Macerals
In coal, the organic equivalent of minerals. These are vitrinite, exinite and inertinite.

Mafic
It is composed of one or more ferromagnesian (iron-magnesium), dark-colored minerals like olivine and pyroxene (in combination with quartz), feldspar or feldspathoid minerals.

Magma
It is molten rock under the crust of the earth.

Magmatite
It is a term which is used to describe any igneous rock.

Magnesia
It is to describe a mineral which is composed of magnesium oxide.

Mantle
It is the middle layer of the earth which lies between the crust and the core.

Marl
It is a calcareous mudstone.

Massive
It is a term which is used to describe a rock or mineral with no particular shape. This can be because it is non-crystalline, it is composed of tiny, unorganized crystals or it is a shapeless fragment of a crystal.

Melting point
It is the temperature which a certain material requires to change to liquid state from solid.

Mesozoic Era
It is a period of the geological time which is from 250 to 65.5 million years ago. It is subdivided into the Triassic, Jurassic and Cretaceous periods.

Metamorphic rock
It is a secondary rock which is formed from an original rock with the help of heat or pressure.

Mineral
It is a naturally occurring, three dimensional, inorganic substance which has a chemical structure that may be exact, or can vary within limits.

Mineral group
It is a scientific recognized selection of minerals which are similar in structure. All the groups have their own specific group name along with the categorization of individual minerals.

Mineraloid
It is a term for amorphous, inorganic substance which lacks a crystal structure. Nature wise they are mineral but lacks crystal structure and hence they cannot be scientifically classified as minerals. Example: Opal, obsidian, etc.

Mineralogy
It is a term used for the study of minerals.

Mineralogist
It is a term used for individual who practice in the subject of mineralogy.

Mining
It is the process of extracting minerals or metal ore from a mine or a mineral deposit.

Miocene
It is a perion of the geological time which extends from 23.8 to 5.32 million years ago.

Mississippi Valley type, (MVT)
It is a type of stratabound deposit of lead and/or zinc in the carbonate rocks. It occurs in the Mississippi valley, USA.

Mohs scale
It is a loose scale of hardness devised by a German mineralogist, Friedrich Mohs in the 19th century. It is used for field collecting, which allows for identification of specimens. The comparative scale of hardness is as follows:
1) talc
2) gypsum
3) calcite
4) fluorite
5) apatite
6) moonstone
7) quartz
8) topaz and beryl
9) corundum
10) diamond

Molecule
It is the fundamental structure in all minerals. They are chemically grouped atoms which are the smallest particles a mineral can be divided without its physical or chemical properties. Chemical group of one element is also considered as molecule.

Molecular arrangement
It is the arrangement of molecules in a substance.

Molecular structure
It is the quantity and method of arrangement concerning the molecules in a particular substance.

Molten rock
It is a liquid rock under the earth's surface at an extremely high temperature. When these type of rocks cool down, it solidifies and forms rocks and minerals.

Molybdates
It is a group of minerals which is composed with the molybdate radical (MoO₄) and a metallic element. These types of minerals are heavy, soft, and brittle and are characterised with the chemically related tungstates, where they can be partially replaced by.

Monochromatic
It means exhibiting only one color. Minerals of monochromatic type occurs in only one color, irrespective of the specimen.

Monoclinic (crystal system)
It is a mineral with three axes of unequal length of two are at right angles to each other and the third lies at an angle other than 90°

Monotropic
It is a mineral which permanently changes over into its paramorph, i.e. once it transforms under suitable conditions, it cannot change back to the original mineral without the crystal structure being destroyed. The enaniotropic minerals can always change back and forth at suitable conditions.

Mother rock
It is the rock where a mineral or minerals is found implanted in.

Multicolored
It is term for exhibiting two or more of colors on a single specimen.

Mudstone
It is a fine grained sedimentary rocks which are similar to the shales in their non-plasticity, cohesion and low water content but it lack fissility.

Multiple oxides (subgroup)
It is those minerals which are compounds of two different metallic elements combined with oxygen. These are the subgroup of the oxides.

Mylonite
It is a rock fragments that has been separated from their original rock from shifting of plates in the fault zones.