a. A coalfield where the coal measures crop out at the surface all around
its margin or boundary. South Wales, England, is a good example of an
exposed coalfield. See also:coal basin; concealed coalfield.
Nelson
b. Deposits of coal that crop out at the surface, as along the rim of a
coal basin. CF:crop coal
a. An area of a rock formation or geologic structure that is visible
(hammerable), either naturally or artificially, i.e., is unobscured by
soil, vegetation, water, or the works of humans; also, the condition of
being exposed to view at the Earth's surface. CF:outcrop
b. The nature and degree of openness of a slope or place to wind,
sunlight, weather, oceanic influences, etc. The term is sometimes regarded
as a syn. of aspect. AGI
c. The proportional mass of a diamond or other cutting medium protruding
beyond the surface of the metal in which it is inset in the face of the
bit. Sometimes incorrectly called clearance. Long
d. The total quantity of light received per unit area on a sensitized
plate or film, usually expressed as the product of the light intensity and
the time during which the light-sensitive material is subjected to the
action of light. A loosely used term generally understood to mean the
length of time during which light is allowed to act on a sensitive
surface. The act of exposing a light-sensitive material to a light source.
An individual picture of a strip of photographs. AGI
See:burnt alum
The process whereby an initially homogeneous solid solution separates into
two (or possibly more) distinct crystalline phases without addition or
removal of material, i.e., without change in the bulk composition. It
generally, though not necessarily, occurs on cooling. Syn:unmixing
AGI
Applied to those sedimentary rocks that solidify from solution either by
precipitation or by secretion.
A general term for the texture of any mineral aggregate or intergrowth
formed by exsolution. It is generally fairly homogeneous, ranging from
perthitic to geometrically regular. See also:eutectic texture
AGI
a. For bulk materials it is usually of troughed design and may be
lengthened or shortened while in operation. Commonly used in underground
mine conveyor work.
b. For packaged materials, objects, or units, the conveyor may be one of
several types including roller, wheel, and belt conveyors. Construction is
such that the conveyor may be lengthened or shortened within limits to
suit operating needs. See also:telescoping conveyor
Explosive charges spaced at intervals in a quarry or opencast blast hole.
See also:deck loading
A conveyor capable of being lengthened or shortened while in operation.
NEMA, 2
Consists of two or more shaker conveyor troughs, nested, to be installed
on the discharge end of the pan line so as to provide for adjustment of
the position of the discharge point. After adjustment, they are locked in
place. Jones, 1
Part of and physically associated with a known mineral deposit, but
outside of the identified parts. Barton
See:tension fault
A fracture that develops perpendicular to the direction of greatest stress
and parallel to the direction of compression; a tension fracture.
See also:extension joint
A joint that forms parallel to the direction of compression; a joint that
is an extension fracture. See also:extension fracture
Ore believed to exist ahead of ore exposed in the face of a drift.
See also:probable ore
a. Instrument used for measuring small deformations, deflections, or
displacements. Obert
b. An instrument for measuring changes caused by stress in a linear
dimension of a body. ASM, 1
See:sintered carbide-tipped pick
In polarized-light microscopy with crossed polars and an anisotropic
mineral in the light train, when the two electric vectors (permitted light
or vibration directions) of a randomly oriented crystal are each parallel
to those of the polars, no light is transmitted and the crystal is at
extinction. Isotropic crystals and anisotropic crystals viewed parallel to
an optic axis remain extinct upon stage rotation, while randomly oriented
anisotropic crystals go extinct four times upon stage rotation of 360
degrees . CF:anisotropy; extinction angle; extinction direction;
inclined extinction; optic axis; parallel extinction;
undulatory extinction.
The angle between a crystallographic direction and a position of maximum
extinction of an anisotropic crystal as viewed with a polarized-light
microscope or polariscope. It can be diagnostic in mineral identification.
Syn:angle of extinction
In polarized-light microscopy, the angular position of extinction with
respect to a crystallographic axis, a crystal face, a cleavage plane, or a
twin plane. CF:extinction
An atmosphere created behind mine seals when the supply of oxygen is
completely cut off, thereby bringing combustion to an end.
Roberts, 1