A former name for the sagenite variety of quartz.
A mineral: SiO2 . It is a high-temperature polymorph of quartz and
tridymite, and occurs as white octahedrons in the cavities of the
fine-grained groundmasses of acidic volcanic rocks. Cristobalite is stable
only above 1470 degrees C; it has a tetragonal structure
(alpha-cristobalite) at low temperatures and an isometric structure
(beta-cristobalite) at higher temperatures. CF:tridymite
Grahamite, a mineral asphalt, from the Cristo Mine, Huasteca, Mexico.
a. See:stalling angle
b. The least angle of incidence at which there is total reflection when an
optic, acoustic, or electromagnetic wave passes from one medium to another
medium that is less refractive. CF:total reflection
c. The angle at which a ray of light in passing from a dense medium, such
as a gemstone, into a rarer medium, such as air, is refracted at 90
degrees to the normal. Any rays reaching the interface at angles greater
than the critical angle are unable to pass into the rarer medium and are
totally reflected.
d. The angle of incidence at which refracted light just grazes the surface
of contact between two different media.
e. The angle of refraction r for which sin r = 1/n, where n is the
refractive index of a transparent material. CF:law of refraction
In prospecting work, an area found to be favorable from geological age and
structural considerations. Syn:favorable locality
The area of coal required to be worked to cause a surface point to suffer
all the subsidence possible from the extraction of a given seam.
See also:subcritical area of extraction
As applied to electric blasting caps, the minimum current that can be
employed to fire detonators connected in series so that the chance of a
misfire will be less than 1 in 100,000. Fraenkel
The point at which the damping constant and the undamped frequency of a
seismometer or seismograph are equal. After deflection, the moving mass
approaches rest position without overswing and the motion is said to be
aperiodic. See also:damping
The density of a substance at its critical temperature and under its
critical pressure; that density of a saturated, granular material below
which, under rapid deformation, it will lose strength and above which it
will gain strength. AGI
a. For any explosive, the minimum diameter for propagation of a stable
detonation. Critical diameter is affected by confinement, temperature, and
pressure on the explosive. Dick, 2
b. The minimum explosive diameter which produces the propagation of a
detonation wave at a stable velocity. It is affected by conditions of
confinement, temperature and pressure on the explosive.
In refraction seismic work, that distance at which the direct wave in an
upper medium is matched in arrival time by that of the refracted wave from
the medium below having greater velocity. AGI
The maximum height at which a vertical or sloped bank of soil will stand
unsupported under a given set of conditions (ASCE, 1958). AGI
a. Minerals essential to the national defense, the procurement of which in
war, while difficult, is less serious than those of strategic minerals
because they can be either domestically produced or obtained in more
adequate quantities or have a lesser degree of essentiality, and for which
some degree of conservation and distribution control is necessary.
See also:strategic minerals; essential mineral. Hess
b. Minerals or mineral associations that are stable only under the
conditions of one given metamorphic facies and will change upon change of
facies. For example, in Eskola's greenschist facies, sericite and
chlorite, albite and epidote are critical mineral associations because
these combinations cannot persist out of the field of the greenschist
facies, although any one of the individual minerals may be found in more
than one facies. Schieferdecker
A point representing a set of conditions (pressure, temperature,
composition) at which two phases become physically indistinguishable; in a
system of one component, the temperature and pressure at which a liquid
and its vapor become identical in all properties.
Syn:decalescence point
a. The maximum feed pressure that can be applied to a diamond bit without
damaging the bit or core barrel. CF:total critical load
b. The minimum load, in pounds per effective diamond cutting point in a
bit face, at which the diamonds cut the rock. Below this load, the
diamonds slide on the rock surface without penetrating the rock, and the
diamonds polish, become dull, and are rendered unfit for further use in
that particular ground unless reset. Long
c. The pressure required to condense a gas at the critical temperature,
above which, regardless of pressure, the gas cannot be liquefied.
AGI
The maximum angle with the horizontal at which a sloped bank of soil or
given height of soil will stand unsupported. Syn:angle of repose;
angle of rest. ASCE
a. The temperature of a system at its critical point; for a one-component
system; that temperature above which a substance can exist only in the
gaseous state, no matter what pressure is exerted.
See also:temperature
b. Transformation temperature.
c. The temperature at which a change takes place in the physical form of a
substance; e.g., the change of diamond to the amorphous form of carbon
begins at a temperature of 1,800 degrees F (982 degrees C) in the presence
of oxygen. Long
d. Synonymous for critical point if the pressure is constant.
ASM, 1
e. The temperature above which the vapor phase cannot be condensed to
liquid by an increase in pressure. See also:temperature
a. Reynolds' critical velocity is that at which fluid flow changes from
laminar to turbulent, and where friction ceases to be proportional to the
first power of the velocity and becomes proportional to a higher power.
Seelye, 1
b. Kennedy's critical velocity is that of fluid flow in open channels that
will neither deposit nor pick up silt. Seelye, 1
c. Belanger's critical velocity is that condition of fluid flow in open
channels for which the velocity head equals one-half the mean depth.
Seelye, 1
The void ratio corresponding to the critical density. ASCE
An asbestiform variety of riebeckite; forms lavender-blue, indigo-blue, or
leek-green silky fibers and massive and earthy forms; suited for spinning
and weaving. Also spelled krokitolit. Syn:blue asbestos; Cape blue.
See:tiger's-eye