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M

Macroscopic       Visible to the unaided eye.

Mafic       Igneous rocks composed mostly of dark, iron- and magnesium-rich minerals.

Magazine       A building, storehouse, or structure where explosive materials are kept or stored.

Magma       The molten material deep in the Earth, from which rocks are formed.

Magmatic Ore Deposit       Formed by differentiation of mineral in magma.

Magmatic segregation       An ore-forming process whereby valuable minerals are concentrated by settling out of a cooling magma.

Magnetic gradient survey       A geophysical survey using a pair of magnetometers a fixed distance apart, to measure the difference in the magnetic field with height above the ground.

Magnetic separation       A process in which a magnetically susceptible mineral is separated from gangue minerals by applying a strong magnetic field; ores of iron are commonly treated in this way.

Magnetic survey       A geophysical survey that measures the intensity of the Earth's magnetic field.

Magnetic susceptibility       A measure of the degree to which a rock is attracted to a magnet.

Magnetite       Magnetic iron ore, being a black iron oxide containing 72.4% iron when pure.

Magnetometer       An instrument used to measure the magnetic attraction of underlying rocks.

Main entry       A main haulage road. Where the coal has cleats, main entries are driven at right angles to the face cleats.

Main fan       A mechanical ventilator installed at the surface; operates by either exhausting or blowing to induce airflow through the mine roadways and workings.

Malleable       Easily hammered and flattened when cold; refers to metals.

Mallet       Wooden hammer for driving stakes.

Man Killer       A hand held tool like an axe or pick that is really too big for the job.

Man trip       A carrier of mine personnel, by rail or rubber tire, to and from the work area.

Manhole       A safety hole constructed in the side of a gangway, tunnel, or slope in which miner can be safe from passing locomotives and car. Also called a refuge hole.

Manway       An entry used exclusively for personnel to travel form the shaft bottom or drift mouth to the working section; it is always on the intake air side in gassy mines. Also, a small passage at one side or both sides of a breast, used as a traveling way for the miner, and sometimes, as an airway, or chute, or both.