Sulvanite, Cu3VS4 (copper = light grey, vanadium = dark grey, sulfur = yellow)
A cubic crystal system with perfect cleavage. Each vanadium atom is surrounded by four sulfur atoms at the corners of a regular tetrahedron. Each sulfur atom is surrounded by three copper atoms at three corners of a nearly regular tetrahedron, and a vanadium atom – in the negative position – at the fourth corner of this tetrahedron.
Sulvanite is a rare copper sulfide generally found in hydrothermal copper deposits containing vanadium as a primary sulfide. Its structure was conclusively determined by Linus Pauling and Ralph Hultgren in 1933, but was first examined by J. Orcel and then by W. F. de Jong in 1928. De Jong, using a mineral sample from Burra in Australia, prepared powder X-ray photographs for his determination. Pauling and Hultgren noticed potential discrepancies in de Jong’s final analysis of the crystal structure and decided to examine the mineral themselves using Laue and oscillation photographs.
Previously, Pauling had utilized Laue and spectral photography for crystal determinations. Laue photography involves the analysis of patterns collected by passing X-rays through a crystal to determine the positions of atoms in the unit cell. Spectral photography incorporates the use of two crystals – one whose structure is known for reference, and the other (unknown) crystal whose structure is being determined. The two crystals are rotated in front of an X-ray beam in a manner such that the reflections from the interaction fall onto a photographic plate. The distances between like planes and the type of repeating unit can then be determined.
For their examination of sulvanite, Pauling and Hultgren decided to use oscillation photography, of which Pauling had become aware while on a trip to Europe in 1930. During his stay in Ludwigshafen, the scientist Hermann Mark shared with Pauling this method of electron diffraction, and showed him the apparatus that was used to carry out the technique. Upon his return to Pasadena, Pauling had a new graduate student, Lawrence Brockway, construct the electron-diffraction apparatus that was eventually used to help determine the crystal structure of sulvanite.
(Image courtesy of the S. N. Bose Project)
Oscillation photography is a type of X-ray diffraction. It is similar in a way to spectral photography, but instead of using a reference crystal, a single crystal is made and oscillated through a small angle on an axis perpendicular to a beam of monochromatic X-rays or particles.
The discovery of a new sulvanite cache in Utah provided the opportunity for Pauling and Hultgren to carry out their examination. A sample was lent to them and they used it to conduct an extensive evaluation of sulvanite’s atomic arrangement. The two initially hypothesized that the structure would be a superstructure of sphalerite. Instead, they found that each copper atom of sulvanite is surrounded by four sulfur atoms at the corners of a nearly regular tetrahedron, and that four atoms of sulfur form a regular tetrahedron around each atom of vanadium.
Pauling and Hultgren were also very interested to find that each sulfur atom in sulvanite is surrounded by three copper atoms which form three corners of a regular tetrahedron. The fourth corner however, is not formed by copper but by a single vanadium atom found in the negative position – that is, the vanadium atom rests in a “pocket” created by the three sulfur atoms. This particular relationship was unusual in comparison to similar sulfur-containing covalent crystal structures that had been determined with certainty at that time, and the new discovery was a pleasant surprise for both of the researchers.
In the mid-1960s, Pauling briefly revisited the structure of sulvanite in his Research Notebooks 24 and 25. Much more on Pauling’s breakthroughs as a structural chemist is available on Linus Pauling and the Nature of the Chemical Bond: A Documentary History.