Tektosilicates- Feldspar Group Min XIVa

Transcription

1 Subject Paper No and Title Module No and Title Module Tag Geology Crystallography and Mineralogy Tektosilicates- Feldspar Group Min XIVa Principal Investigator Co-Principal Investigator Co-Principal Investigator Prof. Talat Ahmad Vice-Chancellor Jamia Millia Islamia Delhi Prof. Devesh K Sinha Department of Geology University of Delhi Delhi Paper Coordinator Content Writer Reviewer Prof. P. P. Chakraborty Department of Geology University of Delhi Delhi Prof. Naresh C. Pant Department of Geology University of Delhi Delhi Prof. Naresh C. Pant Department of Geology University of Delhi Delhi Prof. Santosh Kumar Department of Geology Kumaun University Nainital

2 Table of Content 1. Learning outcomes 2. The classification of silicates 3. Structure and chemistry 1. Learning outcomes After studying this module, you shall be able to: To develop a basic understanding of chemical co-ordination leading to formation of minerals with various structures. To understand the variety of feldspars. To develop appreciation of conditions of formation for alkali- and plagioclase feldspars. 2. The classification of alumino-silicates based on SiO4 tetrahedron Silicon is known to be the second most abundant element after oxygen forming the Earth s crust and mantle. Silicate minerals predominate vast majority of rocks of the Earth s crust as the Si-O bond is considered to be stronger than any other bond prevalent amongst oxygen and any other element. As the [SiO4] tetrahedral dominate silicate structures, the way it arranges itself within a particular structure has long been the way of silicate classification. [SiO4] tetrahedral are either isolated from each other or connected by corner sharing to other [SiO4] tetrahedra. The different frameworks provided by variable corner sharing tetrahedra with other cations fitting into suitable interstices provided by this frame work defines the various types of silicate structures This framework of cations and [SiO4] tetrahedra is of prime importance in understanding the way in which a mineral would adapt to changes in its physical and chemical environment. Many silicate minerals also show substitution of Al for Si in the tetrahedron which is accompanied by compensating replacement in cation content to maintain charge neutrality.

3 Based on the number of the [SiO4] 4- units connected to a single oxygen, silicates has been classified into 6 classes. All these classes represent a unique arrangement of [SiO4] 4- units in the packing of mineral. The Si-O bond lengths and the O-Si-O bond angles determine the shape of the tetrahedron. The X-ray diffraction and neutron diffraction has revealed following things about the tetrahedron: 1. The mean Si-O bond length is 1.62 A. The presence of other cations in the vicinity of the Si-O bond tends to attract the oxygen hence leading to the extension in Si-O bond length. Due to this property, the bond length varies between 1.60 A to 1.34 A. 2. When the tetrahedra is in a structural bond, the bridging oxygen is longer than by about A compared with the non-bridging oxygen. 3. The bond angle in Si-O-Si can vary between about 120 to 180º, depending on the local structural environment as well as the temperature and pressure. The bond angle of a strain free Si-O-Si bond is near 140º. 3. Structure and chemistry 3.1 Framework Silicates All tetrahedra share corners with other tetrahedra, having four bridging oxygens per tetrahedron. The Si:O ratio is 1:2, as in quartz SiO2. If Si is not substituted with any other cation, then all the valence bonds are satisfied and the entire framework has the composition of SiO2 (for example, quartz). However when Al substitutes for Si in the tetrahedra, charge balance is required in the tetrahedron. This results in varied bond lengths, which creates rather large interstitial cation sites, compared to those in other chain silicate minerals. At high temperatures, framework silicates have more expanded structures with the maximum symmetry allowed by the tetrahedral linkage pattern. At lower temperatures, they tend to crumple slightly, reducing the size of the interstitial cavities where any cations would be sited. The crumpling of the structures is achieved by a rotation of the SiO4 tetrahedra.

4 3.1.2 The feldspars The feldspars have a general; formula MT4O8 with between 25% and 50% of the Si replaced by Al in the T sites, and the M sites occupied by ions such as Na +, K +, Rb +, Ca 2+, Sr 2+ or Ba 2+. The composition of most natural feldspars lies in the KAlSi3O8 NaAlSi3O8 CaAl2Si2O8 triangle, in which the shaded region represents the extent of high temperature solid solution. Sanidine, KAlSi3O8, shows an ideal high temperature feldspar structure. The Al and Si are distributed at random so that the average occupancy of each tetrahedron is 25% Al and 50% Si. The basic construction of the framework is of ring of four tetrahedra with alternate pairs of vertices pointing in opposite directions. In the real structure of high sanidine, there is some rotation of the tetrahedra. The structure is monoclinic; space group is C2/m and has the highest symmetry possible in the feldspars. Fig. 1 The extent of solid solution in alkali and plagioclase feldspars at high temperature.

5 Fig. 2 The basic building unit of the feldspar structure is the four-membered ring of tetrahedra with a pair of tetrahedral pointing up and a pair pointing down, (b) The four-fold rings are joined to form a layer in which the rings are related by mirror planes parallel to (010) and diads parallel to the b axis. Two sets of individual tetrahedra are distinguishable in this layer, and are labelled T1 and T2. The T1 tetrahedra are all related to one another by symmetry, as are the T2 tetrahedra. Cations occupy the large oval-shaped cavities between the rings. Structural distortion and Al, Si ordering in the feldspar structure: 1. As the temperature falls, the expanded high temperature framework tends to collapse around the interstitial cation. Larger cations such as K or Ba, resists the collapse, keeping the framework extended. However smaller cations, Na and Ca, are not able to resist the collapse, hence allowing the framework to distort, reducing the symmetry to triclinic. 2. At lower temperatures, the Al, Si tetrahedral tend to order the structure. However, it is an extremely slow process in comparison to the structural distortion, involving breaking very strong Al-O and Si-O bonds.

6 3. The structural distortion is strongly dependent on both Al and Si. For example, the C1 site contains four different T sites, but when it expands to C2/m structure, there are only two distinguishable T sites. The conversion from one structure is result of complete loss of one structure before the formation of another structure. (a) (b) Fig. 3 (a) In sanidine, KAlSi3O8, the oxygen coordination around K + is symmetrical and the unit cell is monoclinic (α=90º). (b) In high albite, NaAlSi3O8, the oxygen coordination around Na + is distorted and the unit cell is triclinic (α=93.4º). The position of Na + ion is smeared over a number of positions and hence represented by an ellipsoid. The number on each oxygen atom is the cation oxygen distance in Angstrom unit (after Liebau, 1985). The alkali feldspar: Albite, NaAlSi3O8 is monoclinic C2/m above 980ºC (monalbite) but collapses around the small Na atom to the triclinic C1 structure below this temperature. At this stage there is little Al, Si ordering, hence it is called high albite, but below 700ºC Al, Si ordering begins and can proceed without any further symmetry change until at low temperature low albite is formed. The Sanidine: High Albite Series Complete solid solution series: KAlSi3O8 NaAlSi3O8 can be synthesized in the lab, but only KAlSi3O8 exists in nature. The K-end member is called high sanidine and the Na end member is called high albite.

7 Crystallography: High sanidine is monoclinic, but high albite is triclinic. High Sanidine Sanidine Anorthoclase High Albite α β γ In the plane polarized light, it have euhedral square shaped or elongated parallel to x-axis. It is colorless and non-pleochroic. Perfect basal and pinacoidal cleavage is present. Under cross polars, it shows very weak birefringence and is anisotropic. It exhibits Carlsbad twinning. The occurrence of sanidine as phenocrysts in lavas of silicic composition distinguishes it clearly. It occurs only in high K-lavas, or as a product of contact metamorphism in xenoliths within basalts. The Orthoclase-Low Albite Series The complete miscible solution exists in this series. At low temperatures, it breaks down to the nearly pure end-members. The initial intermediate members, upon cooling become mixed and show lamellar structure called perthite. Crystallography: The orthoclase are monoclinic with domains of triclinic. Orthoclase Low Albite α β γ In thin section, anhedral grains are commonly seen. Relief is poor to moderate but negative against araldite or quartz. It is colorless and non-pleochroic. It shows basal and pinacoidal cleavage. Under cross polars, its anisotropic character is seen and the birefringence is very weak. It shows first order first order polarization colors, maximum upto first order grey. All members of this series commonly exhibit Carlsbad twinning. These are mainly found in felsic igneous rock associated with quartz, plagioclase, or nepheline.

8 The Microcline- Low Albite Series This series is represented by perthite in which hosts are K-rich end members and lamellae are Na rich. Crystallography: It is triclinic. Monoclinic Low Albite α β γ Under plane polarized light, anhedral grains are common. Perfect basal or pinacoidal cleavage is seen. It is colorless and non-pleochroic. It shows very weak birefringence and shows polarization colors upto first order grey. It is twinned on both albite and pericline laws. In felsic, igneous rock members of this series are associated with quartz, plagioclase, or nepheline. The plagioclase feldspar In pure anorthite CaAl2Si2O8, the small Ca atom is unable to support the expanded monoclinic C2/m structure at any temperature below the melting point. Similarly, the tendency to order Si and Al in the tetrahedra is greater than in the alkali feldspar, since the Si:Al ratio of 1:1 means that in a framework structure any amount of disorder results in the formation of Al-O-Al linkages. In pure anorthite, this disordering temperature is estimated at above 2000 C, well above the melting point. Ordered anorthite is triclinic with space group II. Partial disordering is allowed in anorthite while retaining the II symmetry, and up to 20% disorder may be induced by annealing anorthite just below its melting point. There is complete solid solution between albite (An0) and anorthite (An100) above about 700 C, the result of the substitution Na ++ Si 4+ = Ca 2+ Al 3+.

9 Fig. 4 Summary of the stability regions of the various feldspar structures in the plagioclases. At high temperature, the phase boundaries between C2/m, C1 and I1 are truncated by the melting curve. At low temperatures, the shaded areas represent regions in which the plagioclase consists of intergrowths barely visible by optical microscopy (after Carpenter, 1987). Crystallography: Triclinic Low Albite Anorthite α β γ In thin sections, under plane polarized lights, it is colorless and non-pleochroic. While under cross polars, it shows very weak birefringence and upto first order grey polarization colors. It follows albitic, pericline and Carlsbad twin laws.

10 The Cordierite: The structure of cordierite is based on the six-fold rings of Al, Si tetrahedral (termed the T2 tetrahedra) joined laterally and vertically by T1 tetrahedra, which may also contain Al or Si. The Mg or Fe cations occupy octahedral sites between the rings. Layers are vertically stacked above one another so that the rings form infinitely long channels parallel to the c axis. In each unit cell, there are 9 tetrahedra: 3T1 and 6T2 tetrahedra over which the 4Al and 5Si atoms tends to be distributed. If Al and Si are randomly distributed in all the 9 sites, the resultant structure is hexagonal and each site has occupancy 4/9 Al and 5/9 Si. Natural hexagonal cordierite is called indialite. Fig. 5 One layer of the structure of cordierite showing the showing the six-fold rings of tetrahedra, labelled T2 connected via the tetrahedra labelled T1. The Mg 2+ cations lie in the octahedral sites (reproduced after Putnis). Frequently Asked Questions- Q1. Describe the order-disorder in feldspar? Q2. Discuss the solid solution in feldspars and reasons of its temperature dependence? Q3. How the structure of feldspars and its crystallographic axes are related?

11 Multiple Choice Questions- 1. Sanidine is Ans: a (a) Alkali-feldspar (b) Plagioclase (c) Feldspathoid (d) None of the above 2. The bond angle of a strain-free Si-O-Si in tectosilicates is Ans: c (a) ~100 (b) ~120 (c) ~140 (d) ~ The number of tetrahedral sites in feldspar are Ans: d (a) 1 (b) 2 (c) 3 (d) 4 4. At high temperatures alkali feldspar are Ans: a (a) Monoclinic (b) Triclinic (c) Hexagonal (d) Trigonal 5. This form of feldspar shows highest disorder in tetrahedral site Ans: c (a) Low-albite (b) Microcline (c) Sanidine (d) High-albite

12 Suggested Readings: 1. Klien, Cornelis and Hurlbut, Cornelius S., (1985). Manual of Mineralogy (after James D. Dana), 20 th Edn. John Wiley & Sons, New York. ISBN: , Putnis Andrew (1992), An Introduction to Mineral Sciences, 1 st Edn., Cambridge University Press, UK. ISBN: ,