Chemistry Reference
In-Depth Information
Optical methods allow certain precise determinations. The founder
of these methods was undoubtedly A. Michel-Levy with his many charts
for feldspars (1894-1904). The topics by Roubault et al.
Determination
of minerals of the rocks with the petrograhic microscope
(1963, Fabriès
et al. 1984) and Tröger
Optische Bestimmung des gesteinsbildenden Min-
erale
(1971), provides numerous charts to determine with the microscope,
or at least to approach the composition of many minerals. These methods
are simple and cheap, but they provide only approximations. They are still
widely used for rapid determination of the plagioclases. The invention of the
electron microprobe analysis in 1951 allows almost punctual measures. The
microprobe is now a common tool in all laboratories.
When studying a given geological phenomenon (a magmatic series, a
prograde metamorphism or a metasomatic zoning, for instance), it com-
monly appears that the chemical composition of a given mineral varies more
or less regularly through the various stages of this phenomenon. There are
various laws of substitution playing simultaneously: for instance, a biotite
is simultaneously enriched iron (by the substitution Mg
≥
Fe) and alumi-
num (according the substitution Si Mg
Al Al). In the space of the possible
substitutions that can exist in a given mineral (and that are in principle
independent), there is a law of variation of chemical composition that is
relatively simple involving various possible substitutions: it is called
paired
substitutions
. Variations of the chemical composition of a given mineral
appear to be an excellent marker of a geological phenomenon.
≥
1.2
CLASSIFICATION OF THE MINERALS
There are currently around 4170 known mineral species. Among these min-
erals, about 50 are common rock-forming minerals. The common minerals
of economic importance, forming the ores, are about 70 to 80.
Classically (
Dana's New Mineralogy
, 9th edition, 1997; Strunz,
Min-
eralogical Tables
, 9th edition, 2006) minerals are classified into 9 classes
based on their chemical composition. According to a rather old counting of
2300 minerals, the distribution would be:
1 native elements
50
2 sulfides and sulfosalts
350
3 halides
40
4 oxides and hydroxides
220
5 carbonates [(CO
3
)
2−
]
100
nitrates [(NO
3
)
−
]
8
borates [(BO
3
)
3−
]
100
