The unit cell shown on the attached sheet is that of a common iron sulphide mine

Question

The unit cell shown on the attached sheet is that of a common iron sulphide mineral; cell dimensions are shown on the diagram (note that unit cells are usually shown with the c-axis vertical, but this is not the case for the cel shown here). Answer the questions below concerning this mineral How many formula units does the mineral contain? (NB: only atoms wholly or partly within the cell are counted) a) b)What is the simplest formula of the mineral? c) What is the co-ordination of Fe and S in the cell? d) What is the crystal system of the mineral? e) Look up website www.mindat.org, and determine the name of the mineral from the above information. (Use the option in mindat that allows searching for minerals by chemistry,). f) There are two minerals that have this formula; what is the name of the g) What is the name given to the relationship between these two h) What is the name given to the two sulphur atoms marked 2 and 3 on other mineral and its crystal system? minerals? the diagram (it is a common feature of this class of sulphides)?

Explanation / Answer

1)

(1) type of sulphur atoms = 4

contribution to the lattice = 1/2

so no of (1) type of suphur atoms = 4 x 1/2 = 2

inside crystal sulphur atoms = 2

contribution = 1

so no of inside suphur atoms = 2 x 1 = 2

total suphur atoms = 2=2 = 4

Fe side atoms total = 8 x 1/8( contribution in lattice) = 1

Fe center atom = 1x 1( contribution in lattice) = 1

total fe atoms = 1+ 1= 2

so formula =Fe2S4 = 2 FeS2

so there are 2 formula units in lattice of  FeS2

b) simplest formula is =   FeS2

c). total no of S attached with Fe = 6

Total no of Fe attached with S = 3

so coordination of Fe : S = 3:6

d).=

White Iron-pyrite FeS2 represents the prototype compound of the crystallographic pyrite structure. The structure is ORTHORHOMBIC SYSTEM , The unit cell is composed of a Fe face-centered orthorhombic sublattice into which the S ions are embedded

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