M

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 rock under the crust of the earth.

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.

Magnet

An object that is surrounded by a magnetic field, causing iron or steel materials to be attracted to it. (See magnetism for more information.)

Magnetic

This term can be used to describe materials that give off magnetic fields (i.e. 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 phenomenon of certain metals to attract other metals. (See Magnetic for more information.)

Malleable

Tenacity 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 -- between the crust and the core.

Massive

Term used to describe a rock or mineral that has no particular shape, either because

1. it is non-crystalline
2. it is composed of tiny, unorganized crystals
3. it is a shapeless fragment of a crystal

Matrix

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

Melting point

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

Mesothermal vein

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

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 (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 elements (subgroup)

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.

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.

Metamict

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

Metamiction

The process which a metamict mineral goes through.

Metamorphic

Mineral environment where the minerals are secondary in origin, forming from alteration through heat and pressure. See also metamorphic rock.

Metamorphic rock

Secondary rock that formed from an original rock, through heat or pressure.

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.

Metastability

The act of being metastable.

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. See also meteor.

Meteoric

Term used to describe the extraterrestrial nature of meteors and meteorites.

Meteorite

Meteor that did not fully burn up in the atmosphere and landed on the earth. (See Iron/Iron-Nickel for more information)

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.

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. (See organizing a collection for more information about micromounts.)

Mine

Deposit in which minerals or ore is or was industrially extracted. (See Finding minerals for more information about mines.)

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. See What is a Rock and What is a Mineral for more information.

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. See also series.

Mineraloid

Amorphous, inorganic substance lacking a crystal structure. They are mineral in nature but lack a crystal structure, and thus cannot scientifically be classified as minerals. Many references, though, still classify them as minerals. Two important mineraloids are Opal and Obsidian.

Mineralogy

The study of minerals.

Mineralogist

Individual practicing in the subject of mineralogy.

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 partialy replacing the iron. See also solid solution.

Mohs Hardness scale

A measurement that was devised by Austrian scientist Fredrick Mohs to determine the hardness of a mineral. (See Hardness in mineral properties for more information.)

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 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. (See magma.)

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°. (See Crystals in mineral properties for more information) Animated figure of monoclinic crystal system

Monotropic

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

The iridescent inside of a mollusk shell, which is used as an ornament. Not to be confused with pearl.

Mother rock

The rock a mineral or minerals is found implanted in.

Mottled

Spotted or speckled with different tints or colors.

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.