The Earth Story

The backbone of a rock When you walk outside and pick up a rock, what are you actually holding? When you lean against a brick wall, what are you leaning up against? The chemistry answer to this question is “mostly empty space” – most of the volume of every atom is empty space occupied by moving electrons, so from a chemistry perspective that answer is correct and adequate. Geologists, however, tend to look at bigger systems. We consider lots of atoms and how they behave in bulk, so the geologist would probably give a different answer. For a most solids on Earth, it turns out the geologist’s answer is “oxygen”.

The 2 crystals of quartz coming together at an angle just under 90 degrees is the Japan Law twin - one type of structural intergrowth you can get in quartz. The structure is disrupted along some plane in the crystal, but with a consistent offset that occurs naturally - Japan Law twins aren’t common, but they constantly show this arrangement and would consistently have the same angular and structural offset every time, from one grain to another.

Sulfur

This is a crystal of sulfur found at the 2015 Tucson Gem and Mineral Show. Most crystals of sulfur are formed related to volcanism, and they can commonly be found precipitated from volcanic gases in this setting. This one probably formed from one of the volcanic systems on Sicily, such as Mt. Etna.

The structure of sulfur is extremely complicated for such a simple mineral. The chemical formula is literally just “S”, but the structure involves sulfur bonded into highly distorted rings. The symmetry of the common sulfur, stable at low temperature and pressure, is orthorhombic, but heating it to only 127°C will cause its structure and symmetry to change to monoclinic. There are more than a dozen different structures of sulfur known, not counting high-pressure metallic phases, because of the complex arrangement of atoms in this mineral.

-JBB

Image credit: https://flic.kr/p/rer7SA

References: https://www.minerals.net/mineral/sulfur.aspx http://webmineral.com/data/Sulfur.shtml

The most complex mineral

This pretty little yellow mineral has just been named Ewingite, after Stanford professor of Nuclear Security Rodney C. Ewing. It was found in a closed Uranium mine in the Czech Republic – the same mine that provided material for Marie Curie’s experiments. Over the last century, water flowing through the mine reprocessed the now-exposed ore into new material like this little Ewingite. Although this crystal is small, X-ray data shows that in fact, Ewingite is the most complex mineral currently known to occur on Earth.

The next 2 images show the structure of the compound, obtained by scientists at the University of Notre Dame using X-ray techniques. Uranium and calcium atoms sit in a number of sites, with oxygen atoms and carbonate groups surrounding them. The full structure repeats around a large open cage, capable of bonding to OH groups. The full chemical formula of the mineral is Mg8Ca8(UO2)24(CO3)30O4(OH)12(H2O)138.

The mineral has a tetragonal symmetry, meaning there is a 4-fold symmetry axis in the unit cell. A single unit cell, the unit that is repeated to make up the full crystal structure, is a rectangular polyhedron 35.142Å x 35.142Å, x 47.974Å, where the unit cell of something simple like halite (simple table salt) has dimensions 5.6404 Åx 5.6404 Å x5.6404 Å (an Å is a unit of distance useful for things about the size of an atom, it is 10 to the -10 meters).

Apparently X-ray crystallographers categorize the complexity of materials based on how many “Bits of data” are required to record all of the positions in a single unit cell. The previous record holding mineral was paulingite, a zeolite, with 6766.99 bits of data required for one unit cell. Ewingite’s unit cell required 23,000 bits of data to record all of its positions, more than 3x as complex as the previous record holder. This is not shabby even compared to the most complex material humans have ever created - Al55.4Cu5.4Ta39.1, which required 48,538.63 bits of data to characterize its unit cell.

-JBB

Image credits and original paper: http://ntrda.me/2G93YRv http://bit.ly/2k8BDnB https://www.mindat.org/min-1804.html_ _

Spectacular work with the snowflake photos in this set. (CC licensed). 

One crystal, two colours, no magic...

The beauty of gems and crystals is an interaction between mind, mineral and light, and some minerals have structures that produce interesting optical effects, opal being an obvious example. The beautiful tanzanite crystal in the photo (see http://on.fb.me/1B8IMQy for an intro to this purple wonder) is exhibiting another of these properties, which goes under the name of pleochroism, a word coming from the Greek for many colours. A pleochroic crystal actually appears different colours depending on the angle of viewing, and some gems have a very mild version, while others, including tanzanite, a somewhat more extreme one.

Crystals are regular arrangements of atoms in a symmetrical repeating lattice, and while the number of individual patterns atoms can order themselves into is almost infinite, all minerals crystallise in one of 6 (or 7 depending how you divide things) crystal systems, based on the length of the axes of their unit cell (the smallest 'chunk' of a mineral that can exist) and their angles of intersection. One system is regular and even, known as cubic or isotropic, with equal length axes meeting at right angles, the others exhibit ever greater degrees of wonkiness. This distortion is what allows the gem to play with light in this way and delight our eyes.

As light enters the crystal and encounters the lattice, it flows through it down different pathways following the axes, with the rays splitting into two or three. Some directions will be more densely packed with atoms than others, and the light will move at different speeds through them, and be absorbed differently. Since the colour that we see is what is left behind when the crystal has selectively absorbed some of the wavelengths of the light passing through it, different colours can result in the different rays, which then become apparent when the crystal is rotated, or, as in this case, when a change in growth direction occurred.

Each pathway through the lattice also polarises the light, forcing it to vibrate in a single direction, and this property is the basis of the gadget gemmologists use to see this phenomenon. Two chunks of Polaroid plastic, orientated at right angles to each other are set in a circle and used to look at the gem. Each half of the Polaroid will show one colour. The property is also useful for distinguishing minerals in thin sections, those slices of rock beloved of geologists.

Loz.

Image credit: MIM Museum. http://www.allaboutgemstones.com/gemstone_pleochroism.html http://bit.ly/1CK45Jm http://webmineral.com/help/Pleochroism.shtml#.VSvMq5Om0bg http://www.galleries.com/minerals/property/pleochro.htm7

2014: International year of crystallography Originally at the core of mineralogy, it initially explored crystal form and angles, later moving on to examining optical properties in thin sections of rock in varied forms of polarised light, and then to xray diffraction patterns, which revealed the intimate nature of the building blocks of rocks to the geological world. As the 20th century advanced the discipline became equally central to the life (and congruent pharmaceutical) sciences, once researchers realised that many biochemical constituents of life, such as proteins and DNA, are also crystalline. From the examination of the arrangements of atoms in crystal structures and observation of their properties, prediction became possible, in much the same way that Mendeleev intuited many non discovered elements when constructing his periodic table of the elements. This practise is at the root of the modern pharmaceutical industry. This year has been internationally chosen to celebrate this discipline, so next time you gaze at a beautiful crystal, spare a thought for the toiling crystallographers slaving over a hot microscope or xray diffraction apparatus, and raise your beer to them... Loz Image credit of a lush aquamarine crystal with white albite feldspar and grey muscovite mica matrix from Pakistan 7.8 x 3.8 x 3.5 cm : Rob Lavinsky/iRocks.com Loz http://www.iycr2014.org/