Silicate structure and its classification

SILICATES

1. SILICATE STRUCTURE AND CLASSIFICATION of silicates GOVT HOLKAR SCIENCE COLLEGE GUIDED BY PRESENTED BY Dr. NARENDRA JOSHI SHREYA BOBDE Ms. TANISHKA SONI CLASS-M.Sc. I-SEMESTER

2. CONTENTS INTRODUCTION THE SIO 4 TETRAHEDRON CLASSIFICATION OF SILICATES NESOSILICATE SOROSILICATE INOSILICATES

3. a. SINGLE CHAIN b. DOUBLE CHAIN 7. PHYLLOSILICATE TECTOSILICATE GENERAL FORMULA OF SILICATES 10. REFERENCES

4. INTRODUCTION Silicate makes up 90% of the earth’s crust. Of every 100 atoms in the crust of the earth, more than 46 are oxygen, over 27 are silicon and 7 to 8 are aluminum. The rock forming silicate mineral include olivine, garnet, pyroxenes ,amphibole, mica, feldspar and quartz.

5. THE SIO 4 TETRAHEDRON The structure of silicates consist of four O 2- at the apices of regular tetrahedron surrounding and coordinated by one Si 4+ at the center. The bond between silicon and oxygen ions is estimated by use of Pauling’s electron negativity concept as 50% ionic and 50% covalent bond. Although electron sharing is present in the Si-O bond, the total bonding energy of Si 4+ is still distributed equally among its four closest oxygen neighbours .

6. Strength of any single Si-O bond is equal to just one half the total bonding energy available in the oxygen ion . Each O 2- has therefore the potentiality of bonding to another silicon ion and entering into another tetrahedral grouping, uniting tetrahedral groups through shared oxygen. Such linking of tetrahedra is referred as polymerization. The capacity of polymerization is the origin of great variety of silicate structures.

7. SiO 4

8. Geometry Of SiO 4 Tetrahedron The shape of silicate structure is defined by Si-O bond length and the O-Si-O bond angle. The mean Si-O bond length is 1.62 Å. When [SiO 4 ] tetrahedron are linked in a structure, bond length between the silicon atom and the bridging oxygen atom are on average longer by about 0.025 Å , compared with the Si-O bond length of non-bridging oxygen.

9. When tetrahedra are corner linked, tht O br -Si- O br bond angle defines the orientation of tetrahedra relative to one another. This bond angle can vary between 120 o to 180 o . Depending upon temperature , pressure and local structural environment.

10. CLASSIFICATION OF SILICATE STRUCTURE Class Arrangement of SiO4 tetrahedra Unit composition Mineral example Nesosilicate (SiO 4 ) 4- Olivine , garnet , zircon Sorosilicate (Si 2 O 7 ) 6- Epidote Cyclosilicate (Si 6 O 18 ) 12- Tourmaline , beryl

11. Class Arrangement of SiO4 tetrahedra Unit composition Mineral example Inosilicate (Single chain silicate) (Si 2 O 6 ) 4- Pyroxene , wollastonite Inosilicate (Double chain silicate) (Si 4 O 11 ) 6- Amphibole Phyllosilicate (Si 2 O 5 ) 2- Mica , clay minerals , serpentine , chlorite Tectosilicate (SiO 2 ) Feldspar, quartz

12. NESOSILICATE Nesosilicates are also known as Orthosilicates or Island silicates. These are independent or isolated [SiO 4 ] tetrahedra and bound to each other only by ionic bonds from interstitial cations . Their structure depends chiefly on the size and charge of interstitial cations . Silicon to oxygen ratio in the chemical formula is 1:4.

13. Atomic packing is generally dense, causing the mineral of this group to have high specific gravity and hardness. The crystal habit of nesosilicates is generally equidimensional and cleavage directions are absent. The amount of Al substituion in nesosilicates is very low. E.g. olivine, garnet, zircon , kyanite .

14. Olivine ( Mg,Fe ) 2 SiO 4 Cation positions- 1. The octahederally sites coordinated are known as M1 and M2. 2. M1 is distorted and M2 is somewhat more regular .

15. SOROSILICATE Sorosilicates are also known as pyrosilicates , double island silicates or disilicates . These are characterized by isolated, double tetrahedral, groups formed by two SiO 4 tetrahedra sharing a single optical oxygen. The resulting ratio of Si:O is 2:7. Chemical formula is [Si 2 O 7 ] -6 . More than 70 minerals are known to this group but most of them are rare. E.g. – Epidote group.

16. Epidote A 2 B 3 (SiO 4 )(Si 2 O 7 )O(OH) Contain both SiO 4 as well as Si 2 O 7 group. Chains of AlO 6 and AlO 4 (OH) 2 runs parallel to b axis. A atoms are 8 fold coordination between main chains. B is generally Al, generally substitute by ferric ion.

17. CYCLOSILICATE Cyclosilicates are also known as ring structures or metasilicates . The cyclosilicate contain rings of linked SiO 4 tetrahedra . The resulting ratio of Si:O ids 1:3. The simplest is the Si 3 O 9 ring. Cyclic configuration of this kind may exist as-

18. Each of three tetrahedra shares an oxygen atom. E.g. benitotite BaTiSi 3 O 9 Each of four tetrahedra shares an oxygen atom. E.g. axinite ( Ca,Fe,Mn ) 3 Al 2 BO 3 Si 4 O 12 OH Each of six tetrahedra shares an oxygen atom. E.g. beryl Be 3 Al 2 Si 6 O 18

19. INOSILICATE These are also known as chain silicates. Here SiO 4 tetrahedra are joined together to form chains of indefinite extent. These are of two types:- (a) Single chain structure - Si:O ratio is 1:3. Characterised by pyroxenes. (b) Double chain structure - Si:O ratio is 4:11. Characterised by amphiboles.

20. SINGLE CHAIN structures The SiO 4 tetrahedra form linear single chains with two bridging oxygen atoms per tetrahedron. In pyroxenes the periodicity is 2, i.e. the chain repeats after every two tetrahedra . In pyroxenoids , longer chain periodicity occurs such as in wollastonite (CaSiO3), where the periodicity is 3. The Si:O ratio is 1:3 with general formula (SiO 3 ) n 2n- . The chain is cross linked by cations , generally in octahedral sites.

21. Elongated in ‘c’-crystallographic direction. Pyroxenes are anhydrous minerals.

22. Pyroxene (XYSi 2 O 6 ) Cation positions- M1 sites lies between apices of opposing tetrahedra . Sites are smaller. Almost regular tetrahedra . M2 sites are larger. More distorted and may be octahedra when containing smaller cations or 8 fold sites when occupied by larger cations .

23. DOUBLE CHAIN structure These are also known as the ‘Band Structures’. The alternate tetrahedra are arranged in two parallel ways. These chains are indefinite in extension . The Si:O ratio is 4:11 with general formula (Si 4 O 11 ) n 6n- . Elongated usually in ‘c’-crystallographic direction. Amphiboles are hydrous minerals. Chains are not straight.

24. Half of the tetrahedra have two non-bridging and two bridging oxygens . The other half have one non-bridging and three bridging oxygen. Amphibole formula - W 0-1 X 2 Y 5 Z 8 O 22 (OH, F) 2

25. Cation positions- Sites between tetrahedral bases of adjacent chains are termed as M4.Coordination of M4 is 8 when occupied by larger cations but reduces to 6 when occupied by smaller cations . The smaller sites between the opposed tetrahedral apices are M1, M2 , M3 sites and are octahedral. A site has 10- to 12- coordination with oxygen and (OH ).

26. PHYLlOSILICATE Phyllosilicates are also known as sheet structures. A sheet structure is formed when the SiO 4 tetrahedra are linked by three of their corners. Extend indifinitely in a two dimensional network or “sheet”. The ratio of Si:O is 4:10 with general formula (Si 2 O 5 ) n 2n- . Most of its member have flaky or platy habit. They are generally soft , have low specific gravity and show even elasticity of the cleavage lamellae.

27. All these chracteristics are arises from the dominance in the structure of the infinitely extended sheet of SiO 4 tetrahedra . Most of the minerals of this class are hydroxyl bearing. E.g. mica group. (OH) group located in the center. External ions coordinated to 2 oxygens and 1 (OH). E.g. lizardite .

28. The cations in the octahedral sheet may be divalent or trivalent . On the basis of chemistry and geometry of the octahedral sheets, the phyllosilicates are divided into two major groups:- trioctahedral and dioctahedral . A sheet in which each oxygen or (OH) group is surrounded by three cations , is known as trioctahedral structure. A sheet in which each oxygen or (OH) group is surrounded by two cations , is known as dioctahedral .

29. TECTOSILICATE It is also known as framework structure. When each of the four oxygen atoms of each tetrahedron is shared by another tetrahedron, it results in the formation of tectosilicates . Here every SiO 4 tetrahedron shares all its corners with other tetrahedra giving a three dimensional network. The ratio of Si:O is 1:2. Here the bond is stable and strong. The framework is electrically neutral.

30. There are at least nine different ways in which such a frame work can be built . These modes of geometrical arrangement corresponds to nine known polymorphs of SiO 2 , one of which is synthetic. These are:- Stishovite Coesite Low ( α ) quartz High ( β ) quartz Keatite (synthetic) Low ( α ) tridymite High ( β ) tridymite

31. 8 . Low ( α ) cristobalite High ( β ) cristobalite E.g. feldspar group.

32. feldspar It consisit of continous negatively charged ion. Three dimensional framework made up of corner sharing SiO 4 and AlO 4 tetrahedron and positively charged cations . Cations occupy relatively large interstices within the framework.

33. General formula of silicates General structural formula for silicates as follows X m Y n ( Z p O q ) W r X represents 8 to 12 fold coordination site for large catios like K + , Rb + , Ba +2 , Na + and Ca +2 . Y represents medium sized, two or four valent ions in 6 coordination. Z represents small, highly charged ions in tetrahedral coordination.

34. O is oxygen. W represents additional anionic group such as (OH) - or anions such as Cl - or F - . The ratio p:q depends on the degree of polymerization of the silica (or alumina) tetrahedron or the silicate structural type. m, n and r depends on the need of electrical neutrality.

35. references Andrew Putnis , Introduction to Mineral Science, Cambridge University Press, Page 141 – 182. Dexter Perkins, Mineralogy, Pearson New International Edition, Third Edition , Page 133 – 166. H.H. Read , Rutley’s Element Of Mineralogy, Surjeet Publication, Twenty-Fifth Edition, Page 139-145. William D. Nesse , Introduction to Mineralogy, Oxford University Press, Page 183- 325 Websites-   https://www.britannica.com – Accessed on 27 september 2019

36. https://www.wou.edu/las/physci/ch412/silicate.htm Accessed on 27 september 2019 https://opentextbc.ca/geology/chapter/2-4-silicate-minerals/ - Accessed on 30 september 2019 http://butane.chem.uiuc.edu/pshapley/Environmental/L27/1.html - Accessed on 1 october 2019

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