Glossary of Rock and Mineral Terms - P
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- 1. The fixed orientation of a rock's crystals, based on the Earth's magnetic
field at the time of the rock's formation, that remains constant
even when the magnetic field changes.
2. The study of such phenomena
as indicators of the Earth's magnetic history.
- The science and study of previous life forms, primarily in the form
- Individual practicing in the subject of paleontology.
- An ancient, buried soil whose composition may reflect a climate
significantly different from the climate now prevalent in the area
where the soil is found.
- Paleozoic Era
- The earliest era of the Phanerozoic Eon, marked by the presence of
marine invertebrates, fish, amphibians, insects, and land plants.
- parabolic dune
- A horseshoe-shaped dune having a concave windward slope and a
convex leeward slope. Parabolic dunes tend to form along sandy ocean
and lake shores. They may also develop from transverse dunes
- The crystallization mode of one mineral in retrospect to other
minerals. Paragenesis is applied to determine the conditions and
qualifying factors necessary for a mineral to form.
- Weakly attracted to magnetic fields.
- Magnetic property in certain iron bearing minerals that cause them
to be weakly attracted to magnetic fields.
- A pseudomorph involving two minerals with an identical composition
but different crystal structures. The original mineral forms, but
conditions then cause it to be unstable, so it transforms into the
other mineral with the same chemical structure while retaining the
original crystal shape. An example of this is Aragonite that becomes
unstable and transforms into Calcite but retains the original
- parent isotope
- A radioactive isotope that changes into a different isotope
when its nucleus decays. See also daughter isotope.
- parent material
- The source from which a given soil is chiefly derived, generally
consisting of bedrock or sediment.
- partial melting
- The incomplete melting of a rock composed of minerals with differing
melting points. When partial melting occurs, the minerals with higher
melting points remain solid while the minerals whose melting points
have been reached turn to magma.
- The tendency of certain minerals to split along stressed areas or
along twinned crystals.
- passive continental margin
- A border that lies between continental and oceanic lithosphere,
but is not a plate margin. It is marked by lack of seismic and
- Smooth, round, shiny, organic object composed mainly of calcium
carbonate found in the shells of some mollusks. Not to be confused
- pearly luster
- Exhibiting a luster similar to the inside of a mollusk shell or
shirt button. Many mica's exhibit a pearly luster, and some minerals
with a pearly luster have an iridescent hue. Some minerals may exhibit
a pearly luster on cleavage cracks parallel and below the reflecting
surface of a mineral.
- A soft brown mass of compressed, partially decomposed vegetation
that forms in a water-saturated environment and has a carbon content
of 50%. Dried peat can be burned as fuel.
- A broad surface at the base of a receding mountain. The pediment
develops when running water erodes most of the mass of the mountain.
- A coarse-grained igneous rock with exceptionally large crystals,
formed from a magma that contains a high proportion of water.
- Ornament or piece of jewelry that hangs down, such as from a
necklace or earrings.
- The formation of a crystal penetrating through rock or another
- penetration twinning
- Form of twinning where two or more crystals are intergrown.
Examples: fluorite twin, carlsbad twin, staurolite twin, and
- Pentagonal dodecahedron
- Synonym of pyritohedron.
- perched water table
- A saturated area that lies within a zone of aeration.
- percussion figure
- Six rayed, star-like flaw that forms when some micaceous minerals
are put under pressure.
- An igneous rock composed primarily of the iron-magnesium silicate
olivine and having a silica content of less than 40%.
- Permanently frozen regolith, ranging in thickness from 30
centimeters to over 1000 meters.
- The capability of a given substance to allow the passage of a fluid.
Permeability depends on the size of and the degree of connection among
a substance's pores.
- petrification (petrifaction)
- Process in which organic substances, such as wood and shells, are
replaced by silica.
- petrified wood
- Wood that is petrified, i.e. replaced by silica.
- Any of a group of naturally occurring substances made up of hydrocarbons.
These substances may be gaseous, liquid, or semi-solid.
- Type of geology that deals with the classification of rocks, which
is based on the material they contain. Person who studies in this
discipline is a petrographer.
- Type of geology that deals with the formation, composition, and
source of rocks. Person who studies in this dicipline is a petrologist.
- Measurement used to survey the amount of acid or alkaline present in
a material. pH is measured on a scale of 0 - 14. Neutral is 7; numbers
below 7 are acid and above are alkaline. The lower the number, the
stronger its acidity, and the greater the number, the stronger the
alkalinity. pH stands for "potential of Hydrogen" for
hydrogen causes a material to be acidic.
- phantom growth
- An interesting phenomenon exhibited when a crystal grows, than a new
growth grows over the old crystal in the same direction, leaving an
inscription of the previous growth on the crystal. Additional growth
may be present, leading to the possibility of more than one phantom in
- Large crystal surrounded by much smaller crystals in porphyritic
- phosphates group
- Group of minerals that are compounds of one or more metallic
elements associated with the phosphate radical (PO4). The phosphates
are classified together with the arsenates and vanadates. Most
phosphates are heavy, and none are hard. They are usually brittle and
occur in small crystals or compact aggregates.
- Emission of visible light by a substance, such as a mineral, that is
exposed to ultraviolet light and absorbs radiation from it. The light
appears in the form of glowing, distinctive colors. The emission
continues after the exposure to ultraviolet light ends.
- A foliated metamorphic rock that develops from slate
and is marked by a silky sheen and medium grain size.
- Group of silicate minerals that have each set of tetrahedrons
surrounded by three oxygen atoms, forming a sheet like structure.
- The study and science of energy and motion of matter. The person who
studies this discipline is a physicist.
- Substance that generates an electrical charge when under stress.
- Electricity generated when a piezoelectric substance is put under
- The third dimension of a three dimensional figure, usually
representing its width.
- When in reference to a crystal type, it refers to an elongated
- pinicoidal clevagge
- Type of cleavage exhibited on some prismatic and tabular minerals
where they cleave on the pinicoidal plane, which is the third
dimension aside from the basal and prismatic sides, which they may
also cleave on.
- Tube-like, cylindrical body of igneous rock.
- Aggregate composed of small, spherical particles, larger in size and
commonly more distorted than oolitic minerals.
- Type of mine where a large hole is dug in the ground to extract the
- pitchy (pitchlike) luster
- Luster of a mineral that appears similar to tar. Minerals with a
pitchy luster are radioactive and have gone through the process of
- placer deposit
- A deposit of heavy or durable minerals, such as gold or diamonds,
typically found where the flow of water abruptly slows.
- plagioclase twinning
- Two or more crystals that twin in a repeated pattern ("repeated
twinning") as depicted in the figure below. Named after the
mineral Plagioclase, which most frequently exhibits this form of
- Imaginary line connecting two points on a surface.
- plane of symmetry
- Imaginary lines traced on polyhedrons such as cubes and octahedrons
depicting a point on the polyhedron that exhibits symmetry. For
example, if rotated 180° from that line will yield the same
- Synonym of asteroid
- plastic deformation
- A permanent strain that entails no rupture.
- 1. Small, flat, flaky crystal.
2. The definition of the section of rock present by fault areas.
- plate tectonics
- The theory that the Earth's lithosphere consists of large,
rigid plates that move horizontally in response to the flow of the asthenosphere
beneath them, and that interactions among the plates at their borders
cause most major geologic activity, including the creation of oceans,
continents, mountains, volcanoes, and earthquakes.
- Small, flat, and flaky.
- A dry lake basin found in a desert.
- Pleistocene Epoch
- The first epoch of the Quaternary Period, beginning 2 to 3 million
years ago and ending approximately 10,000 years ago. See also Holocene
- The effect present in a mineral exhibiting two or more separate
colors when viewed at different angles. Pleochroism and dichroism are
synonymous, except dichroism refers only to two colors, but
pleochroism can be more than two. Minerals displaying this
characteristic are said to be pleochroic.
- The solidified "cover" over the opening of a dormant
- Featherlike aggregate in which many small crystals protrude out of a
long, slender one.
- An intrusive rock, as distinguished from the preexisting country
rock that surrounds it.
- plutonic rock
- An intrusive rock formed inside the Earth crust, and
individual crystal grains can be seen.
- pluvial lake
- A lake that formed from rainwater falling into a landlocked basin
during a glacial period marked by greater precipitation than is found
in the region in prior or subsequent periods.
- Cavity in igneous rock in which crystals are usually found.
- Unit of measurement given to small, precious gemstones. One point is
equivalent to 1/100th of a carat. The abbreviation for point is Pt.
- point bar
- A low ridge of sediment that forms along the inner bank of a meandering
- Either tumbled, faceted, or coated to enhance luster.
- Minerals that are polychromatic have many different color variances.
- A three dimensional figure composed of specific shapes.
- A mineral that is identical to another mineral in chemical
composition but differs from it in crystal structure. Eganpmes:
Diamond and graphite or Rutile, Brookite, and Anatase.
- The tendency of minerals with the same chemical composition to form
different crystal structures.
- polysynthetic twinning
- Form of twinning where the crystals intergrow in a repeated pattern.
- polysynthetic twins
- Twinned crystals that are twinned through polysynthetic twinning.
- Containing rounded, tiny holes throughout.
- Containing rounded, tiny holes throughout. Many porous minerals can
- The percentage of a soil, rock, or sediment's volume that is made up
- Describing a rock that contains large, noticeable crystals, usually
- Igneous rock containing large, noticeable crystals, usually
- porphyry copper deposit
- A crystallized rock, typically porphyritic, having hairline
fractures that contain copper and other metals.
- potassium-argon dating
- A form of isotope dating that relies on the extremely long half-life
of radioactive isotopes of potassium, which decay into isotopes of
argon, to determine the age of rocks in which argon is present.
Potassium-argon dating is used for rocks between 100,000 and 4 billion
- The combined influence of gravity and water pressure on
groundwater flow at a given depth.
- potentiometric surface
- The level to which the water in an artesian aquifer would
rise if unaffected by friction with the surrounding rocks and
- precious stone
- Gem or gemstone that is highly appealing and very costly, exhibiting
a powerful luster, high hardness, and rarity.
- The process in which dissolved mineral gets freed from water,
forming a deposit.
- Forming as new, not as an alteration product.
- primary mineral
- Mineral that forms by the combination of elements rather than by
alteration of a mineral.
- primary coast
- A coast shaped primarily by nonmarine processes, such as glacial
erosion or biological processes.
- principle of cross-cutting relationships
- The scientific law stating that a pluton is always
younger than the rock that surrounds it.
- principle of faunal succession
- The scientific law stating that specific groups of animals
have followed, or succeeded, one another in a definite sequence
through Earth history.
- principle of original horizontality
- The scientific law stating that sediments settling out
from bodies of water are deposited horizontally or nearly horizontally
in layers that lie parallel or nearly parallel to the Earth's surface.
- principle of superposition
- The scientific law stating that in any unaltered sequence of
rock strata, each stratum is younger than the one beneath it and older
than the one above it, so that the youngest stratum will be at the top
of the sequence and the oldest at the bottom.
- principle of uniformitarianism
- The scientific law stating that the geological processes
taking place in the present operated similarly in the past and can
therefore be used to explain past geologic events.
- Crystal that is elongated in one direction; the other directions are
- Crystal habit describing a crystal with four or more sides similar
in length and width. Prismatic crystals are usually elongated in one
- prismatic cleavage
- Type of cleavage exhibited on some prismatic minerals in which the
mineral cleaves by breaking off thin, vertical, prismatic crystals off
of the original prism. Example is Acmite.
- A characteristic that distinguishes one substance from another.
- To search for a mineral deposit or mineral in a deposit.
- A positively charged particle that is found in the nucleus of
an atom and has a mass approximately 1836 times that of an electron.
- Assuming a false shape.
- Rhombohedron shaped crystal almost identical to a cube, but its
angles slightly differ from a cube. (May also refer to any crystal
that closely resembles any member of the isometric system but is
- Six sided crystal that assumes a hexagonal shape although it is not
in the hexagonal system. The cause of pseudohexagonal crystals is
orthorhombic crystals that intergrow in three individuals, forming six
- One mineral that chemically replaces another mineral without
changing the external form of the original mineral. There are three
types of pseudomorphs: paramorphs, infiltration pseudomorphs, and
- The act of one mineral chemically replacing another.
- Abbreviation for point, 1/100 of a carat.
- Free of impurities.
- P wave(abbreviation for primary wave)
- A body wave that causes the compression of rocks when
its energy acts upon them. When the P wave moves past a rock, the rock
expands beyond its original volume, only to be compressed again by the
next P wave. P waves are the fastest of all seismic waves. See
also S wave.
- P-wave shadow zone
- The region that extends from 103º to 143º from the epicenter
of an earthquake and is marked by the absence of P waves.
The P-wave shadow zone is due to the refraction of seismic waves
in the liquid outer core. See also S-wave shadow zone.
- Type of instrument that measures specific gravity.
- Shaped as a pyramid.
- Twelve sided polyhedron; all sides are equidimensional and
pentagonal. Minerals shaped as pyritohedrons belong to the isometric
- Being or pertaining to rock fragments formed in a volcanic eruption.
- pyroclastic cone
- A usually steep, conic volcano composed almost entirely of an
accumulation of loose pyroclastic material. Pyro-clastic cones
are usually less than 450 meters high. Because no lava binds
the pyroclastics, pyroclastic cones erode easily.
- pyroclastic eruption
- A volcanic eruption of viscous, gas-rich magma. Pyroclastic
eruptions tend to produce a great deal of solid volcanic fragments
rather than fluid lava.
- pyroclastic flow
- A rapid, extremely hot, downward stream of pyroclastics, air,
gases, and ash ejected from an erupting volcano. A pyroclastic
flow may be as hot as 800ºC or more and may move at speeds exceeding
150 kilometers per hour.
- (used only in the plural) Particles and chunks of igneous rock
ejected from a volcanic vent during an eruption.
- Substance that generates an electrical charge during a temperature
- Electricity generated when a pyroelectric substance is put under
temperatures in which it will generate electricity.
- The separation of metals from ore or from alloys through a process
dependent on heat, or the forming of alloys and purification of metals
through a process dependent on heat.
- Synonym of sorosilicates
- pyroxene (group)
- Group of minerals that contain iron, magnesium, silicon, and oxygen.
They may also contain calcium, sodium, and lithium. The pyroxenes have
prismatic cleavage, and one can note the angle of 87° and 93°
(almost perpendicular), which distinguishes it from amphiboles which
are at 56° and 124°. The pyroxenes are very similar to the
amphiboles, and the minerals belonging to each group many times cannot
be distinguished by ordinary methods. Generally, though, the pyroxenes
occur in shorter, stubbier crystals than the amphiboles. The
amphiboles contain the same elements that the pyroxenes do, except
they have hydroxyl (OH) in their structure, which alters both its
physical and chemical properties.The pyroxene group is in the
inosilicate subdivision of the silicates group.