The unit cell shown on the attached sheet is that of a common iron sulphide mine
ID: 290023 • Letter: T
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
a) From the structure of the unit cell, the number of Fe ions in the corners= (8X1/8)=1
the number of fe ion in the cell (at the centre)=1
the total number of fe=2
the total number of S ions in the cell( at the edges and faces) = (1/2X6)+(1/4X4)=3+1=4
Hence, the formula units the mineral contain=4
b) The number of Fe ions are 2 and the number of S ions are 4, so the formula is Fe2S4.
That is, the formula is FeS2.
c) At the centre, the fe ion is coordinated by 6 S ions, hence the coordination number of fe is 6.
The S ion is coodinated by 3 Fe ions and 1 S ion, hence the coordination number of S is 4.
d) It belongs to Orthorhombic Crystal System since, a, b and c are not equal to each other and the angles between A and b, band c and c and a are equal to 90 degrees.
e) The name of the mineral is Marcasite.
f) The name of the other mineral is Pyrite and it belongs to Isometric Crystal System.
g) The relationship between these two minerals in polymorphism. i.e, they are polymorphs to each other since they have same composition but different structures.
h) The name given to the two sulpher atoms marked a 2 and 3 on the diagram is disulphide bond.
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