Principal Investigator Co-Principal Investigator Co-Principal Investigator Prof. Talat Ahmad Vice-Chancellor Jamia Millia Islamia Delhi

Transcription

1 Subject Paper No and Title Module No and Title Module Tag Geology Crystallography and Mineralogy Nesosilicates (Olivine and Garnet Group Min IX 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. Somnath Dasgupta Department of Geography Jamia Millia Islamia, Delhi Prof. Santosh Kumar Department of Geology Kumaun University Nainital

2 Table of Content 1. Introduction 2. Olivine group 3. Garnet group

3 1. Introduction Silicate minerals constitute about 92% minerals in the Earth s crust and mantle. All silicate minerals have [SiO4] tetrahedra as building blocks in their structure (Fig. 1). Since the tetrahedra are not charge neutral, having negative charge of 4 (Si=+4, each oxygen = -2, in total overall charge= -4), these combine with various positively charged elements to make neutral species. Fig. 1 The SiO4 tetrahedron as the basic unit of silicate structure. Blue: Silicon Buff: Oxygen Single SiO4 tetrahedra are linked together to form various structures and silicate minerals are structurally classified into various groups based on such linking. Basically, different structural patterns are obtained through sharing of oxygen atoms occupying the apices of the tetrahedra. The simplest case is where no sharing takes place and individual isolated SiO4 tetrahedra are bounded to one another via ionic bonds with interstitial cations (Nesosilicates or Orthosilicates). Sharing of two oxygen atoms can produce both single chain inosiliocates (example: amphibole group of minerals) and double chain inosilicates (example: pyroxene group of minerals), while sharing of three oxygen atoms produces sheet silicates or phyllosilicates (example mica group of minerals). Sharing of all the 4 oxygen atoms produces a three-dimensional network structure, known as Tectosilicates (example:

4 quartz and feldspar group of minerals). Note that the above scheme of classification of silicate minerals is not complete and there are other groups of structures, such as Cyclosilicates and Ring silicates. Fig. 2 shows the basic structural characteristics of linking of SiO4 tetrahedra producing different groups. Fig. 2 Different degrees of polymerization of SiO4 tetrahedra with their arrangements in silicate minerals. Isolated or single- tetrahedron is characteristic of Nesosilicates, single chains of pyroxene, double chains of amphiboles, sheet of micas and three-dimensional network (tectosilicates) of quartz and feldspar.

5 Accordingly, the Si: O ratio varies from 1:4 in Nesosilicates, to 1:3 in single chain inosilicates and 4:11 in double chain inosilicates, 2:5 in phyllosilicates to 1>2 in tectosilicates (note that in feldspar additional oxygen is present as part of AlO6 octahedra. Two important groups of minerals falling under Neso- or Orthosilicates are olivine and garnet. 2. Olivine Group The olivine group includes a number of minerals, which crystallize with orthorhombic symmetry. As mentioned above, the structures of all the minerals of the group consist of independent SiO4 tetrahedra linked by divalent atoms in six-fold coordination. As is common in other nesosilicate minerals, Si is not replaced by Al, and the octahedral positions in the structures are occupied almost exclusively by divalent ions. Trivalent ions Al and Fe 3+ are either absent, or present in very small amounts. The name olivine commonly refers to Mg-Fe members of the family, although there are other less common end members in the group. Mg-Fe olivine (Mg,Fe)2SiO4 (α olivine) Characteristic optical and crystallographic parameters (after Deer, Howie & Zussman, 1982) Forsterite Fayalite Mg2SiO4 Fe2SiO4 Orthorhombic (+) Orthorhombic (-) α β γ δ Vγ 82 o 134 o α=y, β=z, γ=x ; O.A.P (001) α=y, β=z, γ=x; O.A.P= (001) D H Colour Mostly green, colourless in thin section Greenish yellow, pale yellow in thin section Pleochroism: Absent α=γ pale yellow, β reddish brown Unit Cell a 4.754A o a 4.821A o b A o b A o c 5.98A o c 6.088A o Z=4 Space group Pbnm

6 Mg-Fe olivine may contain minor amounts of Ni, Co, Ca and Mn. The olivine structure is based on isolated SiO4 tetrahedra which link chains of (Fe,Mg)O6 octahedra (Fig. 3 & 4). There are two octahedral cation sites: M1 and M2 (Fig. 5). Both sites accommodate Fe2 + and Mg 2+ cations and normally there is complete disorder of Fe and Mg over the M1 and M2 sites. Recently, some evidence have been found favoring partial ordering in olivine have been noted. Olivine is orthorhombic and therefore will show parallel or symmetric extinction under crossed polarized light. M1-O distance~2.101 A, M2-O~2.135A. M1 radius=0.781a o, M2 radius=0.812a o. Fig. 3 The olivine crystal structure. SiO4 tetrahedra in blue and violet (Mg,Fe)O6 octahedra in green-yellow. Fig. 4 The olivine structure showing occupation of Mg, Fe in octahedral sites connecting SiO4 tetrahedra. Yellow: Fe, Mg; violet: Si; blue: O.

7 Fig. 5 Olivine structure (100) view. Compositional characteristics There is complete solid solution between forsterite and fayalite (Fig. 6) and natural mineral data attests to that. In many natural occurrences, and particularly in the more iron-rich members of the series there is a little replacement of (Mg,Fe)2 by Mn and Ca. At the magnesium-rich end of the series Cr and Ni, generally in small amounts, are usually present. Fo90 olivine is considered to be the major mineral in the mantle. Fig. 6 Phase diagram in the Fo-Fa series of olivines showing complete solid solution.

8 There is an inverse relationship between Mn content Fo# in natural olivines- Mn is higher in fayalitic olivines. Ni content is higher in forsterite-rich olivines from ultrabasic rocks. Cr content in olivine is the highest in komatiites and in olivine occurring as inclusion in diamond. These olivines are very rich in forsterite. Ca content in olivine is slightly higher in fayalite-rich varieties. In some ultrabasic rocks, platinum group elements are present in Mg-rich olivines. It may be noted that concentrations of the above elements in natural olivines do not exceed 1 wt.% and often found in trace contents. The reader is referred to Deer et al. (1982) for chemical analysis of olivine. Inter-relationships between composition and optical and physical properties Some of these relationships are summarized in Fig. 7 (after Deer, Howie and Zussman, 1982). Refractive indices, optic axial angle and density vary linearly with composition, increasing steadily from Fo to Fa. Estimation of composition of olivine was routinely done on the basis of optical properties prior to the advent of EPMA. Utility of such diagrams has been greatly reduced because it is currently possible to carry out microscale in-situ chemical analysis with the help of EPMA. Olivines are distinct by their high relief, high birefringence and larger optic axial angles. Compositional zoning (Mg-rich cores to Fe-rich rims) is common in olivine in many igneous rocks, which is also distinctly reflected in the optical properties, hence can be studied under microscope. In Mg-rich olivine (010) and (100) cleavages are indistinct, but the (100) cleavage is distinct in Fe-rich varieties. Twinning on (011) and (012) is not uncommon. One rare, but characteristic crystal habit of Mg-rich olivine is spinifex texture. The formation of elongated and dendritic olivines is well displayed in many ultramafic lavas, komatiites, in which the mineral occurs in highly elongate or skeletal crystals showing radiating, branching and parallel growths. This so called spinifex habit of the komatiitic lavas has been described by many workers. The spinifex habit of the olivine in these ultramafic lavas is generally ascribed to the rapid cooling rate of the liquid.

9 Fig. 7 Correlation of optical and physical properties with composition in the Fo-Fa series. Occurrence Olivine is a major constituent of common ultrabasic rocks, such as dunite and peridotite, where the composition is typically >Fo90. It is abundant in ultrabasic nodules occurring in alkali basalts and kimberlites. Olivine is dominant in dunite and peridotite occurring in ophiolite complexes. It is often a phenocrysts phase in olivine basalts, and present in stony meteorites and lunar and Martian basalts. Fayalite is present in ironstones and rhyolites. The most magnesian olivine (almost pure forsterite) occurs in metamorphosed calc-magnesian rocks, particularly in contact aureoles. Olivine alters easily to serpentine and brucite. The gem variety of olivine is known as peridot. High-pressure transformations of olivine α- olivine converts to two high-pressure polymorphs, known as wadsleyite (βolivine) and ringwoodite (γ- olivine) at high pressures. Such transformations are significant in the context of mineralogy of the mantle below the depth of

10 approximately 400 km. Wadsleyite is orthorhombic and with a formula of (Mg,Fe 2+ )2(SiO4), its cell parameters are as follows: a = 5.7Å, b = 11.7Å and c = 8.24Å, space group Pmmb. Wadsleyite has both a single (SiO4) and coupled tetrahedral (Si2O7). Because of oxygens not bound to silicon in the Si2O7 groups of wadsleyite, it leaves oxygens unoccupied, and as a result, these oxygens are hydrated easily. As a result, there are high concentrations of hydrogen atoms in the mineral. Hydrous wadsleyite is a considered a potential site for water storage in the Earth s mantle due to its low electrostatic potential. The structure of wadsleyite is shown in Fig. 8. Natural wadsleyite has been described from Peace River meteorite in Canada and the mineral has been synthesized in the laboratory. Fig. 8 Structure of Wadsleyite (β- olivine). With increasing pressure, wadsleyite transforms to a cubic phase called ringwoodite, also known as γ-olivine or silicate spinel. Ringwoodite has the crystallographic

11 parameter of a=8.113a o with space group Fd3m. Structure of ringwoodite is shown in Fig. 9. Here, Si atoms occupy the tetrahedral sites and Fe and Mg atoms occupy the edge-sharing octahedral sites. Fig. 9 Structure of ringwoodite (γ-olivine). Ringwoodite is also described from meteorites and also from a rare ultra-deep diamond from Brazil, where it occurs as inclusion. Ringwoodite is also capable of hosting (OH) and therefore can contribute towards water content in the mantle. The conversion of α-olivine to β-olivine and to γ-olivine with increasing pressure and temperature is shown in the phase diagram (Fig. 10). High pressure experiments in the system Mg2SiO4-Fe2SiO4 show that the β-olivine is restricted to Mg-rich compositions, while the γ-olivine is stable in Fe-rich compositions as well (Fig. 11).

12 Fig. 10 P-T phase diagram-showing transformations in olivine (after Suito, 1977). Fig. 11 Isothermal P-X phase diagram in the Fo-Fa series showing phase transformations in olivine. Note that wadsleyite is restricted to Mg-rich compositions (after Akimoto et al., 1976).

13 Seismic discontinuities in the Earth s mantle are believed to be caused by different types of phase transitions in the olivine structure. α-olivine is stable up to 410 Km depth, where it is converted to β-olivine, which is subsequently converted to γ- olivine at ~ 525 Km depth (Fig. 13). γ-olivine subsequently breaks down to silicate perovskite and periclase at 660 km seismic discontinuity (Fig. 12). Fig. 12 Olivine phase transitions in the Earth s mantle. Mn-rich olivines Tephroite: Mn2SiO4 Knebelite: (Fe,Mn)2SiO4 There is complete solid solution between fayalite and tephroite, knebelite being the intermediate member (Fig. 13). Minerals of the tephroite-knebelite series crystallize in the orthorhombic system and are optically negative. Entry of Mn 2+ in fayalite enlarges the cell parameters (for tephroite a=4.9a o, b=10.6a o, c=6.25a o ). Some optical parameters in this series are: α= β= γ= δ= Vα=70-44 o

14 Colour: Tephroite is olive green to bluish green, pale green in thin section, strongly pleochroic; knebelite is brown/black, pale yellow in thin section, feeble pleochroism. Olivines in this series contain very little Mg, but may contain appreciable Zn (maximum ZnO=10.68 wt.%). Tephroite and knebelite occur in metamorphosed Mn-rich silicate rocks, particularly in those derived from Mn carbonate protoliths. These minerals also occur in skarns associated with these metamorphic rocks. Relatively rare Mn- bearing olivine is glaucochroite (Ca,Mn)2SiO4, while monticellite has the composition Ca(Mg,Fe)SiO4. Fig. 13 Nature of solid solution in the system Fe2SiO4-Mn2SiO4 (after Maresch et al., 1978).

15 3. Garnet Group The principal end-members of the garnet group of minerals along with their optical, physical and structural characteristics are summarized below (taken from Deer et al., 1982). Therefore, the garnet group has the general composition A3B2Si3O12, where A= Mg, Fe 2+,Mn 2+, Ca and B=Fe 3+, Al, Cr. The garnet group is often subdivided into pyralspite (pyrope, almandine and spessartine) and ugrandite (uvarovite, grossular and andradite) subgroups. There are complete solid solutions within both the subgroups, but limited solid solutions between end members of the two subgroups. Fig. 14 shows a typical garnet unit cell with the dispositions of SiO4 tetrahedra, octahedra of trivalent cations and 8-fold coordinated divalent cations. The structure consists of alternating SiO4 tetrahedra and AO6 octahedra, which share corners to form a three-dimensional network (Fig. 14). Within these there are cavities containing triangular dodecahedra that contain the B cations. Each oxygen is coordinated by one Si, one A and two B cations. Two edges of each tetrahedron and six of each octahedron are shared with dodecahedra and four dodecahedral edges are shared with other dodecahedra.

16 Fig. 14 Garnet unit cell with the dispositions of SiO4 tetrahedra, octahedra of trivalent cations and 8-fold coordinated divalent cations. Chemistry Although the six species of anhydrous garnet (pyrope, almandine, spessartine, grossular, andradite and uvarovite) are the most abundant in natural silicate garnets, another molecule knorringite (Mg3Cr2Si3O12) has been reported in certain kimberlitic garnets. Ti may be present in some andradites, which is referred to as melanite. Rarer elements in garnet include vanadium and zirconium. Presence of structural H2O in rare garnet occurrences has been recognized and the corresponding species is hydrogrossular. The reader is referred to Deer et al., (1982) for chemical analysis of garnets. P-T stability fields of different species of garnet are shown in Fig. 14 (after Huckenholz et al., 1975), which shows that garnet should be common in both the crust and the mantle under favorable bulk chemical conditions.

17 Fig. 14 P-T plot of reaction curves for different garnet end members. I= Invariant point, S= singular point, Ih= hydrous melting, Ia= anhydrous melting point, Sc= congruent melting under hydrous condition. Optical and physical properties There are strong correlations between composition and optical properties in the garnet series. For example, composition of garnets in the space almandine-pyropespessartine can be determined from data on cell edge and refractive indices (Fig. 15) (after Sriramdas, 1957). Utility of such diagrams has been greatly reduced because it is currently possible to carry out microscale in-situ chemical analysis with the help of EPMA.

18 Fig. 15 Relationship between unit cell edge and refractive index parameters within the compositional space pyrope-almandine-spessartine (after Sriramdas, 1957). Moreover, garnet is known to preserve excellent compositional zoning formed during growth and cooling from high temperature (diffusion zoning). Therefore, optical properties of garnet will vary within a single grain. Composition of garnets, obtained from electron probe microanalysis, show remarkably good stoichiometry. Occurrence Garnet group of minerals are very common in metamorphic rocks of practically all bulk compositions (pelitic, basic, quartzofeldspathic, calcareous) and are present in a variety of igneous rocks. Garnet occurs quite commonly as detrital grains in clastic sediments. In natural occurrences garnet is mostly solid solutions of different end members, one notable exception being nearly pure pyrope in ultrahigh pressure metamorphosed pelitic rocks. Almandine-rich garnets (with variable solid solutions of pyrope, spessartine and grossular contents) occur in regionally metamorphosed pelitic rocks (defining garnet isograds in Barrovian zonal sequence) as index mineral. Garnet becomes

19 progressively Mg-rich with increasing grades of metamorphism and occurs abundantly in the granulite and eclogite facies rocks. Almandine-rich garnets also occur in some basic plutonic rocks and as rare phenocrysts in acidic volcanic rocks. Spessartine-rich garnets are widespread in metamorphosed Mn-rich sediments, such as in gondite, and occur rarely in granitic pegmatites. Mn-rich bulk compositions in manganese silicate rocks allow spessartine-rich garnets to form in greenschist facies also. Pyrope-rich garnets (with variable grossular and almandine contents) are common in garnet peridotite xenoliths in kimberlites, in some ultrabasic rocks and in ultrahigh pressure metamorphosed pelitic and basic rocks. Grossular-rich garnets are common in both regionally and thermally metamorphosed calcareous rocks, and, depending on the ambient oxygen fugacity, may contain variable amount of andradite. Andradite-rich garnets are common in thermally metamorphosed calcareous rocks and in skarns associated with such metamorphism. Ti-rich varieties are found in alkaline igneous rocks. Uvarovite-rich garnets (with grossular) commonly occur in serpentinites and in association with chromite deposits in basic-ultrabasic rock series. Hydrossular is reported from altered gabbroic rocks and in some skarn deposits.

20 Frequently Asked Questions- Q1. Enumerate the structural changes in olivine with depth under mantle conditions mentioning their characteristics? Q2. How is the structure of silicate minerals classified? Name the groups? Q3. Mention the essential characteristic features in the structure of garnet group of minerals? Q4. What are the types of zoning observed in garnet and how do these form? Multiple Choice Questions- 1. How many octahedral cation sites are in the olivine structure a) One b) Two c) Four 2. Spinifex texture is produced by a) Rapid cooling b) Slow cooling c) Faster growth rate of olivine 3. Knorringite-rich garnet is found in a) Regional metamorphic rocks b) Contact metamorphic rocks c) Lower mantle rocks 4. Ultrahigh pressure politic rocks contain a) Almandine-rich garnet b) Pyrope-rich garnet c) Hydrogrossular

21 Suggested Readings: 1. Akimoto, S. I., Matsui, Y., & Syono, Y. (1976). High-pressure crystal chemistry of orthosilicates and the formation of the mantle transition zone. The physics and chemistry of minerals and rocks, Bowen, N. L., & Schairer, J. F. (1932). The system, FeO-SiO2. American Journal of Science, 24(141), Deer, W. A., Howie, R. A., & Zussman, J. (1982). Rock-Forming Minerals: Orthosilicates, Vol. 1A. Geological Society of London. ISBN: , Huckenholz, H. G., Holzl, E., & Lindhuber, W. (1975). Grossularite, its solidus and liquidus relations in the CaO-Al2O3-SiO2-H2O system up to 10 kbar. N Jb Mineral Abh, 124, Maresch, W. V., Mirwald, P. W. & Abraham, K. (1978). Nachweis einer Mischuke in der Olivin-reihe Forsterit (Mg2SiO4) - Tephroit (Mn2SiO4). Fortsch Min, 56, Sriramdas, A. (1957). Diagrams for correlation of unit cell edges and refractive indices and the chemical compositions of garnet. Am. Mineral, 42, Suito, K. (1977). Phase relations of pure Mg2SiO4 upto 200 kilobars. In High-Pressure Research-Applications in Geophysics. Academic Press,