Chemistry Reference
In-Depth Information
plutonic rocks. This leaching of the alkalis is accompanied by an introduc-
tion of elements like tin, tungsten, lithium and particularly volatile elements
like boron (tourmaline) and fluorine (topaz, fluorite).
Skarns
are rocks mainly composed of calcic silicates that are developed
at the contact between two chemically incompatible media, most often
between calcic rocks (often carbonate rocks but also basic or ultrabasic
rocks) and a siliceous rocks (igneous rocks often of the family of granites).
Skarns contain the same minerals as calcic carbonate rocks (calcic garnets,
pyroxenes, amphiboles), some minerals like wollastonite and scapolite that
are not exclusive of skarns, are much more common in the skarns; there are
also minerals that are specific to the skarns, like silica under-saturated calcic
minerals.
1.4
PLAN ADOPTED IN THIS GUIDE
Most topics of systematic mineralogy present minerals in the order of the
classical classifications by Dana (1997) or by Strunz (2006) which are based
on the chemical composition and structure. The silicates are classified by the
arrangement of (SiO
4
)
4−
.tetrahedra.
The rock-forming minerals, which are mostly silicates, are presented
here by chemical affinity, which allows us to regroup minerals that have
similar occurrences and that are frequently associated.
1 major framework silicates: quartz, feldspaths, feldspathoids, ubiquitous
minerals which are the main bearers of aluminum and alkalis;
2 major ferro-magnesian minerals: micas, chlorites, amphiboles,
pyroxenes, olivines;
3
aluminous minerals: minerals resulting from the metamorphism rocks of
clay-rich sedimentary (pelites, shales) and peraluminous igneous rocks;
4
calcic, magnesian and calc-magnesian minerals: minerals of more or less
impure metamorphic marbles, calc-silicate-gneisses; skarns and some
calcium-rich igneous rocks;
5
accessory minerals which are the bearers of minor elements like B, P, Ti,
Zr …;
6
some minerals more common in sedimentary rocks; only those which
are recognizable under the microscope will be here discussed;
7
minerals of the ore deposits are mostly opaque minerals whose study
requires the use of the reflected light microscope. Some minerals recog-
nizable with the petrographic microscope are presented.
