Mineral definitions
It is remarkably difficult to define a mineral. Like other well-intended definitions, it is
often derailed by exceptions, becoming ever more abundant with increased complexity
of the definition. The standard definition usually goes along lines like 'a mineral is a
chemically pure, solid substance with a uniform and coherent internal organisation,
formed by natural processes.'
Natural
The 'natural' is ever so often replaced by the term 'mineralogical' giving a wonderful
circular and inherently nonsensical definition or 'geological' which is less obvious but no
more informative. Regrettably, very few if any minerals fit the definition!
Pure
How should we understand 'chemically pure'? Practically every mineral contains traces
of alien elements, and some are even known to form series of mixed minerals - this
includes well-known groups like feldspars and amphiboles. And obviously, if we replace
'chemically pure' with 'varying within fixed limits' we might as well forget about it.
Solid
Solid? Well, we like to think of Mercury as a mineral ... OK, that's the exception then.
But an otherwise abundant mineral like Ice [yes, it is a mineral - pure, solid, internal
organisation] does become mobile and slippery under pressure. That's how glaciers
move. Many other minerals have similar properties, we just don't have the chance to
observe it as readily. Some have a more eccentric lack of solidity, where you can shift
the crystal lattice, even if the mineral is neither soft, liquid or even warm. A single
crystal of Antimonite can be bent, shifting the crystal lattice, but it is a one-time only
process; it can not be bent back [please don't try this on a good specimen!]. You may
also press a corner of a Calcite cleavage rhombohedron against a firm base, and flip the
corner - technically you induce a twin. This only works on very small pieces, but does
demonstrate that even a 'solid' mineral is not solid in the sense it wont move.
Internal organisation
'Uniform and coherent internal organisation' basically means it is crystalline - all the
atoms are arranged in a repetitive pattern. That works fine ... up to a point. We already
mentioned Mercury, that is liquid and thus has no internal repetitive pattern. The same
applies to Opal and all other amorphous minerals (there are a few). With good analytical
methods, you may detect coherent internal organisations (crystal lattices), that are not
uniform. Every now and then, crystals are zoned and two chemically different
substances grow on top of each other, sharing a crystal lattice, that changes slightly in at
least one dimension. This is very common in f.ex. amphiboles, that often differ
chemically from the core through several layers towards the surface of the crystal.
Lost?
So we didn't get too far on that definition part, did we? And you know what? It doesn't
bother me at all! It doesn't bother me, because it is a pseudo-problem. I can define
'Calcite' [you can look that one up yourself] - we know it's chemistry, crystallography,
lattice constants, etc., and even know it can incorporate foreign elements like magnesium
and iron without appreciably affecting the properties. That is, we have a firm definition
of Calcite, and we have a good understanding of how real objects in nature may deviate
from that; the definition of 'Calcite' allows us to distinguish between objects that are
'Calcite' and those that are 'not Calcite' and hence it is a good definition. We have
similar definitions of 'Gold', 'Azurite', 'Quartz', 'Kristiansenite', etc. That is, whenever
we put a name on things, we can go back and look up what that name means.
The problem arises, when we wish to define a group of all these good definitions,
excluding equally good definitions of other objects (sugar, steel, grandpa, ...), and that
definition has to be non-circular and simple ... and be ready to include definitions of
future minerals? Finding the common denominator of over 3,500 definitions,
simultaneously predicting future scientific understanding is not a reasonable expectation.
Regrettably, I can only offer two possible definitions of the concept of 'mineral': I) I
know one, when I see one (rather presumptuous); II) a substance recognised as such by
the International Mineralogical Association (no hard feelings, but still 'old boys
network'). But do note, that even if I can not define 'mineral', I can explain 'mineral'
and I can at any time define any mineral.
Bibliography
Blackburn, William H. & William H. Dennen. 1997. Encyclopedia of mineral
names. Canadian Mineralogist, special publication 1.
Gaines, Richard W., H. Catherine W. Skinner, Eugene E. Foord, Brian Mason,
Abraham Rosenzweig & Vandall T. King. 1997. DanaÕs new mineralogy: the system of
mineralogy of James Dwight Dana and Edward Salisbury Dana, 8th ed.
Ramdohr, Paul & Hugo Strunz. 1980. KlockmannÕs Lehrbuch der Mineralogie,
16th ed.
This page is authored by Claus Hedegaard.
