Oxy-anion A polyatomic anion incorporating oxygen
(and/or hydroxide) alongside another element. e.g.
nitrate NO 3 - . Alternatively known as oxo-anions.
Polybasic Synonym of polyprotic.
Polymer Material whose molecules are built up from
a series of identical smaller units. Thus polyethylene
consists of chains of ≤80 ethylene (ethene, C 2 H 4 ) mol-
ecules bonded together.
Polyprotic Describes an acid whose formula con-
tains more than one hydrogen atom, and whose
dissociation therefore produces more than one
hydrogen ion (Appendix B).
Polymorph One of several alternative crystal struc-
tures that a given substance can adopt.
Positron ( β + ) Anti-matter counterpart of an electron .
Potential energy The energy a body possesses by
virtue of its position in a force field such as gravity.
ppm See parts per million .
Precipitate, precipitation Formation of insoluble
solid from solution, or formation of liquid droplets
from vapour. Requires solution or vapour to be
supersaturated (Chapter 4).
Precise Describes a measurement that is subject to
only a small random error .
Precision The magnitude of the random error
associated with a measurement. (Confusingly, 'high-
precision' describes a measurement that has a low
Primitive A material that has undergone little or no
differentiation or fractionation .
Product A chemical species formed by a chemical
reaction (appearing on the right-hand side of the
Prograde (of metamorphic reactions) Proceeding to
higher grade, or higher temperature.
Proton (p + ) Positively charged massive nuclear
Pseudo-binary, pseudo-ternary System which for
practical purposes approximates to a two-compo-
nent (binary) system, but which in certain details
exhibits more complex behaviour, e.g. crystallizing
phases whose compositions lie outside the system.
Partial (vapour) pressure ( p i ) The pressure in Pa
exerted individually by one volatile component i in
a gas or other phase.
Partition coefficient K i A See Box 9.1.
Parts per million (ppm) Unit of (low) concentration
of an element (not oxide). 1 ppm = 1 g per 10 6 g = 1 μ g
per g = 1 mg per kg. 10 3 ppm = 0.1%. In gas mixtures,
'ppm' usually refers to parts per million by volume
Peridotite Rock type comprising more than 40%
Mg-rich olivine (together with other Mg-rich miner-
als such as pyroxene) making up the bulk of the
Perthite The name for a potassium feldspar crystal
containing exsolution lamellae of sodium feldspar
- see Figures 2.6b and 2.7.
Petroleum Term embracing all naturally occurring
liquid and gaseous hydrocarbon deposits.
pH See Appendix B.
Phase (chemical usage) A part of a system occupying
a specific volume and having uniform 1 physical and
chemical characteristics which distinguish it from all
other parts of the system. For example, olivine crys-
tals, melt and gas bubbles in a crystallizing magma
constitute three distinct phases.
Phase boundary Line or surface on a phase diagram
marking the limit of P-T-X stability of a particular
phase or assemblage.
Photodissociation Break-up of a molecule caused by
a high-energy photon (e.g. Equation 3.8).
Photon Quantum of light. Behaves in some respects
like a particle.
Polarizability Susceptibility of a substance to polari-
zation on the atomic or molecular scale: a measure
of how easily electron density can be deformed in an
Polarization (a) Attraction of negative charge toward
one side of an atom, ion or bond, leaving the other
side with a net positive charge. (b) A light beam
whose electric vector vibrates only in one plane is
said to be plane-polarized.
Quantum number A number - usually an integer -
whose multiple values (0, 1, 2, etc.) identify alterna-
tive electron orbitals/waveforms/stationary states
in an atom (Table 5.3) or the harmonics of any stand-
ing wave (see Figure 5.2).
Undergoing spontaneous nuclear decay.
Or continuously varying, as in a zoned mineral crystal.