Let me introduce you to Polly Halite. No, I’m kidding, it’s not a woman, it’s a chemical, usually spelled polyhalite. It’s been in the news lately because a company called Anglo American has provided an update on its plans to continue digging for it, 1,600 metres beneath the North York Moors.
Potted History
Anglo American’s Woodsmith Mine project has been struggling with viability for a very long time. I’m going to explore the reasons for this, which have roots in a story going back 250 million years. It involves extraction of minerals not just in North Yorkshire, but in neighbouring Teesside and across into County Durham, What the story won’t include is the history of Cleveland Ironstone and Durham Coal. Although these are important, they relate to different geological periods, Jurassic and Carboniferous, respectively. That story can be told another time. Our moment in time is the Permian period.
A tropical sea
Our tale begins during the Permian evaporite succession. Marine transgressions produced a sequence of evaporite deposits. These were laid on the floor of the vast Zechstein Sea (see map below) at a time when the UK was close to the Equator. They form part of a rock layer known simply as Zechstein.
Map licensed under the Creative CommonsAttribution-Share Alike 3.0 Unported license
A rich resource
The chemicals within the Zechstein formation have provided an incredible variety of mineral resources over the past 200 years or so and will continue well into the future. Here are some examples of the treasures that have been unearthed:
Magnesian Limestone
At the very edge of the warm, shallow Zechstein Sea, a compound of calcium magnesium carbonate, crystallised in massive quantities. This rock, commonly known as dolomite, developed into the 60-metre thick Raisby Formation, which can be found in southern County Durham. The minerals of the Raisby Formation have been exploited for centuries. Towards the west, around Ferryhill and Coxhoe, it is quarried for roadstone and refractory bricks. I described a recent visit to the area here. Over towards the coast, particularly in Hartlepool it was known as Magnesian Limestone, and was once used for building purposes. It’s a hard-wearing rock, and many of the town’s finest buildings are made from it. Here’s an example, All Saints Church in Stranton:
Take it with a pinch of salt
In 1859, Middlesbrough iron and steel company Bolckow, Vaughan & Co made a chance discovery. While boring for potable water for its Vulcan works, it struck an enormous seam of Zechstein evaporite in the form of halite (sodium chloride, also known as rock salt). Over the next century, numerous works opened across the Tees basin to exploit this salt, extracted in the form of brine. The saltworks were mainly centred on Port Clarence and Haverton Hill. By 1937, industrial giant Imperial Chemical Industries (ICI) had consolidated these works into its Salt Division. Most of the salt was shipped across to the nearby Cassel works for use as a feedstock in the manufacture of hydrochloric acid.
Anhydrite
Halite wasn’t the only gift to Teesside from the Zechstein layer. Anhydrite (anhydrous calcium sulphate) was first discovered and mined in Hartlepool in 1925. Two years later, larger deposits were discovered seven miles away under ICI’s Billingham works; A mine was sunk in 1927, which remained in production until 1971. 33 million tonnes of anhydrite was extracted from beneath Billingham. It was used primarily as a feedstock to produce sulphuric acid.
North Sea oil and gas
Zechstein formations trapped much of the oil and gas that has provided wealth from the North Sea for more than 60 years. They form the main cap rock to gas fields in the southern basin of the North Sea. Further north, off the east coast of Scotland, Zechstein salt domes form the structure for several oil fields, such as Machar.
Potash
Zechstein is the gift that keeps on giving. Potash (potassium chloride) was first discovered in 1939 at Aislaby, near Whitby, while prospectors were drilling for oil. Aislaby is 25 miles away from Billingham but, as we have seen, the Zechstein rock layer is extensive. So, no oil, but potash, a valuable fertilizer. However, the potash was at too great a depth to be economically mined at the time. But ICI was always on the hunt for fertilizers to supplement its products made from synthetic ammonia at Billingham. And the company had the resources to exploit the find.
The deepest mine in Europe
In 1968, ICI sunk the Boulby mine in a joint venture with mining specialists Anglo American, called Cleveland Potash Ltd. Anglo American would go on to buy the Woodsmith project 52 years later. The joint venture began to extract ore from a depth of up to 1,500 metres. Because of the depth of the mine, it is home to the only deep subterranean science facility in the UK – Boulby Underground Laboratory. The Boulby ore was a mix of potash and halite. The halite was considered a by-product, but it was shipped by rail to Teesside for use as road salt. Which it still is to this day.
From potash to polyhalite
Anglo American sold its stake in Cleveland Potash to Israeli Chemicals Ltd (ICL) in 2002. ICL has owned it ever since. In 2010, the Boulby miners uncovered a commercially recoverable seam of a super fertilizer, polyhalite. The Zechstein formations have delivered the goods yet again. Polyhalite extraction commenced in 2011, and It gradually replaced potash as reserves dwindled. Potash mining ceased in 2018.
My, that’s a big one
Surveyors had discovered polyhalite beneath Yorkshire in the 1930s, but its commercial potential was overlooked. Modern surveys revealed that the mineral that had been discovered at Boulby was part of a much bigger seam; approximately 2.5 billion tonnes of it. The world’s largest known deposit, right under the North York Moors. Mr Zechstein, with your polyhalite formations, you are really spoiling us.
Let’s get Sirius
In 2011, a company called Sirius Minerals plc formed the York Potash Project. The company prospected for the mineral between Whitby and Scarborough. Sirius claimed to have discovered a ‘world class’ potash deposit, enough to supply the UK for 50 years. This would be useful, because the nearby Boulby mine was running out of potash. Four years later, in 2015, the company announced plans to build a huge mine to exploit the potash formation. This was to become the Woodsmith mine.
Ball of Confusion
But a close inspection of the Sirius’ press release revealed that it wouldn’t be mining potash after all. Sirius had been seeking polyhalite all along, had found it, and was now going to exploit it. The London media was understandably confused by events far away in Yorkshire. The Independent reported on a find of polyhalite potash, something that doesn’t exist. The scope of Sirius’ ambitions was breathtaking. It would extract 20 million tonnes of product each year at its new mine, 20 times the amount being extracted at Boulby.
Going Underground
In 2017, the company switched its share listing from the Alternative Investment Market (AIM) to the London Stock Exchange (LSE) in order to raise capital. In the same year, Sirius started excavating two vertical shafts at Sneatonthorpe, near Whitby, to a depth of 1,600 metres, but then hit a snag. Unexpectedly poor ground conditions meant that the Woodsmith Mine Tunnel, the horizontal tunnel towards Teesside, would need to be increased in diameter by 0.6 m, increasing the project budget by £600mn.
An all-too-familiar pattern
The strain on Sirius cash flow showed in September 2019, when the company shelved a £400mn junk bond sale, This was the first of what was to become a pattern with investments in the Woodsmith mine, and one which repeats itself to this day. Four months later, Sirius ran out of cash, and its share price collapsed. The company was bought by Anglo American for £400mn in March 2020. So, 18 years after selling one North Yorkshire mine, the company had bought another one 12 miles away.
So now we’re back on track?
Not quite. In March 2023, less than three years after acquiring Woodsmith, Anglo American had to book a $1.7 billion writedown on its investment. In December the same year, the company confirmed that it was looking to offload its 49% stake in the mine. Things were sinking faster than a mineshaft. In February 2024, Anglo American told investors that it was considering putting the project on hold. It put these plans into action three months later. Construction would slow almost to a halt for the foreseeable future.
Slash and burn
Woodsmith Mine had been burning cash at the rate of more than £2mn per day, and something had to give. Anglo American’s capital investment programme had envisaged a spend of £800mn per annum in 2025 and 2026. The 2025 investment was cut to £160mn, a fifth of its previous planned level. For 2026, it was slashed to zero. By the beginning of 2025, 900 of the 1,600 employees had been laid off, and a target opening date of 2027 was abandoned. It would be 2030 at the earliest now.
Copy and paste
So, fast forward to 18 February 2026 and a curious article by Felicity Collinson, Teesside Live’s Head of News.
Collinson reported that Anglo American’s product, POLY4, reportedly could improve crop yield by up to 5% compared to conventional fertilizer. But was this news? Six years earlier, Anglo American’s rival company, ICL, reported that its product, Polysulphate, could put POLY4 in the shade, increasing crop yields by 30%.
Collinson’s text continued “An under-construction North Yorkshire firm says it is on track to mine crop-boosting fertiliser.” It’s a stretch to say that the firm is “on track” when it has entirely nixed this year’s capital spend.
Double take
That sounds impressive. It’s even more impressive when you realize that Anglo American’s POLY4, and ICL’s Polysulphate are exactly the same thing, polyhalite. POLY4 and Polysulphate are both 100% natural and 100% branding.
Earnings call
The reason for Anglo American’s puff-piece press release came two days later, on 20 February 2026. During the company’s quarterly earnings call, CEO Duncan Wanblad announced that Mitsubishi Finance was considering the purchase of 25% of the company’s stock. This would be conditional on the outcome of an ongoing feasibility study on the Woodsmith mine. Will Mitsubishi be impressed by an article in a local Middlesbrough paper, talking about crop yield improvements that have been known about for a century? Only time will tell. Wanblad also confirmed that his tunnellers had encountered higher water levels than expected, and this had slowed progress.
So, why has Boulby succeeded, and Woodsmith probably never will?
There are three key factors. The first is the sheer amount of capital investment needed to sink a mine of this magnitude. It took ICI sixteen years, from 1968 to 1984, to turn a profit on Boulby. But back then ICI was one of the wealthiest companies in the UK. It didn’t have to keep coming back to investors with cash calls. The Woodsmith project has haemorrhaged cash from day one.
Go by rail
But the second barrier to the future success of Woodsmith is the cost of transport. One of ICI’s key reasons for locating at Boulby was its potential rail connection to the deep water Port of Tees. The mine was situated on the track bed of the coastal Middlesbrough to Whitby railway. The track had been removed in 1964. British Rail reinstated it ten years later, At the same time Cleveland Potash built two dedicated rail terminals in Teesside. These were at Middlesbrough, for rock salt, and Tees Dock, for potash. Both terminals are still in use. Here’s Middlesbrough goods yard in February 2026, with rock salt heaped on the right, and ‘Cleveland Potash’ wagons in the distance, waiting to return to Boulby.
Woodsmith’s logistical nightmare
When it was first conceived, the planners of Woodsmith mine had considered a rail connection. The railway track between Whitby and Scarborough closed in 1964, but a short section at Hawsker was kept open as a potential railhead for a new mine. But the massive volumes being planned would have overwhelmed the fragile local railway network, and this idea was dropped.
So what’s plan B?
As with Boulby, the Woodsmith mine needs to connect to the River Tees. The next option considered was a pipeline. This would have involved dissolving the polyhalite in water, forming a slurry, then pumping the solution 23 miles to the final destination, Redcar Bulk Terminal (RBT). It would then have been processed before loading it aboard ships. But the pipeline would have been overground, and would have bisected the North York Moors National Park. It was out of the question.
So now we’re down to Plan C
Plan C was the final, chosen plan. This involved the 23-mile Woodsmith Mine Tunnel from the mine to a Materials Handling Facility (MHF) at Wilton International in Teesside. The MHF will remove impurities from the polyhalite to produce the finished article. A narrow-gauge railway will run the entire length of the tunnel to permit maintenance. The tunnel is shown on the map below. Woodsmith Mine is at bottom right, and the MHF and RBT are at top left.
Conveyor belts
Polyhalite will be carried on three separate conveyor belts to the final destination at RBT. The first belt will stretch from the mine to Lockwood, about 15 miles away. Material will then be switched to a second conveyor belt for the remaining eight miles to the MHF. The third conveyor belt will take the dried out product on a 2.5 mile overhead journey from the MHF to the RBT. This will involve crossing the A1085 Trunk Road, a railway line and a pipeline network.
Expensive and complicated
So, the polyhalite from Woodsmith will face an expensive and complicated journey to arrive at almost exactly the same point as the product from Boulby. And it will face the same production costs. Heavy machinery operating at a depth of 1,600 m, sometimes way out under the North Sea. The temperature at the Boulby cutting face is 40°C. It’ll be similar at Woodsmith. You have to pay people well to work in conditions like that.
More about polyhalite
Polyhalite is a complex chemical (K2Ca2Mg(SO4)4⋅2(H2O)) that contains four out of six of the essential plant nutrients. Polyhalite’s chemical formula has created a valuable natural fertilizer. It contains, in descending order: Sulphur (48%), Calcium (17%), Potassium (14%) and Magnesium (6%). Impurities make up the remaining 15%. ICL markets its polyhalite under the brand name ‘Polysulphate’, to emphasize its sulphur content.
But polyhalite is not perfect
ICL’s would-be rival, Anglo American, claims that polyhalite is revolutionising crop nutrition. But that’s an overstatement. It doesn’t contain essential phosphorus in the form of phosphate.. And, more importantly there’s no nitrogen, the cornerstone of any nutritious fertiliser.
You badly need your nitrogen fix
If you don’t have sufficient quantities of nitrogen in your soil, you’re always going to struggle. Prior to the large-scale availability of fertilisers, farmers needed to rotate their crops to avoid nitrogen depletion. Once every few years, each field was left fallow for a whole year, to allow the soil to recover as nitrogen was fixed by microorganisms.
Can farmers manage without polyhalite?
Well UK farmers can – they hardly use it. As can be seen in the chart below, they use a great deal of nitrogen and phosphate, neither of which are present in polyhalite. It does contain potassium and sulphur, but farmers can obtain these separately; potassium from potash, and sulphur directly. All without blitzing the soil with expensive polyhalite.
But it sells well abroad, right?
Yes, ICL has developed a niche market for polyhalite, and sells a million tonnes each year to countries where the soil isn’t as bountiful as in the UK. Markets include China and Spain, and the product is sent to ports in the Netherlands and Belgium for onward land transport across Europe. Here are details of the ‘Western Boheme’ setting forth from Teesside with 20,000 tonnes of polyhalite, bound for Brazil.
But this is at a different scale
Anglo American has to sell 20 times more polyhalite than ICL, into a relatively underdeveloped market. Both companies market polyhalite as a sustainable product. It requires little processing after being mined, apart from grinding out those impurities. But you need to take into account: Cutting the product from a rock face 1,600 m underground, trucking it perhaps ten miles to a loading point, transporting it on two conveyors, processing it at the MHF, transporting it on to a third conveyor, loading on to a ship, transporting it 5,000 miles across the ocean, unloading it, then trucking in 500 miles to a farm in Brazil. You’ve got to question that sustainability claim. Then think how much all of this costs. Woodsmith will need to generate serious cash flow.
Waiting for a miracle
If Mitsubishi injects its much-needed capital, will this get Woodsmith over the line? That’s touch and go. But Anglo American knows that, if and when the mine starts production, the polyhalite market will need to expand at an almost exponential rate. If it does, the final chapter is yet to be written in the story of that abundant Zechstein inheritance. One that has been 250 million years in the making.
