The Crystal Structures of Corundum and Hematite

Corundum model, side view.

Corundum, Al₂O₃ (aluminum = silver; oxygen = red). A hexagonal (rhombohedral) crystal system constructed of aluminum atoms that are each surrounded by six oxygen atoms. The oxygen atoms are not bonded at the corners of a regular octahedron.

Hematite model, side view.

Hematite, Fe₂O₃ (iron = orange; oxygen = red). A hexagonal (rhombohedral) crystal system constructed of iron atoms surrounded by six oxygen atoms not at the corners of a regular octahedron.

---

In 1925 Linus Pauling and Sterling Hendricks published a paper detailing the crystal structures of corundum and hematite. It was the fifth crystal structure analysis that Pauling had undertaken. During the early years of his research, Pauling had a tendency to correct the work of others, and the determination of hematite and corundum’s crystal structures was not an exception.

In 1917 the British father and son duo of William Henry and William Lawrence Bragg had studied the structure of ruby using X-rays. Citing this data, they hypothesized in 1924 that each aluminum atom in ruby is equidistant from six oxygen atoms, and that each oxygen atom is equidistant from four aluminum atoms. The Bragg’s used this hypothesis in their later work on theories of birefringence (the refraction of a ray of light into two slightly different and unequal rays) and to explain the intensity of X-ray reflections, in terms of temperature variation, from the faces of ruby crystals.

Hendricks and Pauling were not certain of the Bragg’s methods, and wrote in their analysis of corundum, “an exact knowledge of the arrangement of the constituent atoms in ruby would make the arguments of these papers much more convincing.” (J. Am. Chem. Soc., 47 (1925), p. 781)

Corundum is a gemstone whose varieties include ruby and sapphire. It is an aluminum oxide, and the second hardest mineral known to science after diamond. This property is generally attributed to the strong and short bonds which pull oxygen and aluminum atoms close together, making the crystal unusually hard and very dense.

Corundum model, top view.

Hematite comes in many varieties, each having their own unique name and composition. Hematite is an iron oxide, and very important as an ore of iron. It is also used as a pigment and is collected as a mineral specimen. It is blood red in powdered form, but can be gray, black, red or brown in its solid form. It is also used in jewelry, either as a set stone, or as a piece itself.

Pauling and Hendricks used Laue and spectral photographs, as well as the theory of space groups, to analyze the crystal structures of hematite and corundum. They found that, contrary to the Braggs’ hypothesis, the spacing of atoms in corundum’s atomic structure was not equidistant. Though they confirmed the Braggs’ ratio of oxygen to aluminum atoms, they found that instead of forming the corners of a regular octahedron around aluminum atoms, three of the six oxygen atoms were closer to the aluminum atom than were the other three. Similarly they found that instead of forming the corners of a regular tetrahedron around oxygen atoms, two of the aluminum atoms surrounding each oxygen atom were closer than the other two.

Sir William Lawrence Bragg

Pauling and Hendricks disproved the Braggs’ hypothesis of a constant aluminum-oxygen distance, and found that the Braggs’ value for the distance between aluminum and oxygen was also incorrect. The publication of the Pauling-Hendricks findings and the professional implications of their critique were not missed by the Braggs. Pauling was later told that Lawrence Bragg resented his “intrusion” into the fields of crystallography and mineralogy, and that he considered Pauling to be a competitor. Consequently, many of Pauling’s initial publications, often critiques of the work that others had done, led to the start of what would become a long lasting rivalry between himself and the Braggs.

Pauling later claimed that his view of their relationship was very different, both at the beginning of his academic career and the end of it.   According to Pauling, the work that was initiated to correct the Braggs’ early hypotheses was done in order to strengthen the validity of their subsequent claims. In regards to the influence of atomic arrangements on birefringence, this work was successful.

Pauling had perceived the early relationship between himself and W. L. Bragg as that of professor and student, respecting the work that the Braggs had done, and acknowledging that it had enabled him to study crystallography and chemical bonds upon his entrance to Caltech. In reference to the rivalry perceived by the Braggs, Pauling wrote “I did not think of my own scientific work as being competitive; I found it engrossingly interesting for its own sake.” Overall it seems that Pauling and the Braggs were not merely separated by an ocean, but also by an unfortunate misunderstanding of motives.

Pauling research notebook entries on the structures of corundum and hematite.

More on Pauling and Hendricks’ determination of the structures of corundum and hematite can be found in Pauling’s Research Notebook 4.   The larger story of Pauling’s structural chemistry work, including his relationship with the Braggs, is told in Linus Pauling and the Nature of the Chemical Bond: A Documentary History.

The Crystal Structure of Molybdenite

[Ed note: This is the first in a six part series of posts featuring models of crystal structures solved by Linus Pauling during the early years of his career.  The models were built by two undergraduate students working in the OSU Libraries Special Collections.  More about their work can be found on page 10 of this PDF link.]

Molybdenite model, side view.

Molybdenite, MoS₂ (molybdenum = pink; sulfur = yellow).  A hexagonal crystal system constructed of molybdenum ions bonded to two layers of sulfur atoms through ionic bonding. The sulfur layers do not present strong bonds with other sulfur layers, which creates perfect cleavage.

---

After moving to Pasadena to begin his graduate studies in 1922, it was decided that Linus Pauling would begin working as Roscoe Dickinson’s sole graduate student. Dickinson received the first doctorate from the California Institute of Technology in 1920, having joined the staff there after completing his undergraduate studies at M.I.T. in 1917.

In late September of 1922, Pauling and Dickinson were working on structure determinations of crystals with the use of X-ray diffraction, and Pauling was initially encouraged to determine the structure of the lithium hydride crystal. However, after three weeks of synthesizing the crystals and setting up his photographic apparatus, he was forced to abandon his work upon receiving word that the structure had recently been determined in Holland.

Pauling then created crystals of fifteen different organic substances. He subjected some of them to preliminary stages of X-ray analysis, but none of the samples proved sufficient for the work. After these disappointments, Dickinson helped Pauling work through the process of determining the complete structure of the molybdenite crystal.

Molybdenite, or molybdenum sulfide, is a very soft metallic mineral. Its properties include a bluish-silver color and a greasy feel that can leave marks on fingers. It has a very high melting point, so it is often alloyed with steel to make it stronger and more heat resistant. It is also an important material for the chemical and lubricant industries, and can be used as a catalyst in some chemical applications.

Molybdenite model, top view

Molybdenite crystals bend easily but are not elastic, making X-ray spectral photo analysis, the method used by Pauling and Dickinson, somewhat difficult. Despite these issues, Pauling and Dickinson managed to take a suitable photograph and were able to determine the crystal’s structure. Molybdenite was expected to have an octahedral atomic structure. Pauling and Dickinson discovered that it was instead comprised of six atoms surrounding the corners of a trigonal prism. This surprise demonstrated, among other things, the potential for unpredictability and excitement in chemical experimentation.

Fresh off their laboratory success, Pauling co-authored his first scientific paper with Dickinson. It appeared in a 1923 issue of the Journal of the American Chemical Society under the title “The Crystal Structure of Molybdenite.” The success renewed Pauling’s confidence in his capacity to carry out professional scientific analysis, and instilled in him an understanding of the value of well-planned experimentation. The process likewise provided him with a valuable framework that he began shaping and developing to fit his particular temperament. Throughout the rest of the 1920s and 1930s, he would use this method to write and publish multiple crystal structure papers nearly every year.

Pauling notebook entry documenting work on molybdenite, 1922.

Much of Pauling’s early work on the structure of molybdenite is detailed in his Research Notebook 2.  For more on his amazing achievements in structural chemistry, see the website Linus Pauling and the Nature of the Chemical Bond: A Documentary History.

Linus Pauling, Vitamin C and the AIDS Crisis

Ewan Cameron, Ava Helen and Linus Pauling. Glasgow, Scotland, October 1976.

“Many orthomolecular substances are so free from toxicity that they show beneficial effects over a 10,000-fold range of concentrations. Yet if you take even ten times the amount of aspirin that many patients take, for example, you’d be dead; hundreds of people do die every year from aspirin poisoning. And all of the other major drugs are highly toxic as well.”
– Linus Pauling, December 1986

Today, December 1, 2008, is World AIDS Day. In honor of the fight against AIDS, The Pauling Blog would like to discuss Pauling’s own attempts to find a cure.

Beginning in the early 1930s, stemming from early investigations into the chemical nature of hemoglobin, Linus Pauling became deeply interested in the application of chemistry to medical problems.  Once involved in a long-term study of the substance, he began to recognize the significance of hemoglobin to the health of individuals. In 1949, Pauling coined the term “molecular disease” in reference to a mutation in hemoglobin cells that caused sickle cell anemia.

His interest in medicine did not stop there, however. During World War II, Pauling designed a meter to measure oxygen levels aboard U.S. submarines. He later converted this meter to be used in incubators for premature babies with underdeveloped lungs, saving thousands of lives in the process.

In the early 1970s, Pauling developed an interest in the use of megadoses of vitamins as a means for both preventing and treating disease. He became particularly interested in vitamin C because, even in huge doses, it proved to be non-toxic.  Pauling began recommending its use to prevent colds and other illnesses, eventually suggesting that a high-dosage vitamin C regimen could help cancer patients by fortifying the immune system and potentially destroying carcinogens.

With the onset of the AIDS crisis in the early 1980s, Pauling saw potential for another area in which vitamin C could be put to good use. Though he did not officially endorse vitamin C as a treatment for AIDS until the early 1990s, he commonly noted its possible benefits and lack of side effects.

In 1988, Pauling headed a study on the effects of vitamin C in combating the AIDS virus, measuring the impact that ascorbic acid had on infected T-cells. The results, though not extraordinary, were promising.

In 1990, Pauling and his colleagues published the results of their study in the Proceedings of the National Academy of Sciences, claiming that vitamin C appeared to suppress the growth of the AIDS virus. As was true of Pauling’s previous claims regarding vitamin C and orthomolecular medicine, the studies were at least a source of intrigue to many, though likewise dismissed by a wide cross-section of the medical community.

At the same time that Pauling was embarking on his AIDS research, Ewan Cameron, a researcher at the Linus Pauling Institute of Science and Medicine in Palo Alto, approached Pauling regarding a book he was writing entitled The AIDS Disaster. The book was meant to serve as a comprehensive study of the AIDS virus, describing its history, socio-political context, and related research.

Cameron requested that Pauling serve as co-author, editing the book and providing a chapter on vitamin C and AIDS. Pauling agreed and, in late 1988, the book was completed. Due to a variety of publishing issues, the text never reached bookstore shelves, but several complete versions of the manuscript are held in Cameron’s papers here at Oregon State University.

Until his death in 1994, Pauling continued to emphasize the responsibility of the scientific community to help cure diseases such as AIDS and cancer. He gave frequent lectures on the subject of orthomolecular medicine and continually worked to increase support for medical research.

For additional reading on Ewan Cameron’s AIDS work and research, check out the Cameron Papers finding aid, hosted online by the OSU Libraries Special Collections.  Please also note that a few more of Pauling’s thoughts on the treatment of AIDS with ascorbic acid are linked off of this index page from the Linus Pauling Research Notebooks website.

Thinking Structurally: The Roots of Pauling’s Hemoglobin Work

Pastel drawing of Hemoglobin at 20 angstroms, 1964. Drawing by Roger Hayward.

“Linus Pauling is one of that select group of individuals whose lives have made a discernible impact on the contemporary world. His contributions to molecular chemistry have been substantial and fully deserving of the recognition that he received in the form of a Nobel Prize in chemistry….Pauling continued to do productive scientific work throughout his lifetime, making a second outstanding contribution in his discovery of the molecular processes involved in sickle-cell anemia. This discovery, if made by anyone who was not already the only person to receive two unshared Nobel Prizes, might well have merited a third prize in medicine.”
– Ted Goertzel. “Linus Pauling: The Scientist as Crusader.” Antioch Review, 38 (1980): 371-382. 1980.

Two of Linus Pauling’s greatest scientific discoveries, his work on the nature of the chemical bond and the discovery of molecular disease, both hinged on his distinctly structural approach to scientific problems.

Having written a doctoral dissertation on the determination of the molecular structure of inorganic compounds in crystalline state, Pauling chose hemoglobin as an object of study in part because he knew that it was hemoglobin’s changing structure that allowed it to carry oxygen to the tissues of the body. While Pauling like to joke that he chose to work on blood because it was easy to obtain, the intellectual challenge of explaining the sigmoid curve of oxygen saturation in hemoglobin profoundly sparked Pauling’s scientific interest.

Later, upon learning about the disease sickle-cell anemia, Pauling came to recognize that the potentially molecular and structural basis of the disease could facilitate a deeper investigation into structural studies of the molecule. Hemoglobin, in part because of its association with the bonding and transport of iron atoms, demonstrated extremely changeable magnetic charges and suggested, even from a preliminary acquaintance, the importance of structural changes in chemical function.

By 1934, when Pauling suggested hemoglobin as the organic molecule of choice for his particular research program, he had already laid out a general plan of research that relied heavily on investigations into the structural and electrically-charged nature of organic molecules. In May of 1935, Pauling wrote in his research notebook

At this time I have analyzed the oxygen equilibrium data to make plausible the idea that in hemoglobin the four hemes are arranged at the corners of a square on one side of the globin, being interconnected along the edges of the square, and that in the hemochromogens the hemes are independent of one another; and I have outlined a general program of investigation, consisting mainly of magnetic studies and x-ray studies (anomalous dispersion, radial distribution about iron atoms).

In a way, Pauling had always been thinking structurally about the nature of the hemoglobin molecule, its ability to bind oxygen molecules and, later, its particular pathology in the case of sickle-cell anemia.

In 1937, Pauling delivered an inaugural lecture for the Sigma Xi society of Corvallis, in which he asserted both the importance and the relatively-recent arrival of structural chemistry as a discipline. For Pauling structural chemistry involved “the determination of the structures of molecules – the exact location of the atoms in space relative to one another – and the interpretation of the chemical and physical properties of substances in terms of the structure of their molecules.”

This lecture, entitled “Hemoglobin and Magnetism,” addressed the “new branch of chemistry, modern structural chemistry” through a discussion of some of Pauling’s most recent work on hemoglobin’s magnetic properties.