Polymorf - Knowhwere- Tetrahedral summary

Summary of minerals with AX₄ coordination group structures

The number and type of linkage surfaces of close packed AX₄ coordination groups

comprising the forgoing crystal structures depended on the percent occupation of the

structure's tetrahedral voids with A atoms, the type of packing of the X atoms, and

whether the occupied layers of the lattice were successive or alternating. The range

of linkage varied from AX₄ groups that shared all six possible edges to groups that

shared no surfaces at all. Throughout this progression these AX₄ groups have

supplied all of the close packed atoms necessary to build the lattice structure.

The chemical composition of the crystal also depended on the percent occupation

of the tetrahedral voids. A close examination of the lattice structures demonstrates

that the ratio of tetrahedral interstices to close packed X atoms in a FCC or HCP

crystal structure is 2:1. This enables the chemical formula of the mineral to be

deduced a priori from a knowledge of the occupation rate. This clearly establishes

the structural basis for the chemical composition of these minerals.

.
	% occupation	formula
	100	A₂X
	75	A₃X₂
	50	AX
	25	AX₂
	1/6 (16.7%)	AX₃
	1/8 (12.5%)	AX₄
.
Table 6 - Percent occupation versus formula for AX₄ mineral structures

The following section illustrates the effect that the percent occupation of the

octahedral voids of crystal lattice structures has on the geometry and chemical

composition of minerals.

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Page 49 - Structure matters - Tetrahedral summary