The Crystal Structure of Enargite

Enargite model, front view.

Enargite, Cu3AsS4 (copper = dark silver, arsenic = light silver, sulfer = yellow)

A crystal system classified as orthorhombic pyramidal, with a construction consisting of arsenic or copper atoms surrounded by four sulfur atoms in a tetrahedron.  Each sulfur atom is similarly surrounded by a tetrahedron of one arsenic atom and three copper atoms.

As we mentioned in this post, in the early 1930s Linus Pauling began turning his attention from silicate crystals to sulfide minerals. The work on silicates was approaching completion, and Pauling hoped to form a similar understanding of sulfide structures.

He began his work with sulfides by studying the first crystal structure that had ever been determined in the United States. The mineral was chalcopyrite, and Pauling discovered that an initial analysis of the mineral’s structure was incorrect, and that the correct structure was twice as large and featured a different atomic distribution than was initially reported. Pauling continued to work with sulfide minerals and other crystals during this time, publishing structural analyses of sulvanite, zunyite and binnite. He was also busy working on the nature of chemical bonds, quantum mechanics, and the theoretical study of covalent crystals and bonds.

Pauling began studying enargite in March 1931, but was forced to temporarily halt his examination both because of difficulties with the photographs used for structural analysis, as well as the pressures of more pressing priorities. He eventually finished his analysis, and published the findings with Sidney Weinbaum in 1934.

Enargite is a somewhat rare mineral, composed of copper, arsenic and sulfide. It is used as a minor ore of copper and as a mineral specimen. It has a very uncommon symmetry, belonging to the hemimorphic class of crystals – hemi meaning “half” and morph meaning “shape.” This name references the tendency of these crystals to generally have different shaped tops compared to their bottoms.

Enargite model, top view.

Pauling and Weinbaum used data from Laue and oscillation photographs of crystals taken in the Philippine Islands to investigate enargite’s structure, and found that it very closely resembled that of wurtzite, a zinc sulfide. The duo determined that the structure of enargite consists of arsenic and copper atoms which are each surrounded by four sulfur atoms, each sulfur atom being similarly surrounded by a tetrahedron of one arsenic atom and three copper atoms.

In their study, Pauling and Weinbaum verified the mainstream theorized structure of enargite, but found some slight discrepancies in the assumed arrangement. They discovered, for example, that the crystal was incorrectly classified, being orthorhombic pyramidal instead of orthorhombic bipyramidal, as was initially believed.

Sidney Weinbaum, June 1950.

Despite his success with enargite, Pauling found the pace of his sulfide work to be slow, and sought help from the Geological Society of America in the form of a grant proposal during the spring of 1934. Pauling requested a total of $4800;  $1200 for a new apparatus and $3600 to provide three years wages for a postdoctoral fellow. The new position would designate a point person to carry out the “extensive and laborious graphical and numerical calculations” needed to determine the structure of more complex crystals and minerals. During the three years covered in the proposal, Pauling planned to determine the structures of pyragyrite, proustite, pentlandite, covellite, chalcocite, and the minerals of the niccolite group.

Unfortunately for Pauling, Waldemar Lindgren, chairman of the Projects Committee of the Geological Society of America, refused the request. He reasoning for rejection rested primarily upon the feeling that more money would need to be spent on the apparatus initially, in order for a supplemental grant to be provided.

Pauling's notes on enargite, ca. 1930s.

Pauling defended the relevancy of his first proposal, but altered his request so that it only included funding for the three year postdoctoral fellow’s salary. After receiving no reply from Lindgren, Pauling once again repeated his request. He never received a response, and was deeply hurt as a result, later revealing that the incident had contributed to leading him away from crystal-structure determinations in favor of the field of biology.

Though Pauling went on to determine several more crystal structures, the incident was a turning point in his career. From this point on, his relationship with crystal structure work moved from a general systematic determination of mineral group structures, to an occasional examination of substances that piqued his curiosity.

Several pages of notes on enargite by Pauling and Weinbaum are available in Pauling’s Research Notebook 8.  For more on Pauling’s structural chemistry work, see the website Linus Pauling and the Nature of the Chemical Bond: A Documentary History.