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Salt Crystals

Crystals seem so structurally beautiful. What could be mysterious about the obvious beauty of grains of sugar or lumps of quartz? Then problems arise: For example, you and I grew up with the fact that snowflakes derive their beauty from the crystal structure of ice.  But the existence of molecules was still in question during the 19th century.  We wouldn't analyze crystal structures with X-ray diffraction until the 20th century.

In 1872, the Victorian scientist John Tyndall showed rare insight in a book on The Forms of Water.  He digressed from his description of snowflakes to suggest the way molecules must bond to one another.  We didn't yet know the exact structure of ice, but he knew structure was there.  He knew that had to be what determined the exquisite beauty of snow.

A half century later, even books for young people were saying flatly that the crystal structure of snow resulted from some kind of molecular arrangement of molecules.  

By then, we knew that snowflake crystal structure is based on of a hexagonal prism form.  Those prisms have six-sided symmetry, but only in one plane.  Snowflakes grow as molecules are attracted to points and cusps -- not to flat edges. A kind of fractal growing and splitting spreads outward to give snowflakes their remarkable forms.  

Snowflake Crystals
Still, while we’ve always had snowflakes to goad us toward thinking about crystal structure, we’ve been slow to take the bait. Nicolaus Steno offered one of the earliest insights into crystal form. And that was already the mid-17th century. 

Steno was born in Denmark in 1638. He studied medicine and did fine anatomical work. He was 27 when he became physician to the Grand Duke of Florence. There he converted to Catholicism and within twelve years, he was bishop to the Catholics in the Protestant North.

Just after Steno went to Florence, he wrote a book on geology: Introduction to a Dissertation on a Solid Body Contained Within a Solid.  It was about another kind of structure: stony inclusions. He made clear that fossils inclusions were remnants of once-living creatures, embedded in sedimentary stone. Out of that came our realization that Earth is far older than the Biblical begats suggest. 

In that work, Steno made an offhand remark in a figure caption. He commented that angles between crystal faces don't change with size and shape.  His treatise had really given us a whole new way of looking at geological formation. 

But that one small figure caption is better-known. It placed a mantle that said “father-of-crystallography” upon his shoulders. And that’s not all. In 1987, the Catholic Church declared Bishop Steno as “blessed,” the first step on the way to making him a saint -- a pretty unexpected career lurch for a scientist.

So the evidence accumulated. Example, in 1848, Louis Pasteur made the astonishing discovery that racemic acid crystals took two forms. They had identical chemical makeup, yet they were mirror images of one another. Just as you can't use a right glove on your left hand, you couldn't rotate one shape into the other. 

Pasteur realized that this was possible because the crystals had a molecular structure that could be mirror imaged. Chemists call this property chirality -- from the Greek word for hand.

We were now on the way to setting up the complex rules of chemical structure.  But, how to verify those structures after we’d guessed their form? That’s where Dorothy Crowfoot enters our story. 

Dorothy Crowfoot
Dorothy Crowfoot was born in Egypt in 1910 -- a daughter of British archaeologists who packed her back to England when WW-I began.  Her education was choppy. But then she ran across a textbook that told how to grow copper sulfate crystals.  Ten-year-old Dorothy decided to try it.  

It worked, so she resolved to understand this magical lifelike process.  A geologist friend gave her a box of reagents and minerals and said, "Buy a proper book on analytical chemistry!"  She did.  Then she built a chemistry lab in her attic and set her sights on the male bastion of Oxford University.

But first, she went to Jerusalem to help her parents excavate Byzantine churches and reconstruct mosaic patterns from fragments on the floors.  It took a trick of seeing for which she had a special gift.  After Oxford, she worked in X-ray crystallography at Cambridge.  Then, two things happened on the same day in 1934.  First, the name Crowfoot took on a terrible irony.  She found she had crippling rheumatoid arthritis.  Ever after, she led a very active life working in pain, with hands and feet terribly twisted.

Only hours after she learned that, people in her lab made the first X-ray photo of a protein crystal.  And she realized she could go from a pointillist X-ray pattern -- a broken Byzantine mosaic -- to the 3-D structure of a complex organic molecule.  That day, she said, began in pain and ended in a vision.

Three years later, she married a socialist writer Thomas Hodgkin.  They lived a joyful life of odd disorder, verve, and shared radical causes.  By 1946 Dorothy Crowfoot Hodgkin had learned the molecular structure of penicillin.  That, says her biographer, was like drawing plans for a jungle gym when you've seen only its shadow on the ground.  Her life was, in fact, one eerie feat of spatial visualization after another.

By 1951, she'd figured out the mysterious B12 molecule and for that was given the 1964 Nobel Prize in chemistry.  A headline in the Times mawkishly cried out: "Nobel Prize for British Wife," and she kept on working.  In 1969, she showed how 777 atoms made up the incredibly complex structure of the insulin molecule.

Margaret Thatcher studied chemistry at Oxford under Dorothy Crowfoot Hodgkin.  As prime minister, Thatcher kept up their friendship -- the conservative and the ultra-liberal.  But then, qualities that would draw two of the smartest people in England together for tea had to be ones that transcended mere politics.

So it has been no easy matter to see through the seemingly simple structure of crystals. Yet the first creature to see through those structures did so hundreds of millions of years ago -- and it did so quite literally. 

The Cambrian era Trilobite was an elementary forebear of our cockroach. Trilobites evolved for 300 million years and finally evolved eyes. The many lenses of its eyes were, it seems, tiny calcite crystals.  They evolved this wholly unique form of sight which died out with them. Can you imagine? -- A living organic being with inorganic crystalline eyes! And we realize that, like all the structures we’ve looked at, crystals are stranger than we first expect. 

And there’s more. We’ve talked about cathedrals, music, spider webs, foraging, crystals ... Now our operations research expert, Andy Boyd, reminds us that we’re obliged to invent very sophisticated ways for giving structure to information itself. If we fail to do that, we’ll absolutely flounder in our new information-dense modern world.


K. Libbrecht and P. Rasmussen, The Snowflake: Winter's Secret. (Colin Baxter Photography Ltd., 2004)

S. J. Gould, Hen's Teeth and Horse's Toes. (New York: W.W. Norton and Company, 1983): Chapter 5, The Titular Bishop of Titiopolis.

By the way, Steno's actual title was De Solido intra Solidum naturaliter Contento Dissertationis Prodromus. I've translated the word "Prodromus" as "Introduction" for simplicity's sake. It is a more technical term that means something like "Introductory Discourse." Steno's book was meant to set the stage for a really heavy work, which he never completed.

R. M. Roberts, Serendipity: Accidental Discoveries in Science. (New York: John Wiley & Sons, Inc., 1989). Roberts adds a sobering note about chirality. Thalidomide, which created so many birth defects in the 1950s, is chiral. One form is a harmless morning-sickness suppressant. The mirror image is an active mutagen.  The drug was put on the market with both forms present, and it did untold damage.

S. B. McGrayne, Nobel Prize Women in Science. (New York: A Birch Lane Press Book, 2001): Chapter 10, Dorothy Crowfoot Hodgkin, May 12, 1910 -- Physical Chemist, Nobel Prize in Chemistry 1964.

There is so much more to tell about Dorothy Crowfoot Hodgkin.  She was only the fourth woman ever to win a Nobel Prize, but it was the fifth Nobel Prize to a woman, since Madam Curie had won two.  She was the first English woman to win the prize.  But, before that, at the age of only 36, she became only the third woman to be made a Fellow of the Royal Society.

Linus Pauling tried to bring her to America in 1953, but the State Department wouldn't let her in. That was because both her husband and her mentor at Cambridge (John Desmond Bernal) were members of the communist party.  She was apolitical but strongly sympathetic to a number of socialist causes -- any one of which incurred the wrath of Western governments during the 1950s.  Her enduring friendship with Thatcher is all the more remarkable for that.

R. Fortey, Trilobite! Eyewitness to Evolution. (New York: A. E. Knopf, 2000).

See also the Wikipedia entry on Trilobites. 

All pictures are available to the public as their copyrights have expired except salt crystal photo by John Lienhard.

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