This month’s blog from our Community Geologist, Dr Ian Kille, discusses geological families.
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Christmas plans are in place and, despite the coronavirus demonstrating once more that evolution is very real, I will cautiously be heading down to stay with my parents. They still live in the same house I was brought up in. So for Christmas I will be returning to the chalk, with a layer of London Clay over the top. My brother lives not too far away from my parents, still on the chalk but with gravels from the River Thames covering the London Clay and the chalk deeper beneath his feet. My family’s next generation down are more scattered. My youngest son lives above Triassic sandstones of the Chester Formation, with the back of his house mantled in glacial till and the front in river-gravels from the River Irwell. My elder son is above conglomerates from the Helsby Formation also in the Triassic Period. His elevated position means that there is little except a thin layer of organic matter between him and the rock. In my immediate family it seems that I am the only one who has chosen to live on old volcanic rock, as I live above Devonian andesites from the Cheviot volcano, mantled with a fork-breaking layer of fluvio-glacial cobbles.
Many years ago, when I had just started venturing into the intersection between geology and archaeology, I gave a talk on geology and archaeology in Berwick. At the end I was asked a singularly penetrating question about how much I thought that the geology of a landscape influenced the development of culture. The questioner was a certain Lindsay Allason-Jones. At this point I was blissfully unaware of her illustrious career in the world of Roman antiquity, and to this day wonder at just how inadequate my attempt at answering this question was. It is, however, a question that has stuck in my mind, and it returned to me when writing the introduction to this piece about families and geology. I wondered whether the chosen locations for my family might reflect something of our differing cultural values, with the builder in the family closest to solid rock and our family’s geologist closest to volcanic rock (the chosen specialism of my research). This could be a great game to play over Christmas, it’s easy to find your underlying geology by using the BGS geology app: http://mapapps.bgs.ac.uk/geologyofbritain/home.html Though, thinking about it, it is probably only for those who would want to intersperse their Christmas games with watching back episodes of Star Trek and the Big Bang Theory.
There are many other great family games that can be played by geologists, such as Mine-a-Million, home-made Rock Dominoes, Mappa Mundi with added plate tectonics and an all-time classic, the geologists’ version of rock-paper-scissors. Another sort of game was brought to mind when I was writing a presentation about the history and pre-history of the stones used in Hadrian’s Wall. The presentation was put together from the point of view of a grain of quartz, a mineral which is almost indestructible, despite travelling great distances and being knocked about a great deal. It seemed to me that this was similar in character to Tyrian Lannister in the Wall-related series Game of Thrones, which sees him survive intact through to the end. This led me in turn to observing that quartz has its own family or rather a set of families. So begins the Game of Stones; though to be honest it’s more like a geological version of ancestry than a game.
Quartz is made of silicon and oxygen bonded into an interlocking framework of tetrahedra. Silicon, like its close elemental relative Carbon, is remarkable in its ability to combine with other elements to produce a vast array of compounds. Carbon is the master of this in the biological world, but silicon has the edge in the mineral world. The silica tetrahedra – a silicon atom surrounded by 4 oxygen atoms and looking similar to one of the jacks from the old fashioned game of Jacks – is the building block which is used to make the dynasty of silicate minerals. The different ways the tetrahedra combine create distinct structures which define the many different silicate families. The tetrahedra may be isolated (Nesosilicates) and sometimes combine in pairs (Sorosilicates). They also make rings (Cyclosilicates) single and double chains (Inosilicates) and sheets (Phyllosilicates). They also make three-dimensional frameworks (Tectosilicates). Within each of these families, these familial structures combine with numerous other elements to create huge numbers of different silicate minerals. I feel certain that with careful use of coloured paper, glue and infinite patience that an absolutely fabulous set of these silicate minerals could be reconstructed using paper chains, to make the most original, brightest and best Christmas decorations ever devised.
This month’s Mystery Rock (number 21) for the Hadrian’s Wall Archaeology project is one of the silicate dynasties. Feldspars along with quartz are part of the tectosilicate family. These alkali-feldspar crystals are in a piece of Shap Granite. Shap is a distinctive granite, with a matrix of coarse crystals of various silicates along with these much larger feldspar megacrysts. There are dozens of different types of feldspar defined by the relative amounts of sodium, potassium and calcium bonded within their three-dimensional structure. More importantly, many of these feldspars are beautiful. For example, labradorite, a calcium-rich feldspar, glows with iridescent hues of deep blue, green and silver. Its cousin Orthoclase, a potassium-rich feldspar, glows with the milky iridescence of the moon and unsurprisingly is known as moonstone.
The other families can claim their beauties too. Quartz, another, tectosilicate, is one of my favourites, forming hexagonal prismatic crystals which interlock in fabulous modernist forms, and glint with a brightness that reflects how hard they are. With names like clear, milky, smoky, citrine, rose, amethyst they give hints of their qualities. Quartz also mixes with other minerals to produce jasper, sunstone, moss-agate and another of my favourites, tiger’s-eye, all of which will be familiar as semi-precious stones.
The cyclosilicates are particularly exotic. Tourmaline is one of these ring-structured minerals. Commonly it is lustrous black and known as schorl, but sometimes it comes in bi-coloured crystals, lollipop-like in pink and green. Then there is Beryl, though this Beryl doesn’t have a stripey top, is not a peril and is indifferent to the smell of paint (cf. Katherine Mansfield). However, it not only has a ring structure but ends up literally on a ring in the form of emerald and aquamarine.
The neosilicates with their isolated tetrahedra also make their appearance on rings. Precious olivine, known as peridot is a mossy-green colour. Garnets, most commonly in a mulled-wine purple, are found in less expensive jewellery. Zircon, harder than quartz and more lustrous than diamond, comes in many colours. When I visited Ratnapura, the gem capital of Sri Lanka, many years ago, zircon was the fake gem of choice to pass off as its more expensive cousin emerald and unrelated grandee, ruby.
This is just a taster of the assorted bling which the silicate syndicate has to offer. I’m sure there is a market out there for a genealogy equivalent website for silicates – findmysilicate.com, mysilicon.com or silicatry.com – as there is still so much more to explore. However, for now, I think that all of the Christmas bases have been covered with family and games and many a brightly coloured things. Time to settle into a repeat of the Christmas Repair Shop and contemplate the ancient lava flows beneath me and pour myself another glass of that mulled wine.
A very Happy Christmas to you all and all good wishes for a fulfilling New Year exploring your landscapes wherever you are.