1) The idealized formula of a common rock-forming mineral group is (MI,M2) Si206
ID: 106376 • Letter: 1
Question
1) The idealized formula of a common rock-forming mineral group is (MI,M2) Si206. When the M-sites are occupied by iron (Fe) and magnesium (Mig) the rock-forming mineral has an orthorhombic crystal system. When the M2 site is occupied by calcium (the MI site remains filled by Fe and Mg) the rock- forming mineral has a monoclinic crystal system a) Describe the change in symmetry that occur between the orthorhombic and monoclinic crystal systems, and the crystal chemical causes of the changes in symmetry. (Describe the changes in terms of properties of crystallographic axes and simple operations of symmetry) 8 marks). b) Would you describe the substitution ofCa for (Fe,Mg) in the M2 site as a simple or complex coupled substitution? (2 marks).(Remember to justify your answer).Explanation / Answer
1) in this type of crystal system the rock tries to be symmatry for which it tend to be tabular or prismatic.this monoclinic system of cystal always tend to form long prisms.it can be describe as the system of three nonconcurrent vector in 3-D.
a) for the case of symmatry of the monoclinic crystal system it have the angle of alpha and gamma =90 degree but for the case of beta which is equal to or sometimes greater than 90 degree. so the case of symmetry is 2/m. the lattice point of the crystal get shifted and the symmerty is half.but for the case of orthorhombic it have all the 3 angles of 90 degree. but it not have the symmetry of side faces means a not=b not=c. the shape is sometimes rhoumbus cuboidal type.
b) when the atoms of calcium reside at m2 points of the crystal then the previous two atoms of Fe and mg want to be remains in their initial position so it will lead to complicated coupling of the crystal which make it complex sustitution.
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2)
a) the shape of the crytal will be of tetrahedral type because the intertitial cations will be remain in the void and the other cation which are forming the crystal symmetry and hence the shape will be of tetrahedral.and it is primarily occupy by silicon and aluminium because they are tetravalent and trivalent respectively which will make it fit to be occupy and the effinity towards opposite charge ions is also responsible for the same.
b) the solid is the result of ions substitution of one another in the crystal system and for coupled substitution it is the substitution of two diffrent element together in a crystal compound which remains to neutral electrically.the solid solution will be characterized by coupled substitution because i site is occupied by calcium and sodium here .
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