Minerals with isolated [SiO4]4- tetrahedra Nesosilicates Minerals with isolated [SiO4]4- tetrahedra

The garnet picture gallery

Garnets: A large group of cubic minerals with general formula The garnet minerals Garnets: A large group of cubic minerals with general formula A32+B23+(SiO4)3 A – Ca2+, Mg2+, Fe2+ or Mn2+ B – Al3+,Fe3+ or Cr3+ There are also many other synthetic compositions possible

Spessartine Mn3Al2(SiO4)3 Garnet compositions  Grandite group Grossular Ca3Al2(SiO4)3 Andradite Ca3Fe3+2(SiO4)3 Uvarovite Ca3Cr2(SiO4)3   Pyralspite group Pyrope Mg3Al2(SiO4)3 Almandine Fe2+3Al2(SiO4)3 Spessartine Mn3Al2(SiO4)3 Solid solutions within each group are typical. At high T (above 700oC) there is also solid solution between the 2 groups. Note: Solid solutions are always more extensive at higher Temperatures

The garnet structure I

The garnet structure II - isolated tetrahedra

The garnet structure III - corner-sharing with octahedra

The garnet structure IV - the large A polyhedra added

The garnet structure V - cluster around the octahedron

The garnet structure VI - large A polyhedra added

The garnet structure - building out further……..

The garnet structure - the whole unit cell

Garnet

Spessartine Mn3Al2(SiO4)3 Garnet compositions  Grandite group Grossular Ca3Al2(SiO4)3 Andradite Ca3Fe3+2(SiO4)3 Uvarovite Ca3Cr2(SiO4)3   Pyralspite group Pyrope Mg3Al2(SiO4)3 Almandine Fe2+3Al2(SiO4)3 Spessartine Mn3Al2(SiO4)3 Solid solutions within each group are typical. At high T (above 700oC) there is also solid solution between the 2 groups. Note: Solid solutions are always more extensive at higher Temperatures

Grossular

Andradite

Uvarovite

Pyrope

Almandine

Spessartine

Olivine The name ‘olivine’ refers to a range of compositions between Mg2SiO4 (forsterite) and Fe2SiO4 (fayalite), which form a solid-solution.

The structure of olivine (Fe,Mg)2SiO4

Olivine compositions No compositions exist here

Olivine is orthorhombic and often forms a characteristic crystal shape in a rock because it is often the first mineral to crystallise from the melt. 001 101 100 010

Peridot - gem quality olivine This is a cut crystal Olivine

monticellite

Humite Group The humite group contains four members whose general formula is nMg2SiO4∙ Mg(OH,F)2 where n is 1, 2, 3 and 4. Thus, we get: Norbergite Pbnm Mg3SiO4(OH,F)2 n=1 Chondrodite P21/b Mg5(SiO4)2(OH,F)2 n=2 Humite Pbnm Mg7(SiO4)3(OH,F)2 n=3 Clinohumite P21/b Mg9(SiO4)4(OH,F)2 n=4

norbergite

chondordite

clinohumite

The aluminium silicates : kyanite, andalusite and sillimanite Al2SiO5 Al2SiO5 ???? Why this formula for a silicate with isolated tetrahedra? Could be written Al.AlO(SiO4) because there are two different Al structural sites as well as isolated [SiO4] tetrahedra. They also have been considered as belonging to a separate silicate subclass – subsaturates, because there are conceptually too few oxygens, a naïve viewpoint. Kyanite, andalusite and sillimanite are polymorphs, because they have the same chemical composition but can exist with different crystal structures.

The crystal structures of the Al2SiO5 polymorphs All three structures have straight chains of edge-sharing AlO6 octahedra along the c axis

The crystal structures of the Al2SiO5 polymorphs All three structures have straight chains of edge-sharing AlO6 octahedra along the c axis. These chains contain half of the Al in the structural formula. In kyanite the remaining Al atoms are in 6-fold coordination (octahedra) In andalusite the remaining Al atoms are in 5-fold coordination In sillimanite the remaining Al atoms are in 4-fold coordination (tetrahedra) Density: kyanite > sillimanite > andalusite Therefore kyanite is stable at the highest pressures and lowest temperatures, while sillimanite is stable at high temperatures and lower pressures

The stability of the Al2SiO5 polymorphs The Al2SiO5 polymorphs form in metamorphic rocks Pressure (kbar) 3.55-3.66 3.23 3.16-3.20

The structure of kyanite 1. The AlO6 octahedral chains

The structure of kyanite 1. The AlO6 octahedral chains 2. Add the other Al polyhedra

The structure of kyanite 1. The AlO6 octahedral chains 2. Add the other Al polyhedra 3. Add the [SiO4] tetrahedra

The structure of andalusite 1. The AlO6 octahedral chains

The structure of andalusite 1. The AlO6 octahedral chains 2. Add the other Al polyhedra

The structure of sillimanite 1. The AlO6 octahedral chains

The structure of sillimanite 1. The AlO6 octahedral chains 2. Add the other Al polyhedra

The structure of sillimanite 1. The AlO6 octahedral chains 2. Add the other Al polyhedra 3. Add the [SiO4] tetrahedra

Sillimanite

Andalusite

Kyanite

Staurolite Fe2Al9O6(SiO4)4(O,OH)2

Chloritoid (Fe,Mg)Al2SiO5(OH)2

Titanite (sphene) CaTiSiO5

Titanite, CaTiO[SiO4] Monoclinic, 2/m Tetrahedral Si, Octahedral Ti and 7-fold Ca

Topaz Al2SiO4(F,OH)

Zircon

Silicates with isolated tetrahedra (cont.) Zircon ZrSiO4 an important industrial mineral : main source of Zr and ZrO2 found in small amounts in many different types of rocks, usually as microscopic crystals it is resistant to weathering, and so can form significant concentrations in sand deposits its high density makes it easy to separate from the sand Zircon almost always contains radioactive isotopes (e.g. hafnium, thorium, uranium. The decay of U to Pb is the basis for radiometric dating of rocks, and so zircon is a very important mineral for geochronology.

The structure of zircon Blue tetrahedra contain Si Green polyhedra contain Zr in 8-fold coordination Zircon is tetragonal. This is a view down the c axis

The structure of zircon Blue tetrahedra contain Si Green polyhedra contain Zr in 8-fold coordination Zircon is tetragonal. This is a view down the c axis with the top layer of Zr polyhedra removed

The structure of zircon Blue tetrahedra contain Si Green polyhedra contain Zr in 8-fold coordination Zircon is tetragonal. This is a view down the a axis

The structure of zircon Blue tetrahedra contain Si Green polyhedra contain Zr in 8-fold coordination Zircon is tetragonal. This is a view down the a axis