Most of the minerals ubiquitous at Peck and Pryse evaporation ponds are also found at 

 Barbizon. Bloedite, gypsum, halite, loeweite, mirabilite, polyhalite and thenardite are the major 

 evaporite minerals in these ponds. This reflects the fact that the aqueous chemistry of the ponds 

 are similar. 



Differences between the ponds are observed in the evaporites which only form in one of 

 the ponds. While no quantitative tinalysis was done, the minerals discussed below probably did 

 not occur in large quantity. Sylvite (KCl) and arcanite (K,S04) were unique at Barbizon. This 

 reflects the fact that Barbizon evaporation pond has a greater percentage of potassium (of total 

 cations) than the other ponds. 



• Peck had three unique minerals: calcite (CaCO,), georgeyite (K,SO^ • 5CaS0/ H^O) and 

 syngenite (K,SO/CaSO/HjO). Calcite is a surprise. The other two minerals suggest 

 that proportion of Mg is low in this pond relative to other ponds, hence fewer evaporites 

 incorporate Mg. As evapoconcentration occurs, K and Ca precipitate as georgeyite and 

 syngenite. 



• Pryse had several unique minerals: soda [natron] (Na^CO,* lOH^O), anhydrite (CaSO^), 

 bassanite (2CaSO/HjO), glauberite (NajSO/CaSO,) and langbeinite 

 (KjS0/2MgS0p. Soda is probably a result of biologically increased partial pressure 

 of carbon dioxide. At the time soda was identified, five other carbonate or bicarbonate 

 minerals were also identified. Anhydrite and bassanite seem to be occurring instead 

 of gypsum. The hypersaline conditions of Pryse may increase the solubility of gypsum. 

 As seawater is concentrated, glauberite precipitates, so this mineral is not unexpected. 

 Glauberite would probably occur in other ponds if they were as saline as Pryse. 



• Of all the minerals identified, only two minerals (halite and thenardite) were found 

 in all ponds. 



Salts which precipitate in the pore waters at the sediment-water interface and the 

 overlying water column have also been collected. The morphologies between water column and 

 shoreline salts are easily distinguishable most likely because of the different forms that result 

 from one sample being constantly submerged while the other possibly dries out. Such 

 morphological differences, while indicating mineralogical differences, do not necessarily trans- 

 late into compositional differences. For instance, while thenardite (Na,SO^) and mirabilite 

 (Na^SO^ • lOHjO) are two different minerals, they comprise the same number of moles of Na and 

 SO per mole of the mineral and differ only in the hydration status. The dehydration of mirabilite 

 yields thenardite, and this occurs simply by leaving mirabilite in free air. 



In general, water column samples form much larger crystals and eventually coalesce into 

 salt slabs. This is in contrast to the shoreline salts which are powdery and fine. Shoreline salts 

 generally form as a result of wetting and drying along the shore as a result of wave action and 

 are usually of the dehydrated form. Salts forming this way may then be wind-blown further up 

 the bank and hence avoid redissolution. 



A Note Concerning Mineralogic Nomenclature 



Typically, the number of moles of an element in a mole of mineral is expressed as a lump 

 sum. For example, the common mineral thenardite has the chemical formula Na^SO^ and is 

 composed of two moles of Na and one mole of SO,. Likewise, minerals with more than two 

 components such as glauberite are usually found in reference materials such as the JCPDS 

 Mineral Powder Difraction File as Na,Ca(SO,)j. However, for the purpose of stressing the point 

 that these are mixed salts rather than entirely unique minerals, they are being expressed as 

 combined simple salts so that, for example, glauberite is given the chemical formula 

 Na,SO/CaSO,. Water (H^O) is not considered to be a simple salt and is always expressed in 

 combination with a mineral (e.g., mirabilite, Na,SO/ lOH^O). 



page 5.3 



