Rationalization of the Substructures Derived from the Three Fluorite-Related [Li6(MVLi)N4] Polymorphs: An Analysis in Terms of the “Barnighausen Trees” and of the “Extended Zintl–Klemm Concept” - Inorganic 3D Structures - page 107

tetrahedra at the origins (Fig. 10a , colusite, Fig. 10b , germanite) share their edges
rather than corners with the immediately surrounding tetrahedra. However, the
predominant contacts involve corner-sharing, as in sphalerite.
We note that the split sites of these two structures are different: (As/Sn) for
colusite is in site 6 c (Cu(2) in germanite) and (Fe/As) for germanite is in 8 e (Cu(2)
in colusite). Figure 10c shows the corresponding drawing for a sphalerite-like
component of Li 7 VN 4 involving only the V(1)N 4 , V(2)N 4 , Li(2)N 4 , Li(3)N 4 and
Li(5)N 4 tetrahedra, whereas it is the V(1)N 4 tetrahedra at the origins which share
edges. The close similarities are evident.
Here we are entering the whole complex realm of mineral chemistry where given
mineral structures can contain a variety of different substitutions. According to
Spry et al . [ 34 ]“ ... the elements Cu, V, As and S are essential components of
colusite, whereas Sn, Sb, Fe and Ge are often present in significant quantities. Cu
content can vary somewhat.” The ideal, so-called high-tin colusite has the formula
Cu 26 [V 2 A s 4 Sn 2 ]S 32 , the As and Sn being disordered in the ratio of 2 to 1 on the 6 c
site of

P 43 n , and this is the composition whose structure was reported. “Low-tin”

Fig. 10 (a) The structure of colusite ( a ¼ 10.621 ˚ ) viewed along [111]. (As/Sn)S 4 tetrahedra -
green ,VS 4 tetrahedra - grey , Cu(2)S 4 tetrahedra - light blue , Cu(3)S 4 tetrahedra - red , Cu(1)S 4
tetrahedra - purple .(b) The equivalent structure of germanite ( a ¼ 10.5862 ˚ ). Cu(2)S 4 tetra-
hedra - green , Cu(1)S 4 tetrahedra - grey , Cu(3)S 4 tetrahedra - red , (Fe/As)S 4 tetrahedra - light
blue , Cu(4)S 4 tetrahedra - purple .(c)Li 7 VN 4 [111] ( a ¼ 9.606 ˚ ). V(1)N 4 tetrahedra - grey ,

V(2)N 4 tetrahedra - red , Li(4)N 4 tetrahedra - light blue , Li(2)N 4 tetrahedra - green , Li(5)N 4
tetrahedra - purple