Chemistry Reference
In-Depth Information
skarns (Tilley and Harwood, 1931) also display skarns developed at contact
with basic igneous rocks. The skarns are formed mainly of anhydrous or
water-poor silicates of calcium, magnesium, iron and manganese.
Endoskarns
are skarns formed at the expense of igneous or siliceous
rocks;
exoskarns
are those formed at the expense of carbonate rocks.
Essentially, there is a transfer of silica and iron from the siliceous rock
to the calcic rock and transfer of calcium from the calcic rock to siliceous
rock: Ca, Si, Fe are mobile elements. A metasomatic column (which can
reach several tens of meters of thickness) is so formed; it is made of zones
with sharp boundaries and contrasting mineralogy. The activity of silica
increases from the calcic rock to the siliceous rock and the activity of cal-
cium varies inversely.
Aluminum, magnesium, zirconium (and to a lesser extent, titanium,
phosphorus, rare earths) are inert elements or of very little mobility in
this process. The presence of minerals containing characteristic elements
of igneous rocks (which do not occur in carbonate rocks), such as Al,
Zr, P, allows us to precisely define the boundary between endo-and
exoskarns.
Two main types of skarns may be distinguished by the nature of the
carbonate rock: calcic skarns developed on limestone and magnesian skarns
developed on dolostones. There are also very rare manganese skarns devel-
oped on manganiferous rocks. Skarns that grow on calc-silicate-gneisses
and calcium are called skarnoides or calcic hornfels.
The evolution of skarns can generally be described in several stages:
At the
magmatic stage
(usually there are several sub-stages), the
exchanges occur between the magma and the host rock. The magma is con-
taminated by the input of calcium and crystallizes aluminous diopside. It
may even become under-saturated in silica; this is reflected by the crystalli-
zation of melilite of the åkermanite - Na-melilite series and of feldspathoids.
Minerals developed in the marbles are anhydrous minerals high tempera-
ture (700-900°C): wollastonite, tilleyite, spurrite, rankinite, larnite in calcic
skarns, monticellite, forsterite, åkermanite, merwinite, periclase in magne-
sian skarns. This stage is very rarely preserved in calcic skarns; it is some-
what better preseved in magnesian skarns.
The
main stage
(650-400°C), commonly observed, is a hydrothermal
stage where mainly anhydrous minerals are formed: garnets of the gros-
sular-andradite series, pyroxene of the diopside-hedenbergite-johannsenite
series, wollastonite, calcic plagioclase, scapolite, vesuvianite and, in magne-
sian skarns, forsterite, humites, phlogopite, hastingiste, etc. Endoskarns usu-
ally contain hydrated minerals such as amphibole of the hornblende group.
This stage is often divided into several sub-stages. Several types of skarns are
distinguished: skarns where dominant mineral is hedenbergite (iron as Fe
2
,
called “reduced skarns”), skarns with magnetite (FeO
·
Fe
2
O
3
) and skarns
with andraditic garnet (iron as Fe
3+
called “oxidized skarns”).
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