Chemistry Reference
In-Depth Information
Most common tourmalines are iron tourmalines.
Tourmaline is a clastic mineral reworked in sediments. It is common
in metapelites without necessarily coming from former clastic tourmalines:
indeed, boron contained in sea water is adsorbed by clays and recrystal-
lization, even under diagenetic or anchimetamorphism conditions, induces
the formation of tourmaline (which can also nucleate on former clastic
tourmalines).
Boron is an hygromagmaphile element which is concentrated in the
evolved magmas and pneumatolytic and hydrothermal fluids. Tourmaline is
a mineral that crystallizes in evolved granites, pegmatites and aplites. Most
often it is an iron-tourmaline. Some evolved granites and pegmatite contain
lithium-tourmaline. A single crystal can also show a zonation between iron-
tourmaline and lithium-tourmaline.
Percolation of boron-bearing hydrothermal fluids produces crystalliza-
tion of tourmaline in the host rocks that are more or less completely replaced,
the textures being preserved. The result may be massive tourmalinites. This
boron metasomatism occurs particularly around some ore veins, for exam-
ple in tungsten and tin deposits.
Magnesian tourmaline occurs mainly in metamorphic rocks derived
from more or less impure calcareous-dolomitic sediments. It is probable
that these magnesian tourmaline are more commonly calcic (uvite, (Ca, Na)
Mg
3
Al
6
) than sodic (dravite, NaMg
3
Al
6
).
3.5.2.2 Axinite
Axinite is a rare sorosilicate made of almost plane layers composed of dou-
ble Si
2
O
7
tetrahedra linked by two tetrahedra containing boron, alternating
with layers made of chains of 6 octahedra containing aluminum linked by
polyhedral sites containing CaO
6
and CaO
5
(OH); calcium can be replaced
by Fe, Mg, Mn. Hence the formula:
H(Ca, Fe, Mg, Mn)
3
Al
2
BSi
4
O
16
The axinite appears in calcareous (or mafic) rocks which underwent
boron metasomatism (contact metamorphism and skarns). It is also a min-
eral of the Alpine clefts with epidote, albite, etc.
3.5.2.3 Datolite
Datolite CaSiO
4
B (monoclinic) is a nesosilicate which mostly appears as
a hydrothermal (and/or secondary) mineral in the vesicles and in veins in
basic rocks (and in skarns and serpentinites) with calcite, prehnite, axinite,
zeolites, etc.
