Evaporite Minerals

The meltwater absorbed by stony meteorites also causes weathering which manifests itself by the formation of secondary minerals, by alteration of the isotopic composition of certain elements, and by the addition of halogens and similar compounds that are deposited by snow and from the air (Gooding 1986a, b, 1989 Lipschutz 1982, 1986 Velbel 1988 lull et al. 1988 Velbel and Gooding 1990). 

The stony meteorites collected during the 1977/78 field season on the icefields adjacent to the Allan Hills included seven specimens that contain deposits of white efflorescences on their surfaces and in cracks. Marvin (1980) reported that these deposits were composed of the hydrated carbonates and snlfates of magnesium and calcium listed in Table 18.6. In spite of the presence of these deposits, only two of the seven specimens have rust stains. The absence of heavy rust stains is difficult to reconcile with the prevalence of Mg-salts 

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The efficiency of the weathering of rocks in using carbonic acid produced in the carbon cycle is affected by various hydrologic, environmental, and cultural controls. The fact that the principal anion in fresh surface water worldwide almost always is bicarbonate attests to the overriding importance of this process. Exceptions are systems in which evaporite minerals are available for dissolution by groundwater or where human activities are major sources of sulfate or chloride inflow. 

Calcareous minerals and evaporite minerals (haUdes, gypsum) are very soluble and dissolve rapidly and, in general, congmendy, ie, yielding upon dissolution the same stoichiometric proportions in the solution as the proportions in the dissolving mineral and without forming new soHd phases (Fig. 

Evaporite deposition is a much more episodic process and thus difficult to quantify. Because seawater is significantly undersaturated with respect to common evaporitic minerals, like gypsum and halite, evaporites are only formed when restricted circulation develops in an ocean basin in which evaporation exceeds precipitation. A geologically recent example is the Mediterranean Sea of 5-6 Myr ago. At this time excess evaporation exceeded the supply of ocean water through shallow inlet(s) from the Atlantic Ocean. As salinity increased, first CaS04, then NaCl precipitated. Over time, salt deposits 2-3 km thick formed. This thickness represents about 40 desiccations of the entire... 

Fig. 2.6. Example of a flow-through path. Titrating water from a unit volume of seawater increases the seawater s salinity until evaporite minerals form. The product minerals sink to the sea floor, where they are isolated from further reaction.

Since the experimental studies of van t Hoff at the turn of the century, geochemists have sought a quantitative basis for describing the chemical evolution of seawater and other complex natural waters, including the minerals that precipitate from them, as they evaporate. The interest has stemmed in large part from a desire to understand the origins of ancient deposits of evaporite minerals, a goal that remains mostly unfulfilled (Hardie, 1991). 

White and yellow evaporite minerals form thin crusts on the surface of and within fissures in the tailings. XRD analyses identified the hydrated iron sulphates, melanterite (FeS04-7H20), Zn-melanterite ((Zn,Fe)S04-7H20),... 

Metals are temporarily attenuated as evaporite minerals on the surface or in secondary oxides and hydroxides in the tailings. The evaporites will re-dissolve in wet weather conditions and the secondary minerals become unstable with acidification of the tailings releasing these metals into the environment. 

Evaporites are important sources of economic minerals that have been exploited for at least the past 6000y. For example, the evaporite mineral trona (NaHCOj -Na2C03 2H2O) was used by the ancient Egyptians to preserve mummies. Evaporite salts continue to be used for food preservation, construction, road deicing, and in industrial processes. The marine evaporites of Saskatchewan (Canada) are the world s largest source of potash (KCl), which is used as an agricultural fertilizer. In the United States,... 

Diagenesis and catagenesis can alter the evaporite minerals after burial. For example, high temperatures, pressures, and pore-water salinities characteristic of deep burial lead to the conversion of gypsum into anhydrite. Thus, evaporite mineralogy reflects not only the environmental conditions under which the evaporite was formed, but also those under which diagenesis and catagenesis occurred. 

In this chapter, we consider the mineral composition of the hydrogenous minerals and how they fitrm. The evaporite minerals have already been covered in Chapter 17. The hydrothermal minerals (polymetallic sulfides) are discussed further in Chapter 19. 

Chlorine is released as HCl, which dissociates upon dissolution in water to generate Cl (aq). Sulfur is released as either H2S or SO2. Both are transformed into S04(aq) through chemical reactions involving oxidation by O2 and dissociation/dissolution in water. The amounts of primary magmatic volatiles that have been degassed thus far are given in Table 21.5. About half of the chlorine has been retained in the ocean and the other half has been converted into evaporite minerals. In comparison, virtually... 

Equations such as equation (1) above imply that the oxidative dissolution of pyrite is congruent, directly liberating Fe2+, SO4, and H+ to solution. However, in the common circumstance that water is insufficiently abundant to immediately transport the oxidation products away from the mineral surfaces, pyrite oxidation more commonly results initially in the accumulation of various hydroxysulphate evaporite minerals. These minerals form efflorescent crusts, typically white and yellow in colour, on the surfaces of pyrite-rich coals and mudstones (Fig. 1), and they effectively store the oxidation products in a readily soluble form until some hydro-logical event delivers sufficient water to dissolve and transport them away. Because pyrite often occurs in mudstones, where Al-bearing clay minerals are in contact with acidic pyrite oxidation waters, A1 is frequently released from the clays and is also stored in these hydroxysulphate phases. When these minerals finally dissolve, they result in abrupt and extreme increases in dissolved acidity. For this reason, they have been termed acid generating salts (AGS) (Bayless... 

These clays occur in limestones, dolomites, evaporites, shales, siltstones, and hydrothermal deposits. All the sedimentary material appears to have a diagenetic origin. Although the physical environments vary, the chemical environments should be similar. Saline or even super-saline conditions are implied by the presence of evaporite minerals associated with some of the deposits. In the other deposits it is possible that temporary evaporitic conditions (e.g., tidal flats) existed long enough for brucite to precipitate between the layers of expanded-layer minerals. It appears plausible that the parent material was a montmorillonite-like mineral (probably detrital in most cases). 

Clifford 2000). Many metallic ore and evaporite minerals contain methane, though not in hydrate form. For instance, the Red Dog zinc ores mentioned above contain methane inclusions in association with saline inclusions. In many instances, microscopic methane and carbon-dioxide-rich fluid inclusions form clathrates within their tiny hydrous envelopes. Some salt deposits are so enriched in high-pressure free gas inclusions and/or gas hydrates that mining, crushing under boot, or other means of physical disturbance causes popping or even explosive decrepitation. 

Thus, in an analogous fashion to carbon, if total exogenic sulfur has remained constant, then the 834S values of sulfate in seawater, and consequently in evaporite minerals, are functions of the ratio of oxidized sulfur to reduced sulfur in the sedimentary reservoirs of this element. If the size of the sulfate reservoir increases, then the sulfide reservoir shrinks, and the mean 34s/32s ratios of each reservoir changes. 

Friedman G.M. (1980) Dolomite is an evaporite mineral Evidence from the rock record and from sea-marginal ponds of the Red Sea. Soc. Econ. Paleontologists and Mineralogists Spec. Pub. 28. Tulsa, OK. 69-80. 

Kullerud, G. Sulfide phase relations. 50th anniv. Sympos. Mineralogy and Geochemistry of non-marine evaporites. Miner. Soc. Amer., Spec, public. No. 3, 199-210 (1970)... 

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