Olivines, Pyroxenes and Amphiboles

geo-Silicate-tetrahedron-3D-ballsgeo-Pyroxen-chainThe most simple type of silicates are the “nesosilicates”, those with isolated tetrahedrons SiO44- (left). Each tetrahedron can bond iron or magnesium, both bivalent, forming the minerals known as Fayalite Fe2SiO4 and Forsterite Mg2SiO4, respectively. As the Fe and Mg percentage in the crystal lattices is variable, the minerals are referred to as Olivine “solid solution”, also know as Peridot. The mineralogical formula is (Fe,Mg)2SiO4, with variable content of the two minerals.

By adding one silica molecule to Olivines we obtain the silicates known as Pyroxenes, where tetrahedrons are spatially organized in chains, so that they’re called “inosilicates” (right). Bonded cations may be various: (Fe,Mg,Ca,Na,Al)Si2O6. For example, by adding silica SiO2 to the magnesian Olivine “Forsterite” we have geo-Tremolite-chainMg2SiO4 + SiO2 = Mg2Si2O6, which is a Pyroxen known as Enstatite. Doing the same thing to the ferrous Olivine we would obtain Ferrosilite. In nature, we can find some calcium atoms in place of one iron or magnesium atoms. This would lead to Hedembergite or the more common Diopside. CaMgSi2O6.

Going on like that we could ideally obtain the double-chain inosilicates called “Amphiboles” (left), whose more complex formula contains the hydroxylic ion Si8O22(OH)27- obtained by adding silica and water to a Pyroxene in high temperature and pressure environments typical of metamorphism.


Nesosilicates |————— Inosilicates —————|
OLIVINES +SiO2 -> PIROXENES +SiO2+H2O -> AMPHIBOLES
(rombic) (rombic) Ferromagnesian
geomin-forsterite geomin-enstatite geomin-Antofillite
Mg2SiO4 +SiO2 -> Mg2Si2O6 +SiO2+H2O -> Mg7Si8O22(OH)2
Forsterite Enstatite Antophyllite
     
geomin-Fayalite geomin-Ferrosilite geomin-grunerite
Fe2SiO4 +SiO2 -> Fe2Si2O6 +SiO2+H2O -> Fe7Si8O22(OH)2
Fayalite Ferrosilite Grunerite

(monocline) (monocline) Calcic
geomin-diopside geomin-tremolite
CaMgSi2O6 +SiO2+H2O -> Ca2Mg5Si8O22(OH)2
Diopside Tremolite
   
geomin-hedembergite geomin-actinoto
CaFeSi2O6 +SiO2+H2O -> Ca2Fe5Si8O22(OH)2
Hedembergite Actinote
 
Sodic
Na,(Al,Fe)Si2O6 +SiO2+H2O -> Na2(Mg3Al2)Si8O22(OH)2
Jadeite Glaucophane
   
Na,FeSi2O6 Na2Fe2+3Fe3+2Si8O22(OH)2
Aegirine Riebeckite 
   
  Lithium-aluminic
LiAlSi2O6  
Spodumene  

(complex) (complex)
geomin-onfacite geomin-orneblenda
(Ca,Na)(Mg,Fe2+,Al)Si2O6
+SiO2+H2O -> NaCa2(Mg,Fe)5Si8O22(OH)2
Omphacite (diopside+jadeite) Horneblenda
(simplified)
   
 
(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6  
Augite