A low, outspread, gently sloping mass of loose rock material, shaped in
plan view like an open fan or a segment of a cone; deposited by a stream
(esp. in a semiarid region) at the place where it issues from a narrow
mountain valley upon a plain or broad valley, or where a tributary stream
is near or at its junction with the main stream, or wherever a
constriction in a valley abruptly ceases or the gradient of the stream
suddenly decreases; it is steepest near the mouth of the valley where its
apex points upstream, and it slopes gently and convexly outward with
gradually decreasing gradient. CF:alluvial cone; bajada. Syn:fan;
detrital fan; talus fan; dry delta. AGI
A small alluvial plain bordering a river, on which alluvium is deposited
during floods. CF:alluvial plain
The exploitation of alluvial deposits by dredging, hydraulicing, or drift
mining. See also:placer mining
See:placer
A level or gently sloping tract or a slightly undulating land surface
produced by extensive deposition of alluvium, usually adjacent to a river
that periodically overflows its banks; it may be situated on a flood
plain, a delta, or an alluvial fan. CF:alluvial flat
river plain; bajada. AGI
A surface underlain by alluvium, which slopes down and away from the sides
of mountains and merges with a plain or a broad valley floor; an alluvial
surface that lacks the distinctive form of an alluvial fan or a bajada.
See also:bajada
Stream tin, or cassiterite pebbles in the gravel along the courses of
valleys and rivers on the bedrock. Generally, the purest tin ore.
See:stream tin
In placer mining, the minerals recoverable from the alluvium. These
include cassiterite, gold, diamond, gemstones, zirconia, rutile, monazite,
and platinum. Pryor, 3
a. The deposition or formation of alluvium or alluvial features (such as
cones or fans) at places where stream velocity is decreased or streamflow
is checked; the process of aggradation or of building up of sediments by a
stream along its course, or of covering or filling a surface with
alluvium. AGI
b. A hydraulic effect on solids suspended in a current of water, whereby
the coarsest and heaviest particles are the first to settle out, and the
finest muds the last, as gradient or velocity of a stream is decreased.
AGI
See:alluvium
a. A general term for clay, silt, sand, gravel, or similar unconsolidated
detrital material, deposited during comparatively recent geologic time by
a stream or other body of running water, (1) as sediment in the bed of the
stream or on its flood plain or delta, (2) as a cone or fan at the base of
a mountain slope; esp., such a deposit of fine-grained texture (silt or
silty clay) deposited during time of flood. Syn:alluvial deposit;
alluvion. AGI
b. A driller's term for the broken, earthy rock material directly below
the soil layer and above the solid, unbroken bed or ledge rock. Etymol:
Latin alluvius, from alluere, to wash against. Plural: alluvia; alluviums.
AGI
Sp. A deep-red ocher originally from Andalusia, Spain, similar to Indian
red. Used as a pigment and in polishing glass and metals. Also spelled
almagre. Standard, 2
a. An isometric mineral, 8[Fe32+ Al2 Si3 O
12 ] ; pyralspilite subgroup of the garnet group, with Fe replaced
by Mg, Mn, and Ca; in red to brownish-black dodecahedral and trapezohedral
crystals, or massive; Mohs hardness, 7-1/2; occurs in medium-grade
metamorphic rock and felsic igneous rocks; used as a gemstone and an
abrasive. Formerly called almandite; alamandine; almond stone.
b. A violet or mauve variety of ruby spinel; a reddish-purple to
purplish-red spinel.
c. A reddish-purple sapphire (almandine sapphire).
A violet-colored magnesium spinel. Syn:ruby spinel
Former spelling of almandine.
A Spanish hematite. Osborne
A furnace in which the slags of litharge left in refining silver are
reduced to lead by being heated with charcoal. Fay
Former name for almandine.
A lamprophyre chiefly composed of biotite or phlogopite and melilite as
essential minerals, commonly with olivine, calcite, and clinopyroxene.
Perovskite, apatite, nepheline, and garnet may be present. Its name
(Rosenbusch, 1887) is derived from Alnoe, Sweden. Also spelled allnoeite;
alnoeite. AGI
Trade name for fused crystalline alumina or artificial corundum used as an
quartz and beta quartz. Hess
b. Adj. Of or relating to one of two or more closely related minerals and
specifying a particular physical structure (esp. a polymorphous
modification); specif. said of a mineral that is stable at a temperature
lower than those of its beta and gamma polymorphs (e.g., "alpha
cristobalite" or "alpha -cristobalite," the low-temperature tetragonal
phase of cristobalite). Some mineralogists reverse this convention, using
alpha for the high-temperature phase (e.g., "alpha carnegieite," the
isometric phase of carnegieite stable above 690 degrees C). AGI
c. In crystallography the angle between the b and c axes.
A white, anhydrous, nonhygroscopic powder, Al2 O3 , produced
when precipitated Al(OH), is calcined at 1,000 degrees C. It is the
natural product of the Bayer process and other processes used (or
proposed) to treat bauxite, clay, or other aluminum-bearing materials.
Newton, 1