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While crystallographers were working on crystals, chemists were developing atomic theory. This model has passed the test of time and is accepted as the atomic arrangement in anhydrite today.

Blue spheres are Ca +2 ions and yellow tetrahedra are sulfate (SO 4) groups. Wasastjerna in 1925 based on X-ray studies. This arrangement was first described by J.A. Figure 13.2 shows the arrangement of atoms in anhydrite (CaSO 4). Within a few decades after the discovery and development of X-ray diffraction techniques, most of the basic principles of crystal structures were well known. Before their pioneering work, scientists could not test competing hypotheses for the nature of crystal structures. We cannot overstate the importance of the discover of X-rays and the subsequent studies by Röntgen, von Laue, and the Braggs.

Compositional variations in silicate minerals are directly related to the nature of the crystallographic sites in their structures.ġ3.1 The Impact of X-ray Crystallography 13.2 The atomic arrangement in anhydrite.

Atoms pack tightly together in some minerals in others, atoms are in networks with geometric shapes.

The strength of an ionic bond is an ion’s charge divided by its coordination number.

Most common cations bond to 3, 4, 6, or 8 anions alkalis and other large ions bond to more.

An atom’s coordination number is the number of other atoms that it bonds to.

The key controls on ionic bonds are ionic size and charge.

Ionic crystals contain anions packed around cations and vice versa.

We can model many crystal structures as being spherical atoms held together by ionic bonds.

Minerals have highly ordered and repetitive atomic arrangements.

The development of X-ray diffraction techniques allowed mineralogists and chemists to directly investigate the nature of crystal structures.

13.1 The atomic arrangements in quartz and sodalite 13 Atomic Structures