Chemistry Reference
In-Depth Information
Zirconium associated with titanium, can enter into the octahedral site
in andraditic garnet. There is then substitution of tetrahedral silica Al and/
or Fe 3+ :
Al Si
Zr (Al, Fe 3+ )
Such zirconian garnet is called kimzeyite: Ca 3 (Zr, Ti) 2 (Si, Al) 3 O 12 .
Garnet of some skarns may contain some few percent of Sn. This element
is probably in tetrahedral site in substitution to silica. He may enter into octa-
hedral Y site by substitution Fe 3+ Si Sn Al 3+ and/or in X site as Sn 2+ .
Vanadium can enter in the octahedral site replacing Al (goldmanite end
member: Ca 3 V 2 Si 3 O 12 ).
Hydrogrossular are grossular in which part of the SiO 4 4- tetrahedra is
replaced by OH; their formula is thus Ca 3 Al 2 Si 2 O 8 (SiO 4 ) 1-x (OH) 4x . The iron
content generally remains very limited.
Vesuvianite
Vesuvianite (also called idocrase) is a mineral whose structure and chemical
composition are similar to those of hydrogrossular. It differs from it by the
addition of atoms of aluminum, iron, and magnesium along an axis of sym-
metry of order 4, which gives to it a tetragonal symmetry. Silica occupies 10
tetrahedral SiO 4 sites as in the garnets, and also 8 Si 2 O 7 double tetrahedra
sites; vesuvianite is thus classified in the sorosilicates.
The main chemical difference with hydrogrossular is the presence of mag-
nesium, an element that probably stabilizes vesuvianite in its paragenesis.
The general formula of vesuvianite is:
Ca 19 (Fe, Mg) 3 (Al, Fe) 10 (Si 2 O 7 ) 4 (SiO 4 ) 10 (OH, F) 10
Composition of vesuvianite varies considerably by the proportions of
iron, magnesium and aluminum. Calcium can be replaced by small amounts
of Na, K, Li, Mn, U, Th and LREE; iron and aluminum may be replaced by
Cr, and Ti; silica by Be.
Vesuvianite is a mineral of medium to high temperature (360-
650/700°C). Its stablility field strongly depends on the fluid pressure and
the nature of the fluid. At higher temperatures it is replaced by grossular-
monticellite and wollastonite.
Melilite group
Melilite group minerals have a structure in sheets consisting of octahedral Y
sites forming a square centered pavement (an octahedron at each corner and
one in the center) linked by double tetrahedra Si 2 O 7 (Z sites). These sheets
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