The basic formula of olivine is (Mg, Fe)_2 SiO_4, which is a nesosilicate define

Question

The basic formula of olivine is (Mg, Fe)_2 SiO_4, which is a nesosilicate defined by a three dimensional structure with isolated (SiO_4)^4- tetrahedra. The tetrahedral are linked to each other via two near-identical 6-fold M-site coordination polyhedra that may be occupied by iron (Fe) and/or magnesium (Mg). a) In contrast to the pyroxene group minerals, olivine cannot accommodate Ca^2+ in its structure. What are the differences in atomic structure between Ca (Z = 20), Mg (Z = 12) and Fe (Z = 26)? b) Describe why olivine will only accommodate Fe and Mg, but cannot accommodate the larger Ca^2+ ion. c) Explaining your answer, would you describe the substitution of between Fe and Mg in the olivine as a simple or coupled substitution? d) Olivine is a mineral with an orthorhombic crystallographic system, where the crystallographic axes may be defined by the relationship a notequalto b notequalto c, and all axes intersect at 90 degree. Sketch the primitive Bravais lattice of the orthorhombic cell and name and describe the non-translational operations of symmetry present. e) Olivine is highly susceptible to weathering and will readily break-down to a material called iddingsite (MgO* Fe_2 O_3*, 4H_2 O) in a reaction that may be written as: Olivine + water rightarrow Quartz + Iddingsite (Mg_3 Fe)_2 SiO_4 + H_2 O rightarrow Si O_2 + (MgO* Fe_2 O_3* 4H_2O) Using information provided in the reaction, and your knowledge, what type of weathering processes are most important in the weathering of olivine? f). Olivine crystallizes at high temperatures in mafic to ultramafic magmas and in some circumstances may be separated from the magma. This causes a change in the composition of the magma, and the composition of rocks that crystallize from it. Name and describe a process that may lead to olivine's removal from a crystallizing magma? g) Using appropriate diagrams, predict how the composition of the magma will change in response to the removal of olivine from the magma? h) In which type of plate tectonic environment would your expect mafic to ultramafic magmas to be produced, and describe the process that causes partial melting of the upper mantle to produce such magmas?

Explanation / Answer

1 (a) Olivines are an important rock-forming mineral group. Magnesium-rich olivines are abundant in low-silica mafic and ultramafic igneous rocks and are believed to be the most abundant constituent of the Earth’s upper mantle. Olivine also occurs in high-temperature metamorphic rocks, lunar basalts, and some meteorites. The name olivine derives from the unusual yellow-green to deep bottle-green colour of the magnesium-iron olivine series. Typically the name olivine is given to members of the forsterite-fayalite solid-solution series. In addition to these magnesium and ferrous iron end-members, the olivine group contains manganese (tephroite), calcium-manganese (glaucochroite), calcium-magnesium (monticellite), and calcium-iron (kirschsteinite) end-members. Gem-quality forsterite olivine is known as peridot. Because of its high melting point and resistance to chemical reagents, magnesium olivine is an important refractory material which means it can be used in furnace linings and in kilns when other materials are subjected to heat and chemical processes. The composition of most olivines can be represented in the system Ca2 SiO4 – Mg2SiO4 -Fe2 SiO4. All olivines crystallize in the orthorhombic crystal system. Olivine is classified as a nesosilicate which has isolated SiO4 tetrahedrons bound to each other only by ionic bonds from interstitial cations. Within a silica tetrahedron, any single Si-O bond requires half of the available bonding electrons of the O2- ion, meaning that each O2- may bond with a second ion, including another Si4+ ion. The result of this is that the silica tetrahedra can polymerize, or form chain-like compounds, by sharing an oxygen atom with a neighboring silica tetrahedron. The olivine being a silicate is, in fact, subdivided based on the shape and bonding pattern of these polymers, because the shape influences the external crystal form, the hardness and cleavage of the mineral, the melting temperature, and the resistance to weathering. These different atomic structures produce recognizable and consistent physical properties. The simplest atomic structure involves individual silica anions and metal cations, usually iron (Fe) and magnesium (Mg), both of which exist most commonly as ions with charge of +2. Therefore, it takes two atoms of Fe2+ or Mg2+ (or one of each) to balance the -4 charge of the silica anion. Olivine is the most common silicate of this type, and it makes up most of the mantle. Because these minerals contain a relatively high proportion of iron and magnesium, they tend to be both dense and dark-colored. Because the tetrahedra are not polymerized, there are no consistent planes of internal atomic weakness, so they also have no cleavage. When silicate anions polymerize, they share an oxygen atom with a neighboring tetrahedron. Commonly, each tetrahedron will share two of its oxygen atoms, forming long chain structures. These chains still have a net negative charge, however, and the chains bond to metal cations like Fe2+, Mg2+, and Ca2+ to balance the negative charge. These metal cations commonly bond to multiple chains, forming bridges between the chains. Single-chain silicates include a common group called the pyroxenes, which are generally dark-colored. Because the bonds within the tetrahedra are strong, planes of atomic weakness do not cross the chains; instead, pyroxenes have two cleavage planes parallel to the chains and at nearly right angles to each other.

(b) Usually Mineral/melt partition coefficients should decrease with increasing ionic radius owing to the energy required to strain the crystalline lattice to accommodate the substituent ion. Thus partition coefficients for large cations can be calculated from those of Mg or Ca if this strain energy can be calculated. The energy required to expand a continuous isotropic medium to incorporate an ion of given size can be calculated from the site size at minimum energy, the medium's bulk modulus and Poisson's ratio, and the cation's ionic radius. Assuming the site size is 0.057nm (that for a close packed mineral) and olivine bulk moduli and Poisson's ratios of 95.3 GPa and 0.25 respectively it is possible to calculate olivine/melt partition coefficients for the divalent cations. It is observed that most of these elements can be calculated with a precision comparable to that of the experiments in which they were measured. The poor fit for the Co and Fe partition coefficients may reflect crystal field effects.

(c) Olivine is a silicate mineral with the general composition (Mg,Fe)2SiO4. Having a very high crystallization temperature, it is formed early in the Bowen reaction series. It has no cleavage planes and is black to dark green in color. The absence of cleavage planes contributes to a glassy appearance, and any fractures will be con-choidal rather than smooth and planar. The dark color is associated with the high content of iron and magnesium, making olivine an example of a ferromagnesian mineral. In Olivine the Fe2+ and Mg2+ freely substitute for each other. The Fe2+ and Mg2+ ions have almost the same size, 0.074 nm and 0.066 nm respectively, so the substitution makes very little structural change. The chemical formula (Mg,Fe)2SiO4 implies any mixture of iron and magnesium. It is said to be a family of minerals rather than one which has a definite composition.

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