SEDIMENTARY ROCKS/Evaporites 95 evaporation After halite has precipitated, the next mineral to appear is carnallite (hydrous magnesium and potassium chloride) or sylvite (potassium chloride); usually there are no magnesium sulphates The uppermost parts of some deposits, especially from the Atlantic-marginal Cretaceous salt basins of Brazil and Gabon, also contain the mineral tachyhydrite (hydrous calcium chloride) This highly soluble salt (which dissolves in atmospheric moisture) must represent the final evaporative stages of a brine However, if this brine were concentrated seawater, then all the available calcium should have been extracted at a much earlier evaporative stage, during gypsum and early-stage halite precipitation Sylvite should not precipitate during simple seawater evaporation Some sylvite can be explained as a later alteration product of carnallite, but in other cases textural evidence indicates that sylvite is a primary mineral The absence of magnesium sulphates cannot be explained by later diagenetic changes (for instance, converting them to carnallite) because textural evidence also suggests that much carnallite is primary The Missing Sulphate The brines that precipitated potash deposits low in magnesium sulphate did not lack magnesium (carnallite and bischoffite contain this element), and so the problem is to explain why marine-derived evaporites are impoverished in sulphate in their later evaporative stages Explanations of this missing marine-derived sulphate are unsatisfactory One hypothesis suggests that sulphate is removed from the brine by the addition of river water containing additional calcium This calcium strips the brine of its remaining sulphate by the precipitation of additional gypsum Simple calculations indicate that the amount of river water needed would be enormous and more than enough to dilute the brines so that no evaporites would form in the first place A commonly proposed explanation is that sulphate is removed by the activity of sulphate-reducing bacteria The sulphate is reduced to hydrogen sulphide, which is then lost to the atmosphere Hite convincingly argued, by analogy with Holocene environments, that sulphate reduction would be confined to the uppermost metre of sediment, and so the sulphatereducing capabilities of evaporite basins would be limited His calculations showed that bacteria would be unable to remove all the seawater sulphate and, furthermore, that the amount of organic carbon required to reduce the marine sulphate could not be supplied, even if evaporite basins were extraordinarily productive The problem of the missing sulphate exists only if seawater was the original feedstock It is clear from the presence of tachyhydrite in some sequences that either the seawater was substantially modified or some other water was evaporated to generate the potash salts In essence, no brine containing more sulphate than calcium (including all modern seawater-derived brines) can generate tachyhydrite and evaporites deficient in magnesium sulphate, whereas these sequences can be generated if waters are used where more calcium than sulphate is present By definition, any concentrated water that has excess calcium (calcium > sulphate ỵ bicarbonate) is a calcium chloride brine Hardie believed that evaporites that are impoverished in magnesium sulphate formed by the evaporation of calcium chloride brines that were generated in rift or transtensional basins – basins with high heat flows and active hydrothermal groundwater circulations Calcium chloride brines are being expelled today in these types of basin Groundwater circulates deep in the crust and reacts with hot host rocks Hydration of host-rock minerals removes water, concentrating the groundwater into a brine The hot brine reacts with calcium-bearing minerals, commonly feldspars (exchanging sodium for calcium), and some of the expelled calcium reacts with any groundwater sulphate to precipitate anhydrite, thus stripping the brine of its contained sulphate The final product is a brine that is depleted in sulphate and enriched in calcium When heated, these brines become buoyant and may be expelled to the surface, where they evaporate Evaporites as Hydrothermal Deposits in Rift Basins Rift and transtensional basins are ideal locations for evaporites to form Commonly they are isolated or have very restricted access to the ocean Even if they are not located in arid climatic zones, they may develop evaporites by virtue of uplifted rift shoulders or transpression ridges causing orographic aridity Calcium chloride waters entering such basins as hot springs can evaporate and generate evaporite sequences without magnesium sulphate minerals Many evaporites deficient in magnesium sulphate are located in present-day and ancient rift basins When hydrothermal waters are the only feedstock, the resultant evaporites may be entirely deficient in calcium and other sulphates Evaporite sequences should have little, if any, gypsum and anhydrite It is also possible, however, that the main water in the basin is seawater but that this is substantially modified by the addition of relatively small volumes of