

Crystal containing an indefinite amount of two or more elements or a slight amount of one element replacing another. An example is Siderite, which is iron carbonate (FeCO3), but commonly contains small amounts of other elements, such as zinc and manganese partially replacing the iron.

Moho (abbreviation for Mohorovihcic´)

The seismic discontinuity between the base of the Earth's crust and the top of the mantle. P waves passing through the Moho change their velocity by approximately one kilometer per second, with the higher velocity occurring in the mantle and the lower in the crust.

Mohs hardness scale

A measurement that was devised by Austrian scientist Fredrick Mohs to determine the hardness of a mineral.

molecule

The fundamental structure in all minerals. Molecules are chemically grouped atoms that are the smallest particles a mineral can be divided without changing its chemical or physical properties. A chemical grouping of one element is also considered a molecule.

molecular

Having to do with molecules.

molecular arrangement

The arrangement of molecules in a substance.

molecular structure

The quantity and method of arrangement concerning the molecules in a particular substance.

mollusk

Family of marine creatures, which includes the oyster and snail, which have a soft fleshy body surrounded by a calcareous shell.

molten

Hot liquid that results from the melting of solid material at great temperatures.

molten rock

Liquid rock at extremely high temperatures under the surface of the earth. When molten rock cools down it solidifies and forms rocks and minerals.

Moment-magnitude scale

A recently developed alternative to the Richter scale used to measure more accurately the amount of energy released by large earthquakes. This scale involves measurement of an earthquake's seismic moment.

moraine

A single, large mass of glacial till that accumulates, typically at the edge of a glacier.

molybdates

Group of minerals composed with the molybdate radical (MoO4) and a metallic element. These minerals are heavy, soft, and brittle. The molybdates are usually categorized with the chemically related tungstates, in which they can be partially replaced by.

monochromatic

Exhibiting only a single color. Minerals that are monochromatic occur in only one color, no matter what specimen.

monoclinic (crystal system)

Any mineral that falls under the following specifications belongs to the monoclinic crystal system: Three axes, all of them are unequal in length. Two of them are at right angles to each other, while the third is lies at an angle other than 90°.

montropic

Mineral that permanently changes over into its paramorph, meaning that once it transforms under suitable conditions it cannot change back to the original mineral without the crystal structure being destroyed. Enaniotropic minerals can change back and forth when conditions are suitable.

mother-of-pearl

1) The iridescent inside of a mollusk shell, which is used as an ornament. Not to be confused with pearl.
2) Exhibiting a luster similar to the inside of a mollusk shell or shirt button. Many mica's exhibit a pearly luster, and some minerals with a pearly luster have an iridescent hue. Some minerals may exhibit a pearly luster on cleavage cracks parallel and below the reflecting surface of a mineral.

mother rock

The rock a mineral or minerals is found implanted in.

mottled

Spotted or speckled with different tints or colors.

mudcrack

A fracture that develops at the top of a layer of fine grained, muddy sediment when it is exposed to the air, dries out, and then shrinks.

mudstone

A detrital sedimentary rock composed of clay-sized particles.

multicolored

Exhibiting two or more colors on a single specimen.

multiple oxides (subgroup)

Minerals that are compounds of two different metallic elements combined with oxygen. The multiple oxides are a subgroup of the oxides.

mylonite

Rock fragments separated from their original rock from shifting of plates in the fault zones.