![When tetrahedra are corner-linked, the Si-O bond angle
defines the orientation of the tetrahedra relative to one
another. This bond angle can vary between 1200 and 1800
depending on the local structural environment as well as
temp. and pressure. The bond angle of a strain-free Si-O-
Si bond is near 1400.
When Al substitutes Si in a tetrahedron, the [AlO4]
tetrahedra is slightly larger than a [SiO4] tetrahedra coz.
Al-O bond (1.75Å) is larger than the Si-O bond. When
SiO4and AlO4 are linked in a structure, this size
difference is accommodated by a change in the T-O-T
bond angle (T= tetrahedral cation)](https://image.slidesharecdn.com/silicatestructure-130212140159-phpapp02/75/Silicate-structure-4-2048.jpg)
![Inosilicates: double chains- amphiboles
Hornblende:
(Ca, Na)2-3 (Mg, Fe, Al)5
[(Si,Al)8O22] (OH)2
M1-M3 are small sites
M4 is larger (Ca)
A-site is really big
Variety of sites →
great chemical range
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2
light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)
little turquoise ball = H](https://image.slidesharecdn.com/silicatestructure-130212140159-phpapp02/75/Silicate-structure-21-2048.jpg)
More Related Content
Silicate structure
- 1. STRUCTURE OF THESILICATES 1.Approx. 90% of the mineral content of the earth’s crust is of silicates where Si-O bonding, coupled with different cations and anions, formed different minerals. 2.The fundamental unit on which the structures of all silicates are based consists of four O2- apices of a regular tetrahedron surrounding and coordinated by one Si4+ at its centre.
- 2. STRUCTURE OF THESILICATES 3. The Si-O bond is partly ionic (generated due to attraction of opp. charged ions) and partly covalent. This tetrahedral groups can be linked with adjacent tetrahedral groupings through sharing of one or all four oxygen atoms (Polymerization). 4. The mean Si-O bond length is 1.62Å. Other cations near the oxygen atom of a Si-O bond also attract the oxygen and tend to lengthen the Si-0 bond( greater co-ord no; longer bond length). The strength of the Si-O bond limits the range of the bond lengths from 1.60Å to 1.64Å
- 3. STRUCTURE OF THE SILICATES The bond length between Si atoms and the bridging The bond length between Si atoms and the bridging oxygen atoms (OBR) are on average longer by about 0.025Å compared with the Si-O bond length to the O nb. [especially when SiO4 are linked in structures. The OBR-Si-OBR bond angle is also shows slightly smaller value when bridging oxygens are not involved. This suggests that that the Si atoms are displaced from the centres of the tetrahedra (awayfrom O BR) coz of repulsive force between two Si atoms. The aforesaid sharing of oxygens gave rise to diverse structural configurations for silicates with various Si-O ratios.
- 4. When tetrahedra arecorner-linked, the Si-O bond angle defines the orientation of the tetrahedra relative to one another. This bond angle can vary between 1200 and 1800 depending on the local structural environment as well as temp. and pressure. The bond angle of a strain-free Si-O- Si bond is near 1400. When Al substitutes Si in a tetrahedron, the [AlO4] tetrahedra is slightly larger than a [SiO4] tetrahedra coz. Al-O bond (1.75Å) is larger than the Si-O bond. When SiO4and AlO4 are linked in a structure, this size difference is accommodated by a change in the T-O-T bond angle (T= tetrahedral cation)
- 5. SiO44- complex ion •A group of ions that is so tightly bound together that they act like a single unit. • Building block of silicate minerals • 1 silicon ion + 4 oxygen ions arranged in a triangular pyramid • Electrical charge of -4
- 7.
- 8. Tetrahedron Viewing Top point base View from the top, Flat base of tetrahedron Side view looking down. facing you. Top point Top point of of tetrahedron pointing tetrahedron away from you. facing you
- 9. Nesosilicates (island silicates) SiO44-tetrahedron forms ionic bonds with cations such as Mg2+, Fe2+ Example: Olivine Mg2SiO4 - Fosterite Fe2SiO4 - Fayalite (Mg, Fe)SiO4 Solid solution Mg2+ or Fe2+
- 10. Olivine: nesosilicate structure(island silicate) View from the side (“wall” of structure) Why Mg OR Fe? • Same size • Same electrical charge Peridot
- 11. Corner Sharing Baseto Base side view O2- Si4+ 3-D Side View © S. Brachfeld 2003
- 12. Inosilicicates: Single Chains(Pyroxene) SiO3 Chain forms ionic bonds with cations above the tip and below the base Boxed region when view from How many Si How many O 2 Si 6O Yellow tetrahedron in front Gray behind and to the side Cation, forming ionic Stand here bond with tetrahedron look up the chain chain
- 13. Corner Sharing Tipto Tip O2- Si4+ tesy of Donna Whitney, University of Minnesota Dept. Geology Face sharing
- 14. Assembling a SingleChain Silicate (Pyroxene) Build xl in this direction © S. Brachfeld 2003
- 15. Pyroxene- Where’s thecleavage? breaking at the molecular scale What human eyes see at the macro scale 90˚ cleavage © S. Brachfeld 2003
- 16. Double Chain Silicates(Amphibole) PAIR of SiO4 chains that link by corner sharing in 2 directions
- 17. Amphibole formula islong: lots of space for small and medium cations Cations include Na+, K+, Ca2+, Mn2+, Fe2+, Mg2+, Fe3+, Al3+, Ti4+ Stand here look up the chain
- 18. 60˚-120˚ Cleavage inDouble Chain Silicates (Amphibole)- (actually 56˚ - 124˚) breaking at the molecular scale What human eyes see at the macro scale 120 ˚ © S. Brachfeld 2003
- 19. Nesosilicates: independent SiO4tetrahedra b c projection Olivine (100) view blue = M1 yellow = M2
- 20. Inosilicates: single chains-pyroxenes b a sinβ Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
- 21. Inosilicates: double chains-amphiboles Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 M1-M3 are small sites M4 is larger (Ca) A-site is really big Variety of sites → great chemical range Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na) little turquoise ball = H
- 22. Stishovite Tectosilicates Coesite Low Quartz β- quartz α- quartz Liquid Cristobalite Tridymite 001 Projection Crystal Class 32