GLOSSARY - Chemical Fundamentals of Geology and Environmental Geoscience - page 253

Magma Any sort of igneous melt, including sus-
pended matter such as crystals and/or vapour
bubbles.
Mass spectrometer See Box 10.3.
Megascopic Synonym of macroscopic.
meq Abbreviation for milliequivalent.
Metalloid A metal having both metallic and non-
metallic properties.
Metastable Describes a mineral preserved outside its
P-T field of stability owing to the slowness of the
reaction converting it to a more stable mineral (e.g.
aragonite at surface conditions - Fig. 1.4).
Meteoric Describes water ultimately derived from
natural precipitation.
Micronutrient A trace element required by an organ-
ism for its metabolism to function.
Microscopic Visible only under the microscope. Too
small to see or measure with normal apparatus.
Milliequivalent (meq) A mass of substance equal to
one-thousandth of its equivalent mass .
Miscible Two compounds are said to be miscible if
they can be combined in any desired proportions to
form a single, stable, homogeneous phase.
Mole (abbr. 'mol'), molar Refers to an amount of a
compound (or element) whose mass, expressed in
grams, is numerically equal to its relative molecular
(or atomic) mass. E.g. the relative molecular mass of
H 2 O = 2 + 16 = 18; a mole of water is therefore defined
as 18 g water. An amount of water weighing 54 g
therefore constitutes 54/18 = 3 moles.
Mole fraction The fraction of the total number of
molecules in a phase that is represented by a specific
component. Mole fraction is a dimensionless
number .
Molecular weight See relative molecular mass.
Monatomic Gases consisting of separate atoms, not
combined in molecules.
Monovalent (of an atom or element) Having a
valency of 1.

Network-modifying element An element or group
whose ions (e.g. Na + , OH - ) pack in between the
framework components of silicate crystal structure.
Neutron (n) Uncharged massive nuclear particle,
marginally heavier than the proton .
Neutron number ( N ) The number of neutrons in a

nucleus.
Noble gas The gaseous elements in the extreme
right-hand column of the Periodic Table are collec-
tively known as the noble (or inert ) gases on account
of their unreactivity.
Nucleon A term embracing both protons and neu-
trons in a nucleus.
Nuclide A substance consisting of atoms with par-
ticular values of Z and N , i.e. a specific isotope of a
specific element.

Opaque, opacity Property of a material through
which light cannot pass
Orbital Term used in wave mechanics to identify
the spatial characteristics of the waveform adopted
by an electron in an atom (analogous to the classi-
cal orbit of a planet migrating around the Sun). See
Chapter 5.
Order of magnitude Estimate of a quantity to the
nearest power of 10. Two measurements are said to
'differ by an order of magnitude' when one is more
than ten times the size of the other. 10,000 is two
orders of magnitude (i.e. 10 2 ) greater than 100.
Organic chemistry Branch of chemistry dealing with
compounds in which carbon is combined with
hydrogen and other elements.
Organo-metallic Organic compound incorporating
metal atoms.
Oxidation Original meaning: a chemical reaction
resulting in the addition of oxygen to an element or
compound. Current meaning: a chemical reaction
involving the removal of electrons from an atom or
ion (Box 4.7), or an increase in the number shared
with other atoms. Cf. reduction .
Oxidation potential Eh See Box 4.1.
Oxidation state (of an atom in a molecule/com-
pound) The hypothetical charge the atom would
possess if the compound were held together by
purely ionic bonds.
Oxide Compound in which an element is chemically
bound to oxygen. See Figure 9.3.

Native Describes an element (e.g . gold) occurring
naturally in the uncombined elemental or metallic
state (not as a compound).
Network-forming element An element whose rela-
tively covalent bond with oxygen allows it to form
part of the O-Si-O framework (e.g. chains, sheets) of
a silicate crystal structure.