SECTION 5 



MINERALOGY OF PRECIPITATES 



Introduction to Pond Evaporite Mineralogy 



This section presents data on evaporite minerals identified by x-ray powder 

 diffraction CXRPD) analysis. Data on two types of samples are reported: (1) precipitated salts 

 collected from shorelines in ponds and (2) salts formed within the pond water column. Evaporite 

 mineral samples were obtained during monitoring trips (1) in the winter of 1987 at Barbizon 

 pond, (2) winter, spring and fall of 1987 and spring 1988 at Peck pond, and (3) winter and fall of 

 1987 and spring of 1988 at Pryse pond. The types of minerals precipitated in the water column 

 are strongly regulated by the initial chemistry of the inflow drainage water and degree of 

 evapoconcentration. Most of the mineral samples obtained came from shorelines on which salts 

 precipitated as the pond waterline receded and hence are not predicted by the brine chemistry 

 model unless the pond water is taken to dryness. 



Analytical Procedure 



Mineral identification was performed with a Diano XRD 8000 X-ray diffractometer 

 equipped with a strip chart recorder. Cu K-a radiation was used to determine the diffraction 

 maxima of the sample. Samples were scanned between 2 and 60' 26. Minerals were identified 

 using a computer program that converts the 20 values to diffraction spacings and compares the 

 d-spacings of the sample to known mineral d-spacings. Known mineral diffraction spacings were 

 compiled from the Mineral Powder Diffraction File, Joint Committee on Powder Diffraction 

 Standards (JCPDS). All minerals reported had both the 100% intensity peak identified and at 

 least three d-spacing matches with known minerals. Gypsum I and II, and Loewite I and II 

 denote identification through differing d-spacings. 



The samples have been subjected to a fairly rigorous and thorough analysis. The 

 minerals identified probably account for 99% of the salt samples. The dominant minerals reflect 

 the composition of the water as expected for shoreline salts and the wide variety of other 

 components suggest that evaporite formation is a non-competitive process at the shoreline. 



Pond Mineralogy 



Table 5.1 presents the evaporite minerals identified in field samples collected from the 

 shorelines and, when avtiilable, from within the water column of the three ponds. At all three 

 ponds, halite (NaCl) was the only chloride evaporite identified, nahcolite (NaHCO,) and 

 nesquehonite (MgC033HjO) were the only carbonate evaporites identified, and arcanite (K,SO^) 

 and thenardite (NajSO^) were the only sulfate evaporites identified. Other minerals detected 

 were present in only one or two ponds and not in the third pond. 



In Peck pond, the most diversity in evaporites occurred during the winter, after 

 evaporation had been the greatest and before water was seasonally added to the pond. The least 

 diversity in evaporites occurred during the spring, after new water (either rainfall or agricultural 

 drainwater) diluted the pond waters. 



Four minerals are ubiquitous in Peck pond: burkeite (Na2C03»2NajSO^), halite (NaCl), 

 mirabilite (Na,SO/ 10H,O) and thenardite (Na,SO^). Bloedite (Na,S0/MgS0/5H,0), gypsum 

 (CaS0/2H,0), nahcoHte (NaHCO,) and polyhalite (K,SO/2CaSO/MgSO/2HjO) were iden- 

 tified in two of the three samplings. 



Pryse pond did not follow the same pattern of diversity as at Peck. The most diversity 

 in Pryse occurred during winter. The least diversity occurred during the fall. One reason for this 

 might be that Cell 2 does not receive new water right away, hence this cell remains dry longer 

 than would be expected. When the cell does receive water for dilution, the water comes late in 

 the season. 



Three minerals are ubiquitous in Pryse pond: burkeite, halite, and thenardite. Bloedite, 

 gypsum, loeweite (2Na,SO/2MgSO/5HjO), mirabilite, nahcolite, polyhalit* and sodium car- 

 bonate sulfate (NajCOj'Na^SO,) were identified in two of the three samplings. 



p€ige S.I 



