The story of strontium

An explosive start, a volcanic interlude, a learned Scottish cleric, an English genius, and a couple of Bronze Age skeletons.

An updated version of this story can be found here.

It was the road sign that got me intrigued: “Strontian: the village that gave its name to the element Strontium”. This is a village in Ardnamurchan with just a few hundred people: although it has both a post office and secondary school, which helps to make it a focal point for a wider area, it’s not even close to being a small town. And yet it has this unusual claim to scientific fame.

Strontian: "the village that gave it's name to the element Strontium"

The road sign approaching Strontian

This set me on a trail, as I wanted to know how the element came to be specifically there in the first place – and how it was recognised to be a new element as opposed to another previously known one. I have found it a fascinating quest – but telling the tale requires starting at the beginning, because – as all astronomers know – without the stars there would be no strontium in the first place…

An explosive birth

Supernova in the nearby galaxy M95. It's only during these events that elements heavier than iron - like gold, lead or strontium - are formed.

Supernova in the nearby galaxy M95 (circled). It’s only during these events that elements heavier than iron – like gold, lead or strontium – are formed. Image from here.

The story starts about five billion years ago, in the glowing gases of the nebula out of which the Sun would eventually be formed. It is likely that the critical event was a supernova explosion: this is when a massive star, many times the size of the Sun, blows up. All of the naturally occurring elements which are heavier than iron are formed only within the spectacularly intense heat of a supernova explosion – and then scattered far and wide into the surrounding nebula of glowing gas. Strontium is one of the elements that would have formed. By sending shock waves through the nebula, the explosion may also have been the trigger that led to the formation of the Sun and other stars.

However, much as this might explain how strontium got formed, I still wanted to know how it ended up specifically on the Ardnamurchan peninsula.

When Scotland flowed with volcanic lava

The key era was around 400 million years ago, which geologists term the ‘Devonian’: it was a time when trilobites scuttled along the ocean floor; giant ferns dominated the land; and the earliest insects began to appear.

Scotland was a very different place: it was located south of the equator, attached to what is now Scandinavia, and was volcanically active. During this era, the Caledonian mountains were built up, which would have looked much more alpine than they do now.

One of the multitudinous events that took place then was that a crack appeared in the Earth’s crust near where Strontian is presently located. Into it seeped searingly-hot molten magma from the Earth’s mantle. There’s nothing geologically unusual about this – except for its part in this tale.

The centre of the photo shows the Whitesmith mine near Strontian, right on the long but narrow rock seam formed about 400m years ago (the Devonoan era). Image by David McCallum from http://www.mineraltown.com/Reports/7/7.php?idioma=2.

The centre of the photo shows the Whitesmith mine near Strontian, right on the long but narrow rock seam formed about 400m years ago (the Devonian era). Image by David McCallum from here.

When hot molten magma cools, different elements solidify at different temperatures: they will either sink within the cooling magma, or stick to the walls of the crack. Strontium has a melting point of 769K, but in its molecular form, strontium carbonate (which became known as ‘strontianite’), it solidifies at 1050K. Other elements like lead or tin, which solidify at much lower temperatures, rise to the top.

The Caledonian mountains have long since ceased being volcanic. Instead, a much more recent process has been important: the Ice Ages had a dramatic effect, with glaciation eroding the mountains down until what remains now are just the cores of the old, Alpine-like ranges.

Searching for minerals in the wilds

In the early eighteenth century, the first glimmerings of the Industrial Revolution were appearing. Thomas Newcomen had pioneered the first steam engine – but it would be another fifty years before James Watt’s historic improvements. Ardnamurchan – a peninsula south-west of Fort William which was easier to reach by boat than by land – was reputed to be a wild and lawless place, but it had geological riches that promised much for mineral speculators.

In the hills above Strontian, a strip of rock was discovered from which lead ore could be extracted. In 1729 a Royal Charter was obtained to work the mines, but over the following years the mines had a long, turbulent and unproductive history: there was never enough lead ore, or any other economically important mineral, to turn a regular profit; the management was usually ineffective. Moreover, the owner had miners imported from northern England which exacerbated conflict with the Highlanders. In 1760 the mines experienced the first of several closures, and became derelict for a time.

Liechtenstein stamp from 1994 featuring Strontianite.

Liechtenstein stamp from 1994 featuring Strontianite.

Four years later, the Rev John Walker, a minister from Moffat, was asked by a group of three organisations, including his employers the Church of Scotland, to visit the western Highlands, to assess their ‘moral and physical condition’. Walker was not a gifted pastor, but he was already developing a reputation as a brilliant scientist, achieving prominence in the nascent fields of botany, geology and chemistry. He would later become Professor of Natural History at Edinburgh, achieving an international reputation, and inspiring hundreds of students from many countries.

This trip in 1764, the first of several over the following years, enabled him to combine his work with his hobbies. His report on the economy of the area was thorough, articulating both the need and potential for developments in agriculture and education – but the tour also gave him the chance to continue to build up his growing collection of mineral specimens. When he visited the Whitesmith mine above Strontian, his skills meant he was uniquely equipped to be able to recognise the new mineral that would later be called strontianite. His own chemical analysis convinced him that it contained ‘a new earth’, but it would require others to establish that this was indeed the case.

An elementary genius

The eighteenth century was a time when the science of chemistry was beginning to develop as an academic discipline. In particular it was believed that different minerals might be composed of basic elements, each element conveying distinct properties to the whole. A key task therefore was to be able to identify and isolate these elements. The first breakthrough was the separation of oxygen by Joseph Priestley in 1774.

It was some time before Walker’s new mineral could be adequately analysed. For a while, it was thought to be the same as another mineral from the north Pennines, called witherite, which was visually very similar. On this basis a sample was analysed by an Irish physician called Adair Crawford. He was a meticulous scientist, particularly known for his work on animal heat. In 1790 he established that the mineral from Strontian – which he named strontianite – was distinct from witherite. He deduced that there was probably a different chemical element involved.

Doing a flame test distinguishes clearly between the strontium (red) and the barium (yellowish green). Original images here.

Around the same time, Thomas Hope – a former student of John Walker – was doing a more thorough chemical analysis, to give a much clearer indication of what the ‘new earth’ might be. He found that the strontianite and witherite were virtually identical in their chemical reactions, but that the strontianite was about 15% lighter. A more marked distinction was that strontianite burned with a bright red colour, whereas the witherite burned with a faint greenish hue.

What neither scientist was able to do, though, was to separate either of the two earths from the rest of the minerals in which they were found. This was achieved later by the man who did so much to develop and shape the early science of chemistry – Humphrey Davy.

Sir Humphrey Davy

Sir Humphrey Davy by John Linnell.
UK Government Art Collection 2010, licensed under the Open Government Licence.

Davy was a brilliant experimental scientist who acquired celebrity status because of his gift for dramatic presentations of his scientific discoveries. He was particularly well known for his invention of a safe miner’s lamp, but he also developed the technique of electrolysis, which was a major breakthrough in chemistry.

Davy’s insight was that the newly discovered phenomenon of electricity might have something to do with the way elements combine and therefore how chemicals form. His conviction was that electrolysis could therefore be used to separate these elements.

By 1808, he had already used this technique to identify several new elements, such as sodium, calcium and potassium. However, extracting the elements from witherite and strontianite proved difficult. He tried various methods to induce the elements to separate – finding, for example, that combining each of the minerals with ‘red oxide of mercury’ facilitated the process. However, once they formed, they reacted quickly with the oxygen in the air, and violently when in water, reverting back to their original state: sulphuric acid was a better medium. Eventually he was able to produce a metallic sample in sulphuric acid, which sank without reacting. Once he accomplished this, he recognised the need to name the new elements: that from strontianite became strontium, and that from witherite was called barium.

Deciphering the Bronze Age

The underlying reason why strontianite had been confused with witherite was that the key elements of each mineral (strontium and barium respectively) are chemically very similar. They are now known to belong to a group of elements, along with calcium, magnesium and radium, in which each behave in a like manner.

This product utilises the chemical similarities between calcium and strontium - whether it is effective, I have no idea.

This product utilises the chemical similarities between calcium and strontium – whether it is effective, I have no idea.

Thus, if strontium is absorbed into the human body, it is processed as if it were calcium: in human bones there is one strontium atom to every 1500 calcium atoms. This is the key to its technological usage: its similarity to calcium means that it is not a lucrative mineral to mine, but it does have medical and archaeological benefits. For example, strontium-enriched bones seem to be stronger than normal bones, which provides a method of treatment for those with bone diseases such as cancer.

Like the other 94 naturally occurring elements,  strontium is defined by the number of protons in its nucleus: in this case, 38 protons. However, there are different varieties (or isotopes) of strontium, defined by the numbers of neutrons in their nuclei: for example, the most abundant of these is called strontium-88, because it has 50 neutrons in addition to its 38 protons.

The four varieties that are found in nature are all stable. However, there is another version, strontium-90, which is a product of nuclear weapons testing – and is itself highly radioactive, decaying with a half-life of 29 years. As it is easily absorbed into the human body, it is a major reason for the incidence of bone cancer in regions with significant nuclear fallout.

The different varieties of strontium come in varying proportions, depending on location. This is reflected in the bones and teeth of both humans and animals, which absorb the strontium irrespective of the variety. This therefore provides archaeologists with an important tool, because bones and teeth from ancient human skeletons can be analysed to find out where they originated.

Archaeologists have used strontium analysis on Bronze Age skeletons found near Stonehenge to show just how mobile people were at that time.

Archaeologists have used strontium analysis on Bronze Age skeletons found near Stonehenge to show just how mobile people were at that time.

This is dramatically illustrated by two that were found within a few miles of Stonehenge. For example, there was a teenage boy with an amber necklace, buried 3,500 years ago – the unusual ornament indicating a high rank. Analysis of his teeth showed that he originated from the Mediterranean. Another skeleton, from 4,300 years ago, hailed from the Alpine foothills of Germany. This shows just how mobile people were in the early Bronze age – and that Stonehenge was probably significant to people across Europe, and not just within Britain.

The almost-uniqueness of Strontian

Strontianite on display in the Tourist Information Centre at Strontian

Strontianite on display in the Tourist Information Centre at Strontian

On the Swedish island of Rosaro, there is a village called Ytterby. At a nearby mine, the mineral yttria was discovered from which no fewer than four elements were discovered, between 1794 and 1878: yttrium, ytterbium, terbium and erbium. This is the only other settlement which has had a naturally occurring element named after it. (Germany, after which germanium is named, is a country, not a settlement!) Strontian’s distinction, while not quite unique, is nevertheless sufficiently unusual that it is surprising that not more is made of it. Even in the village itself it deserves a bit more acknowledgement than a road sign: strontium may not be a well-known element, but its story would certainly grace a museum in Strontian!

4 thoughts on “The story of strontium

  1. We have just returned from a holiday in Strontian and like you we were disappointed to find so
    little was made of the chemical connection. The tourist information centre have a small display
    but no leaflets. What they need are printed copies of your wonderful blog.
    It was a coincidence that found us lodged in Moffat on the way home. There’s plenty of stuff on
    J.Walker about there not referring to the Rev John I think.
    Thanks for your blog. Next year we may holiday in Ytterby.

    • Thanks Roger for your kind comments – much appreciated. How interesting that you went to Moffat on the way home. I’d be interested to hear about Ytterby – and especially if they do better with tourist information! Blessings, Rich

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