Colourful Compound Interest

I discovered Andy Brunning’s Compound Interest last year and got absolutely hooked on it – and I don’t even teach chemistry! If, perchance, you do teach chemistry and don’t yet know what CI is all about, then you probably should check it out. (And if you want to use the material in the classroom, you can download the high-resolution PDF files.) The topics range from general chemistry to material science, chemical warfare and everyday compounds. You’ve got answers to many questions you always wanted to ask but never had time to find out for yourself, like, “is it worth (not) to refrigerate tomatoes?”. The Undeserved Reputations section is a perfect antidote to the “oh my God, our food is still full of chemicals” stream of rubbish published by your Facebook friends.

Here are ten the top ten some of my CI favourites.

Metal Ion Flame Test Colours Chart

1. This is how (I’d like to think) I’ve got interested in chemistry. We used to have a gas hob in our kitchen. I loved the fact that the flame was blue. One day, my brother told me that you can make the flame bright orangey-yellow if you sprinkle it with table salt or bicarbonate of soda. “Why?”, I asked. “Sodium”, was the answer. Unsatisfactory as it was, it stayed in my memory. Yes, chemistry won’t be of any interest to me if not for flame and colours.



Colours of Transition Metal Ions in Aqueous Solution

2. When they are not busy burning or, better still, exploding stuff, your archetypal chemists are often imagined (and therefore portrayed; or is it the other way round?) as hiding behind the test tubes filled with colourful solutions. Which is just as well. The test tubes filled with colourless solutions would be really boring.



What Causes the Colour of Gemstones?

3. Who didn’t dream of finding a treasure, that is, a pirate’s chest filled with gold and jewels? Wait. I still dream of that. I remember how surprised I was when, back in elementary school, I read in some book that ruby and sapphire are basically the same mineral corundum, the only difference is in a type of impurity. Well it’s quite an important difference then. Without impurities, most gemstones would be colourless.



The Chemistry of The Colours of Blood

4. My interest in bioinorganic chemistry (even though at the time I didn’t know at it was called that) was also awakened in school, when I learned that some animals have blue blood. I also discovered that, contrary to what anatomy textbooks show, veins do not carry blue blood in humans. I am not sure if I was relieved or disappointed. Later, already in the university, I read about a Soviet-developed fluorocarbon-based blood substitute nicknamed “Blue Blood”. Fascinating stuff.



The Chemicals Behind the Colours of Autumn Leaves

5. I remember, as a child, reading, or rather browsing, an illustrated book about plants (translated from English), with many beautiful colour photographs. “This apple is yellow because of anthocyanin”. Next page: “This apple is yellow because of carotene”. Next page: “This apple is green because of chlorophyll”. The realisation dawned that, apple-wise, being green is not only necessary but sometimes sufficient.

   But what about leaves? When autumn comes, chlorophyll starts to break down and we get to see other pigments in them. Apart from caroteinoids and flavonoids, there are also coloured chlorophyll degradation products, termed “rusty pigments”.



The Chemistry of Stain Removal

6. Sometimes, however, we want to get rid of all these beautiful colours. The infographic shows the chemical methods of achieving that, although I am not sure that “enzymatic stains” is a correct name for stains caused by blood or grass (yes, haem and chlorophyll again!).



The Atmospheres of the Solar System

7. Alchemists associated seven metals with seven planets (which included the sun and the moon). At the time, it seemed to be quite reasonable. Now that nobody expects Mercury to be made of mercury (and, for that matter, Pluto to be made of plutonium), precious little is known about composition of these planets. About their atmospheres, we’ve learned a bit more. Hey, isn’t it amazing that Mercury’s atmosphere has by far highest percentage of molecular oxygen (42%) compared to any other atmosphere in Solar system? We still won’t be able to breathe there though, because its atmosphere is way too thin (its surface pressure is less than 10⁻¹⁴ bar).



The Metals in UK Coins

8. Compared to gemstones, coins are so much duller, especially now that we don’t come across either gold or silver coins any longer. Continuing the alchemical tradition, we can say that modern British coins of 20 pence and higher are mostly from Venus (that is, copper), while 1 p, 2 p, 5 p and 10 p coins are mostly from Mars (i.e. iron). Of course, you can find much more metal variety in commemorative coins.



The Metal Reactivity Series

9. In contrast to their salts, aqueous complexes and gemstones, pure metals do not offer a great variety of colours. Copper is red, gold is yellow and caesium is yellowish; the rest are coming in many shades of grey. But their chemical behaviour is wildly different, as this infographics shows. You don’t need a sophisticated lab equipment or fancy reagents, just water and some (diluted) acids. If there’s no reaction whatsoever, you’ve got a precious metal. Easy!



Analytical Chemistry – Infrared (IR) Spectroscopy

10. Did I tell you that my first love, as far as the world of analytical chemistry is concerned, was vibrational spectroscopy? If not, I’m telling you now. I’ve never got to do any experiment worthy of a publication, because if I did, believe me, it would have been awesome. This infographics reminded me of happy days of my studenthood when I knew and cared more about amide bands (bless them) than about money or my future career.

Tetracalcium octachromium(3+) strontium octacarbonate hexadecahydroxide sulfate pentaicosahydrate

The Polar Bear peninsula in Western Australia is one of the many places on this planet I never heard before. The reason I mention it now is that a new mineral named putnisite was discovered there, and this mineral caused a bit of a stir recently, for being “completely unique and unrelated to anything”. In fact, if you Google “Polar Bear peninsula”, all you find is putnisite.

In 2007, specimens of an unknown mineral forming purple crystals (a) were collected at the Polar Bear peninsula while prospecting for nickel and gold. The specimens were eventually forwarded to Peter Elliott, a research associate with the South Australian Museum, for examination.

(a)

Elliott et al. [2] report the composition and crystal structure of this unique mineral, named in honour of mineralogists Christine and Andrew Putnis of the Institut für Mineralogie, Universtität Münster, Germany. The compositional name for putnisite I come up with is “tetracalcium octachromium(3+) strontium octacarbonate hexadecahydroxide sulfate pentaicosahydrate”. Curiously, Mindat and Mineralienatlas give the molecular formula containing only 23 molecules of water.

(b)

The crystal structure (b) was determined from single-crystal X-ray diffraction data. Cr(OH)₄O₂ octahedra (red) link by edge-sharing to form an eight-membered ring. At the centre of each ring lies a decacoordinated Sr²⁺ cation (purple). The rings are decorated by carbonate triangles (green), each of which links by corner-sharing to two Cr(OH)₄O₂ octahedra. Rings are linked by Ca(H₂O)₄O₄ polyhedra (blue) to form a sheet parallel to the (100) plane. Adjacent sheets are joined along the [100] direction by corner-sharing sulfate tetrahedra (yellow).

1. Mills, R. (2014) New mineral shows nature’s infinite variability. Phys.org.
2. Elliott, P., Giester, G., Rowe, R. and Pring, A. (2014) Putnisite, SrCa₄Cr³⁺₈(CO₃)₈SO₄(OH)₁₆·25H₂O, a new mineral from Western Australia: description and crystal structure. Mineralogical Magazine 78, 131—144.

Ochre, a great universal

From Architecture of First Societies: A Global Perspective by Mark Jarzombek:

Known scientifically as hematite, ochre is a reddish iron-containing rock that was used as a coloring substance made by grinding the stone into a powder that when mixed with fat or water formed a paste. Homo erectus had already begun to work with ochre, thus securing it as a key, nonfunctional element in human life.

Ochre was a great universal of all First Societies. The Blackfeet of the American Plains referred to it as nitsisaan or “real paint” and profusely daubed it on their ceremonial garments. Its color was thought to represent the sun and the energy that permeates all things, making a person rubbed with it appear holy and powerful. Its redness and brilliance signaled supernatural potency overlapping with a range of cosmological concepts revolving around rain, fertility, hunting, and death. The nineteen-century painter George Catlin painted the Sioux worshipping at a red boulder in the open grasslands of the Great Plains.

George Catlin, Sioux Worshiping at the Red Boulders, 1837—1839

Ceremonial uses of ochre still exist today, such as among the Maasai in Kenya during certain initiation rituals, by Amazon tribes and by Aboriginal people in Australia.

The !Kung, who live in the Kalahari desert of Botswana and who are among the oldest of the surviving First Society people in the world, use the pigment in rituals dealing with a woman’s first menstruation. A female initiate, on emergence from seclusion, would present the women of her kin group with lumps of ochre for decorating their faces and cloaks and also for adorning the young men to protect them when out hunting.

Icosahedrite

In 1982, Dan Shechtman observed unusual diffraction pattern in aluminium—manganese alloy [1, 2]. Almost 30 years later, he was awarded The Nobel Prize in Chemistry 2011 “for the discovery of quasicrystals”.

Earlier this year, the first naturally occurring quasicrystal was described. Icosahedrite Al₆₃Cu₂₄Fe₁₃ is a new mineral found in southeastern Chukhotka, Russia. It is named “for the icosahedral symmetry of its internal atomic structure, as observed in its diffraction pattern” [3].

1. Shechtman, D., Blech, I., Gratias, D. and Cahn, J. (1984) Metallic phase with long-range orientational order and no translational symmetry. Physical Review Letters 53, 1951—1953.
2. Fernholm, A. (2011) Crystals of golden proportions. Nobelprize.org.
3. Bindi, L., Steinhardt, P.J., Yao, N. and Lu, P.J. (2011) Icosahedrite, Al₆₃Cu₂₄Fe₁₃, the first natural quasicrystal. American Mineralogist 96, 928—931.

Don’t trust your eyes

Ultramarine differs from other inorganic pigments in that it does not contain any transition metals. It is the sulfur species that confer the colour: S₃^•− (blue ultramarine), S₂^•− (yellow ultramarine) or S₄ (red ultramarine). Green ultramarine contains both S₂^•− and S₃^•− [1].

You’d think that by now they should know what the structure of ultramarine is. But no. In the Inorganic Crystal Structure Database I’ve found only two structures called “ultramarine” (ICSD 27523 and 27524), both associated with a paper from 1936 [2]. The composition of ICSD 27523 is given as Na₈Al₆Si₆O₂₄S_2.5·(H₂O)_0.6. The structure (see figure below) is an aluminosilicate cage containing sodium cations and beautiful octahedral sulfur clusters. Wait a minute. S₇ clusters? Never heard about those before.

A more recent structure of deuterated lazurite, Na_7.5Al₆Si₆O₂₄S_4.5·(D₂O)_0.5 (ICSD 63022), also featuring S₇ octahedra, provided an explanation. The authors [3] wrote that

to accommodate the indications emerging from the difference maps, the octahedral model of sulfur occupancy was modified to reproduce a more even density inside cage by adding a further sulfur to the hollow octahedral shell, with sulfur occupancies rescaled accordingly to maintain S₃ overall.

I feel relieved if slightly disappointed.

1. Landman, A.A. (2003) Aspects of solid-state chemistry of fly ash and ultramarine pigments. Doctoral Thesis, University of Pretoria.
2. Podschus, E., Hofmann, U. & Leschewski, K. (1936) Röntgenographische Strukturuntersuchung von Ultramarinblau und seinen Reaktionsprodukten. Zeitschrift für anorganische und allgemeine Chemie 228, 305—333.
3. Tarling, S.E., Barnes, P. and Klinowski, J. (1988) The structure and Si,Al distribution of the ultramarines. Acta Crystallographica B44, 128—135.

Selenite

1. In chemistry, selenite, [SeO₃]²⁻, is a diconjugate base of selenous acid, H₂SeO₃.
2. In mineralogy, selenite is a variety of gypsum, CaSO₄·2H₂O.
3. In science fiction, e.g. in The First Men in the Moon by H. G. Wells, the native inhabitants of the Moon are referred to as “Selenites”.

All three words are derived from Σελήνη, Greek for the Moon. Ironically, only fictitious Selenites have a “real” lunar connection.

(1)

(2)

Selenite (1) — not to be confused with selenate or selenide — is named similarly to other oxoanions of “ous” acids, such as sulfite or nitrite. The systematic name recommended by the Red Book is trioxidoselenate(2−). Now, Berzelius gave the element selenium its name by analogy with tellurium, which, in its turn, was named after Tellus, Latin for Earth. (Do you follow the logic?) The Mineral Information Institute gives an alternative explanation:

This is a reference to the silvery-gray color of metallic, non-crystalline selenium.

A similar line of thinking is responsible for naming of selenite (2):

From the Greek σελήυη, for “moon”, in allusion to the moon-like white reflections of the mineral or to the quality of the light transmitted by semi-pellucid gypsum slabs of cleavages used as windows.

Fine, but not as touching as this childhood belief:

When we were studying chemistry and the teacher talked about selenium, I thought that selenium was named after a Mexican pop star Selena who died during my childhood.

Speaking of Mexico: the world’s largest natural crystals, some as long as 11 meters, consist of selenite (2) and are found in Cueva de los Cristales in Chihuahua, Mexico.

Antarcticite

Antarcticite is a mineral form of calcium dichloride hexahydrate. It was first discovered in Don Juan Pond in Antarctica, which is probably the saltiest (47% w/v) body of water on earth. Looking at crystal structure of antarcticite (below), one can see that both name “calcium dichloride hexahydrate” and formula CaCl₂·6H₂O are misleading, for there are two kinds of water in it. The structure comprises the alternating layers of (i) trigonal planar triaquacalcium(2+) ions and (ii) water and chloride ions. I suppose it should be named “triaquacalcium dichloride—water (1/3)” or “triaquacalcium dichloride trihydrate”, with formula [Ca(OH₂)₃]Cl₂·3H₂O.