a. All the area occupied or favorable for occupation by living organisms.
It includes parts of the lithosphere, hydrosphere, and atmosphere.
AGI
b. All living organisms of the Earth and its atmosphere. AGI
A bedded, blanketlike mass of rock composed mainly of the remains of
sedentary organisms; an organic layer, such as a bed of shells or corals,
or even a coal seam. CF:bioherm
a. A monoclinic mineral, K2 Mg6 (Si6 Al2 O
20 )(OH,F)2 ; mica group, with Fe2+ replacing Mg and
Fe3+ replacing Al; in masses with perfect basal cleavage; dark
brown, dark green, black; a common rock-forming mineral in crystalline
rocks, either as an original crystal in igneous rocks or as a metamorphic
product in gneisses and schists; a detrital constituent of sedimentary
rocks.
b. A general term to designate all ferromagnesian micas. Syn:black mica;
dark mica; magnesia mica. See also:iron mica
A gneiss in which biotite is the prominent dark mineral.
A jet black igneous rock consisting essentially of biotite. Near Libby,
MT, such a rock has been altered to vermiculite by hot waters.
Johannsen
The heat-transfer ratio hr/k, where h is the heat-transfer coefficient, r
is the distance from the point or plane under consideration to the
surface, and k is the thermal conductivity. The Biot number is a useful
criterion in assessing thermal-shock resistance. Dodd
An electrode that is not mechanically connected to a power supply but is
placed in an electrolyte, between the anode and the cathode, such that the
part nearer the anode becomes cathodic and the part nearer the cathode
becomes anodic. Syn:intermediate electrode
A closed crystal form consisting of a positive and negative pyramid.
CF:pyramid
A geophysical measuring device such as a magnetometer, plus the housing in
which it is towed behind an aircraft. AGI
a. Mixed screened anthracite passing a 1/2-in (12.7-mm) screen, but
retained on a 1/8-in (3.2-mm) screen. May be subdivided into buckwheat,
rice, and barley. See also:anthracite coal sizes; barley;
bird's-eye coal. CTD
b. Eng. Applied to various rocks with small spots, in some places to a
concretionary slate, and in Guernsey to a spotted variety of diorite or
gabbro. Arkell
Sometimes applied to anthracite coal when very small fractures are
numerous and freshly broken surfaces display rounded or oval eyelike
forms, many of which have convex surfaces.
See also:anthracite coal sizes; bird's-eye. AGI
A very fine-grained limestone containing spots or tubes of crystalline
calcite.
A name given by prospectors and miners to a fine-grained igneous rock
having small phenocrysts, particularly if they are quartz, from a fancied
resemblance to birds' eyes. Hess
A quarryman's term for slate containing abundant deformed or squeezed
concretions. Holmes, 2
A fine-coal dewatering machine that consists of a tank or truncated
conical shell, which is revolved at the desired speed by means of a drive
sheave. A screw conveyor rotates inside the cone or bowl at a slightly
lower speed in the same direction of rotation. The feed entrance, in the
center of the large end of the truncated cone, is high enough to allow
formation of a pool of slurry. Adjustable effluent-discharge parts are so
located in the large end of the bowl that the level of liquid is
maintained at the desired height. The solids are steadily moved forward by
the screw conveyor as fast as they are deposited, being carried above the
level of the pool for an interval before leaving the bowl. Discharge of
both solids and effluent is continuous.
See:birefringent
See:birefringent
a. The numerical difference between the refractive indices of a mineral.
This difference results in a display of interference colors when thin
sections or small fragments of anisotropic minerals are viewed between
crossed polars. Isometric minerals and amorphous materials are isotropic
and have the same refractive index in every direction; they have no
birefringence and show no interference colors. See also:colors
b. The property of anisotropic crystals to split a beam of light into two
polarized rays that traverse the crystal at different velocities as they
pass through it and produce characteristic optical effects that are
recognizable with the proper instruments or, in some cases (e.g.,
calcite), by the eye alone. Syn:double refraction
CF:transmitted light
Said of a crystalline substance that displays birefringence; such
materials have more than one index of refraction. Syn:birefractive;
birefracting.
See:boule
A monoclinic mineral, Na4 Mn14 O27 .9H2 O ;
black or dark-brown, named for Birness, Scotland.