Thaumasite
Thaumasite is a soft, colourless mineral which forms fibrous or prismatic crystals and is primarily of interest to micromineral collectors.
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Information about Thaumasite
Thaumasite is a rare calcium silicate carbonate sulphate mineral and a member of the ettringite group – a chemically unusual family of minerals that are as well known to civil engineers and concrete chemists as they are to mineral collectors.
Thaumasite itself occupies a particularly unusual position in mineralogy, having been described as the first known natural carbonate sulphate ever discovered, and its chemistry remains genuinely singular even by the eccentric standards of the ettringite group.
It typically forms as elongated prismatic to acicular hexagonal crystals, often in radiating or sub-parallel groups, and also as fibrous masses and chalky encrusting material. It is typically colourless to white, occasionally very pale yellow, with a vitreous to silky lustre and transparent to translucent crystals.
Well-crystallised specimens from the N’Chwaning and Wessels mines in South Africa – the best collector locality by a considerable margin – can show doubly-terminated colourless crystals of extraordinary clarity, which have occasionally been faceted into collector gemstones despite the mineral’s low hardness. The specific gravity is unusually low for a mineral with such a heavy-sounding chemistry, reflecting its very high water content of twelve water molecules per formula unit.
Thaumasite forms in several quite different geological settings. In its natural mineral setting it occurs as a late-stage hydrothermal mineral in sulphide ore deposits, and through the low-temperature alteration of basalts and volcanic tuffs by circulating groundwater, typically alongside zeolites, calcite, apophyllite, and gypsum.
However, its most widespread and practically significant modern occurrence is as a destructive breakdown product of concrete – a setting very far removed from the collector’s cabinet and of considerable importance to civil engineering.
Uses and History
Thaumasite has no industrial applications as a mineral in its own right – quite the reverse, in fact: its formation in concrete structures is a recognised engineering hazard.
The mineral was first described in 1878 by the Finnish-Swedish mineralogist Nils Adolf Erik Nordenskiöld, from specimens collected at the Bjelke Mine at Åreskutan in Jämtland, Sweden – its type locality. The name comes from the Greek word thaumazein, meaning “to be surprised” or “to wonder” – a reference to Nordenskiöld’s astonishment at the mineral’s highly unusual chemistry, combining carbonate, sulphate, and silicate anions within a single structure in a combination previously unknown in nature.
As a destructive phase in concrete, thaumasite forms when Portland cement concrete is exposed to sulphate-bearing groundwater at low temperatures – typically below around 15°C – in the presence of calcium carbonate from limestone aggregate or filler.
Under these conditions the calcium silicate hydrate phases that give concrete its strength are progressively replaced by thaumasite, which has no binding properties whatsoever, eventually converting the concrete to a soft, white, structureless pulp. This process – known in civil engineering as the Thaumasite Form of Sulfate Attack, or TSA – has caused serious structural deterioration to roads, bridges, foundations, and retaining walls built on sulphate-bearing ground in the UK and elsewhere. A prominent UK example was the failure of sections of the A50 road in Staffordshire and the M5 motorway in Worcestershire in the 1990s, which prompted major government-funded investigations into TSA and significant revisions to concrete specification standards in the UK. The cool, wet, limestone-rich geology of Wales and the west of England makes these regions particularly susceptible to TSA, and the issue remains of active concern to the construction industry.
As a mineral, the finest collector specimens come from the N’Chwaning II and Wessels mines in the Kalahari Manganese Field of the Northern Cape Province of South Africa, where colourless doubly-terminated prismatic crystals of exceptional clarity – quite unlike the dull white material typical of most other localities – have been collected in limited quantities. The Swedish type locality at Bjelke, and the nearby Långban locality at Filipstad in Värmland, have produced the classic historical material.
Mineralogy
Hazards and Warnings
No specific health risks have been formally recorded for thaumasite as a mineral specimen. Mineral collectors should wash their hands after handling specimens as a matter of good practice.
Almost all rocks, minerals (and, frankly, almost all other substances on earth) can produce toxic dust when cutting, which can cause serious respiratory conditions including silicosis. When cutting or polishing rocks, minerals, shells, etc, all work should be done wet to minimise the dust, and a suitable respirator or extraction system should be used.
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