Garnets and aluminum silicates

Another typical metamorphic mineral family are the Garnets. They are Nesosilicates, that is they are structured by isolated groups of SiO44- tetrahedrons; in this case the groups are made of 3 tetrahedrons: (SiO4)35-. Aluminum is often found bonded to those groups to form the typical Garnet radical: Al2(SiO4)33-, but ferric iron (III) is also common in Andradite, as chrome is in Uvarovite. This radical may also bond magnesium, iron and calcium.

Mg Fe Mn Fe3+ Al Cr
geomin-piropo geomin-almandino geomin-spessartite geomin-andradite geomin-grossularia geomin-uvarovite
Mg3Al2(SiO4)3 Fe3Al2(SiO4)3 Mn3Al2(SiO4)3 Ca3Fe2(SiO4)3 Ca3Al2(SiO4)3 Ca3Cr2(SiO4)3
Pirope Almandine Spessartite Andradite Grossular Uvarovite
<——— Calcic Garnets ———>

Another group of nesosilicates is an important indicator of a metamorphic rock’s formation temperature: the aluminum silicate Al2SiO5. It exists in three polymorphs, never to be found together in the same rock, since they are stable at different pressure and temperature conditions: Andalusite forms at low pressure (LP) but variable temperatures; Cyanite only forms at high pressure (HP > 4 Kbar); Syllimanite forms at high temperature (HT) but variable pressure.

geomin-cianite geomin-andalusite geomin-sillimanite
Cyanite Andalusite Syllimanite

geomin-Silicate-double-tetrahedra-3D-ballsIn some cases, the tetrahedron groups are formed by Si2O76- pairs (left) and are called Sorosilicates. In the Epidote group the tetrahedrons are bonded to calcium, ferric iron and aluminum, and also to a SiO4 group. Most common Epidotes are the Zoisite (orthorombic) and the Clinozoisite (monocline):

Fe3+ Al
geomin-epidoto geomin-zoisite
Ca2(Al,Fe)3O(SiO4)(Si2O7)(OH) Ca2Al3O(SiO4)(Si2O7)(OH)
Epidote Zoisite