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Glossary of Rock and Mineral Terms - G
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- gabbro
- Any of a group of dark, dense, phaneritic, intrusive rocks that are
the plutonic equivalent to basalt.
- gamma rays
- Form of electromagnetic radiation which contains the shortest wavelength of
the spectrum. Gamma rays easily penetrate matter because of their short
wavelength.
- gangue material
- Worthless material in which valuable metals or gems occur.
- garnet paper
- Sandpaper composed of tiny, hard, Garnet crystals, glued on to it for use as
an abrasive.
- geiger counter
- Instrument that measures intensity of radiation. It is very useful for
detecting radioactive minerals.
- gem
- A cut mineral or pearl refined for use as an ornament.
- gemology
- The science and procedures involved in mining, faceting, and marketing of
gemstones. A person studying in this discipline is a gemologist.
- gemstone
- Any mineral or naturally occurring substance in an raw, uncut state that is
capable of being a gem.
- geochronology
- The study of the relationship between the history of the Earth and time.
- geode
- Hollow rock that is filled or partially filled with crystals.
- geologic
- Having to do with geology.
- geologic time scale
- The division of all of Earth history into blocks of time distinguished by
geologic and evolutionary events, ordered sequentially and arranged into eons
made up of eras, which are in turn made up of periods, which are in turn made
up of epochs.
- geology
- The scientific study of the Earth, its origins and evolution, the materials
that make it up, and the processes that act on it.
- geophysics
- The branch of geology that studies the physics of the Earth, using
the physical principles underlying such phenomena as seismic waves,
heat flow, gravity, and magnetism to investigate planetary
properties.
- geyser
- A natural spring marked by the intermittent escape of hot water and
steam.
- glacial abrasion
- The process by which a glacier erodes the underlying bedrock through contact
between the bedrock and rock fragments embedded in the base of the glacier.
See also glacial quarrying.
- glacial drift
- A load of rock material transported and deposited by a glacier. Glacial
drift is usually deposited when the glacier begins to melt.
- glacial erratic
- A rock or rock fragment transported by a glacier and deposited on bedrock of
different composition. Glacial erratics range from a few millimeters to
several yards in diameter.
- glacial quarrying
- The process by which a glacier erodes the underlying bedrock by loosening
and ultimately detaching blocks of rock from the bedrock and attaching them
instead to the glacier, which then bears the rock fragments away. See also glacial
abrasion.
- glacial till
- Drift that is deposited directly from glacial ice and therefore not sorted.
Also called till. See also glacial drift.
- glacier
- A moving body of ice that forms on land from the accumulation and compaction
of snow, and that flows downslope or outward due to gravity and the
pressure of its own weight.
- glassy (luster)
- Synonym of vitreous luster.
- gneiss
- A coarse-grained, foliated metamorphic rock marked by bands of
light-colored minerals such as quartz and feldspar that alternate with bands
of dark-colored minerals. This alternation develops through metamorphic
differentiation.
- graben
- A block of rock that lies between two faults and has moved downward
to form a depression between the two adjacent fault blocks. See also horst.
- glauber's salt
- Salt made of sodium sulfate (Na2 SO4 ·10H2O)
used in the manufacture of paper and glass and in stomach medications.
Glauber's salt is extracted from the mineral Glauberite
- globular
- The term globular is used as a synonym of botryoidal, but sometimes
describes any rounded agglomeration, such as botryoidal, reniform and
mammilary.
- goniometer
- Instrument that measures crystal angles.
- grain
- 1. Individual component belonging to a grainy aggregate.
2. Weight used in reference to measuring pearls. A grain is .05 grams or ¼
carat.
- grainy
- Crystal aggregate resembling a cluster of grain.
- graded bed
- A bed formed by the deposition of sediment in relatively still water, marked
by the presence of particles that vary in size, density, and shape. The
particles settle in a gradual slope with the coarsest particles at the bottom
and the finest at the top.
- graded stream
- A stream maintaining an equilibrium between the processes of erosion and
deposition, and therefore between aggradation and degradation.
- gradient
- The vertical drop in a stream's elevation over a given horizontal distance,
expressed as an angle.
- gram
- Weight measurement used to measure less valuable gems or rough stones. It
corresponds to the measurement of the metric gram.
- granitic
- Containing Granite
- granite
- A pink-colored, felsic, plutonic rock that contains potassium and
usually sodium feldspars, and has a quartz content of about 10%. Granite is
commonly found on continents but virtually absent from the ocean basins.
- granular
- Same as grainy.
- gravel
- Accumulation of small, smooth, rounded rock deposited by a stream or river.
- gravity
- 1. The force of attraction exerted by one body in the universe on another.
Gravity is directly proportional to the product of the masses of the two
attracted bodies.
2. The force of attraction exerted by the Earth on bodies on
or near its surface, tending to pull them toward the Earth's center.
- gravity anomaly
- The difference between an actual measurement of gravity at a given
location and the measurement predicted by theoretical calculation.
- greasy (luster)
- Luster of a mineral that appears coated with grease. Some minerals are
coated with chemicals to induce a greasy luster.
- groin
- A structure that juts out into a body of water perpendicular to the shoreline
and is built to restore an eroding beach by intercepting longshore drift
and trapping sand.
- group
- 1) An aggregate of crystals.
2) Synonym of crystal group.
3) The classification order that minerals are arranged in based on their
chemical structure.
4) A scientifically recognized selection of minerals similar in structure. All
groups have a specific group name, and individual minerals may also be
categorized. A mineral in the group may have the same name as group name. An
example is the Olivine Group, which contains the minerals Forsterite, Olivine,
and Fayalite.
- grouping
- Same as aggregate.
- guyot
- A seamount, the top of which has been flattened by weathering,
wave action, or stream erosion.
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