Glossary of Rock and Mineral Terms - P

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paleomagnetism

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.

paleontology

The science and study of previous life forms, primarily in the form of fossils.

paleontologist

Individual practicing in the subject of paleontology.

paleosol

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 through deflation.

paragenesis

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.

paramagnetic

Weakly attracted to magnetic fields.

paramagnetism

Magnetic property in certain iron bearing minerals that cause them to be weakly attracted to magnetic fields.

paramorph

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 orthorhombic shape.

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.

parting

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 volcanic activity.

pearl

Smooth, round, shiny, organic object composed mainly of calcium carbonate found in the shells of some mollusks. Not to be confused with mother-of-pearl.

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.

peat

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.

pediment

A broad surface at the base of a receding mountain. The pediment develops when running water erodes most of the mass of the mountain.

pegmatite

A coarse-grained igneous rock with exceptionally large crystals, formed from a magma that contains a high proportion of water.

pendant

Ornament or piece of jewelry that hangs down, such as from a necklace or earrings.

penetration

The formation of a crystal penetrating through rock or another crystal.

penetration twinning

Form of twinning where two or more crystals are intergrown. Examples: fluorite twin, carlsbad twin, staurolite twin, and swallowtail twin.

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.

peridotite

An igneous rock composed primarily of the iron-magnesium silicate olivine and having a silica content of less than 40%.

permafrost

Permanently frozen regolith, ranging in thickness from 30 centimeters to over 1000 meters.

permeability

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.

petroleum

Any of a group of naturally occurring substances made up of hydrocarbons. These substances may be gaseous, liquid, or semi-solid.

petrography

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.

petrology

Type of geology that deals with the formation, composition, and source of rocks. Person who studies in this dicipline is a petrologist.

pH

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 a crystal.

phenocryst

Large crystal surrounded by much smaller crystals in porphyritic igneous rock.

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.

phosphorescence

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.

phyllite

A foliated metamorphic rock that develops from slate and is marked by a silky sheen and medium grain size.

phyllosilicates

Group of silicate minerals that have each set of tetrahedrons surrounded by three oxygen atoms, forming a sheet like structure.

physics

The study and science of energy and motion of matter. The person who studies this discipline is a physicist.

piezoelectric

Substance that generates an electrical charge when under stress.

piezoelectricity

Electricity generated when a piezoelectric substance is put under stress.

pinicoid

The third dimension of a three dimensional figure, usually representing its width.

pinicoidal

When in reference to a crystal type, it refers to an elongated crystal.

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.

pipe

Tube-like, cylindrical body of igneous rock.

pisolitic

Aggregate composed of small, spherical particles, larger in size and commonly more distorted than oolitic minerals.

pit

Type of mine where a large hole is dug in the ground to extract the valuable material.

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 metamiction.

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 twinning.

plane

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 shape.

planetoid

Synonym of asteroid

plastic deformation

A permanent strain that entails no rupture.

plate

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.

platy

Small, flat, and flaky.

playa

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 Epoch.

pleochroism

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.

plug

The solidified "cover" over the opening of a dormant volcano.

plumose

Featherlike aggregate in which many small crystals protrude out of a long, slender one.

pluton

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.

pocket

Cavity in igneous rock in which crystals are usually found.

point

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 stream.

polished

Either tumbled, faceted, or coated to enhance luster.

polychromatic

Minerals that are polychromatic have many different color variances.

polyhedron

A three dimensional figure composed of specific shapes.

polymorph

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.

polymorphism

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.

porcelaneous

Containing rounded, tiny holes throughout.

porous

Containing rounded, tiny holes throughout. Many porous minerals can be dyed.

porosity

The percentage of a soil, rock, or sediment's volume that is made up of pores.

porphyritic

Describing a rock that contains large, noticeable crystals, usually feldspars.

porphyry

Igneous rock containing large, noticeable crystals, usually feldspars.

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 years old.

potentia

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 sediments.

precious stone

Gem or gemstone that is highly appealing and very costly, exhibiting a powerful luster, high hardness, and rarity.

precipitation

The process in which dissolved mineral gets freed from water, forming a deposit.

primary

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.

prism

Crystal that is elongated in one direction; the other directions are about equal.

prismatic

Crystal habit describing a crystal with four or more sides similar in length and width. Prismatic crystals are usually elongated in one direction.

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.

property

A characteristic that distinguishes one substance from another.

prospect

To search for a mineral deposit or mineral in a deposit.

proton

A positively charged particle that is found in the nucleus of an atom and has a mass approximately 1836 times that of an electron.

pseudo

Assuming a false shape.

pseudocube

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 slightly asymmetrical.)

pseudohexagon

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 sided trillings.

pseudomorph

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 incrustation pseudomorphs.

pseudomorphism

The act of one mineral chemically replacing another.

pt.

Abbreviation for point,  1/100 of a carat.

pure

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.

pyncnometer

Type of instrument that measures specific gravity.

pyramidal

Shaped as a pyramid.

pyritohedraon

Twelve sided polyhedron; all sides are equidimensional and pentagonal. Minerals shaped as pyritohedrons belong to the isometric system.

pyroclastic

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.

pyroclastics

(used only in the plural) Particles and chunks of igneous rock ejected from a volcanic vent during an eruption.

pyroelectric

Substance that generates an electrical charge during a temperature change.

pyroelectricity

Electricity generated when a pyroelectric substance is put under temperatures in which it will generate electricity.

pyrometallurgy

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.

pyrosilicattes

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.