Research ArticleThe Sensitivity of Temperature to Tachyhydrite Formation:Evidence from Evaporation Experiments of Simulated BrinesBased on Compositions of Fluid Inclusions in Halite

Huaide Cheng ,1,2 Qingyu Hai,1,2 Jun Li,1,2,3 Jianguo Song,1,2,3 and Xuehai Ma1,2,3

1Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lake,Chinese Academy of Sciences, Xining 810008, China2Key Laboratory of Salt Lake Geology and Environment of Qinghai Province, Xining 810008, China3University of Chinese Academy of Sciences, Beijing 100049, China

Correspondence should be addressed to Huaide Cheng; chenghuaide@isl.ac.cn

Received 26 February 2019; Accepted 11 June 2019; Published 3 July 2019

Academic Editor: John A. Mavrogenes

Copyright © 2019 Huaide Cheng et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

An average of concentrations of Na+, Mg2+, Ca2+, K+, and Cl– in fluid inclusions, from the Khorat Plateau evaporite primary halite,was employed. The evaporation–crystallization sequence and paths were obtained under various temperature conditions for thequinary system, Na+, K+, Mg2+, Ca2+//Cl–-H2O. The results showed (1) a halite, sylvite, and carnallite stage at 25°C; (2) a halite,sylvite, carnallite, and bischofite stage at 35°C; and (3) a halite, sylvite, carnallite, bischofite, and tachyhydrite stage at 50°C.These results indicated that (1) a hot state is favorable for tachyhydrite formation, (2) tachyhydrite occurs in the lateevaporation stage, and (3) the stability field of tachyhydrite increases with increasing temperature. The crystallization paths wereplotted by the application of Jänecke phase diagram at 25°C, 35°C, and 50°C involving the system Na+, K+, Mg2+, Ca2+//Cl–-H2O. The crystallization sequence predicted on the Jänecke phase diagram showed a good agreement with the experimentalcrystallization sequences and paths. Tachyhydrite precipitate more easily from a high Ca concentration solution during the lateevaporation stage with increasing temperature under the same relative humidity condition. The evaporite mineral succession inthe Khorat Plateau, Sergipe, and Congo basins agrees well with the mineral precipitation sequences predicted from their ownfluid inclusions in halite. This is confirmed by the simulation of the Jänecke phase diagram at 50°C involving the system Na+,K+, Mg2+, Ca2+//Cl–-H2O. The precipitation of tachyhydrite was sensitive to the temperature, and that the thermal resource mayoriginate from a temperature profile in the solar pond. This study presented a simulated approach that can help inunderstanding similar cases that studies the sensitivity of temperature to salt formation.

1. Introduction

Only one modern environment, namely, the salt pan of theGavkhoni Playa Lake, southeast of Isfahan in central Iran,is known to contain tachyhydrite (see Table 1 for mineral for-mulas). Besides a minor deposit in Germany, tachyhydrite iscommon in Cretaceous graben and half-graben basin evapo-rite deposits in Brazil, western African, and Thailand [1].Because fractional crystallization of seawater in a series ofpreconcentration brines does not result in a CaCl2-rich brineand thus complete evaporation of marine water cannot form

tachyhydrite [2], tachyhydrite is present mostly in nonmar-ine settings particularly in hydrothermal regimes [3–7].Through studies on Khorat basin reformation and its evolu-tion, Vysotskiy [8] proposed that (1) chloride brines becameenriched in calcium in late stages mainly through chloride–calcium brine outflow from underlying rocks along faultsand weakened zones, (2) the evaporation formation wasaccompanied by a thermal setting that was favorable totachyhydrite, and (3) formed tachyhydrite beds are rift grab-bers or gentle depressions of the syneclise type. Evidencefrom a regular upward increase in bromine and a constant

HindawiGeofluidsVolume 2019, Article ID 7808036, 16 pageshttps://doi.org/10.1155/2019/7808036

strontium concentration through the upper zone of tachyhy-drite in the Khorat Plateau [1] showed that (1) the marinebrines became enriched in calcium in late stages mainlythrough the structural setting in fault-bounded troughs withlarge local relief; (2) tachyhydrite crystallization was uninter-rupted by sudden influxes of marine waters and where pri-mary salts were preserved without alteration; and (3) a“dry-lake” stage was never reached even though extreme vol-ume reduction is required for tachyhydrite crystallization. ElTabakh et al. [7] suggested that hydrothermal CaCl2 watersentered the restricted marine basin and created the right con-ditions for tachyhydrite precipitation, and they cite graniticintrusions [9] as possible evidence for thermal activity duringthe time of tachyhydrite formation. Some researchers pro-posed a tachyhydrite formation mechanism by multicom-ponent water–salt system theory [1, 6] and a phase theory[5, 10–13]. We have also attempted to characterize the forma-tion of tachyhydrite by evaporation experiments of simulatedbrines based on compositions of fluid inclusions in halite.

The sequences and types of crystallized salts and changesand relative changes in brine composition with the amount ofwater evaporated during isothermal brine evaporation aretheoretical bases for our study on geochemical brine evolu-tion, and the fundamental information that they provideallows us to explain the origin or formation condition ofevaporites. We employed an average of concentrations ofNa+, Mg2+, Ca2+, K+, and Cl– in fluid inclusions, from theKhorat Plateau evaporite primary halite (39 fluid inclusions)[14]. The Na+, Mg2+, Ca2+, K+, and Cl– concentrations andthe Jänecke coordinates (moleΣ 2KCl +MgCl2 + CaCl2 =100) of the fluid inclusion are listed in Table 2. The purposeof this study is to (1) obtain a crystallization sequence fromevaporation experiments of simulated brines based on themajor-ion composition of fluid inclusions at 25°C, 35°C,and 50°C; (2) simulate a crystallization path by the applica-tion of the Jänecke phase diagram at 25°C, 35°C, and 50°Cfor the quinary system, Na+, K+, Mg2+, Ca2+//Cl–-H2O;and (3) discuss the formation condition of tachyhydrite.In addition, we paid more attention on the sensitivity oftemperature to tachyhydrite formation and attempted toelucidate the relationship between tachyhydrite occurrenceand temperature.

2. Experimental

2.1. Reagents and Materials. Sodium chloride (NaCl), potas-sium chloride (KCl), magnesium chloride (MgCl2·6H2O),and calcium chloride (CaCl2) were of reagent grade andwere purchased from Sigma-Aldrich. All reagents were usedas received without any further purification. Double-distilledwater (18MΩ cm) was used in all experiments and foranalytical measurements. Standard glassware was used inall experiments.

2.2. Evaporation Procedure. A synthetic brine sample basedon compositions of fluid inclusions in halite from the KhoratPlateau primary halite was used as a starting point for exper-imental isothermal evaporation at 25°C, 35°C, and 50°C, andits average concentrations were given in Table 2. Duringevaporation, brine samples were taken in sequence at differ-ent densities for chemical analysis. The precipitated salt inthe saturated brine was filtered and stored for X-ray diffrac-tion and chemical analysis. X-ray diffraction analysis wascarried out on the same day as the solid samples were col-lected to avoid any alteration by atmospheric conditions.Herein, tachyhydrite is hardly to be kept in solid at roomtemperature during the preparation of X-ray diffraction sam-ple, because tachyhydrite will be transformed to bischofiteand a Ca-rich mother liquor once it is exposed at atmosphereduring the process of its crystallization. Moreover, tachyhy-drite will be precipitated in the late of evaporation. Therefore,the indirect way to identify tachyhydrite mineral is to deter-mine the compositions of terminal precipitated mineralsand its saturated mother liquor. Because the tachyhydriteoccurs in the late stage of evaporation and the Jänecke coor-dinates of the terminal saturated mother liquor locate on thetachyhydrite stability zone of the phase diagram involvingthe system Na+, K+, Mg2+, Ca2+//Cl–-H2O, it can be consid-ered that tachyhydrite should exist in the precipitated salts.Then, the tachyhydrite can be identified indirectly by deter-mining the ionic compositions of terminal precipitated min-erals, especially Ca and Mg. Density measurements werecarried out using a specific gravity balance.

Evaporation experiments were conducted in a constanttemperature water bath with an accuracy of ±0.1°C. Therewas no stirring of solution. The evaporation vessels were1000ml, 500ml, 250ml, and 100ml beakers. The masses ofsolution at the beginning of the experiment were 1000.00 g.When a kind of new mineral was precipitated out, the solu-tion and solid would be completely separated. Then, theevaporation of filtered solution continued. The entire evapo-ration process lasted 80 days. In the absence of saline mineralprecipitation, the evaporation process will continue foranother 10 days to ensure complete evaporation. Our exper-iment was conducted in March and April at an experimentalsite in Xining, Qinghai, China. According to local climaticconditions, the local air relative humidity ranges from 36%to 42% in the fall. The solid-phase minerals and the liquor-phase solutions were separated using a Sartorius Polycarbon-ate Filter Holder (8 μm membrane).

The saturated initial brine used for evaporation wasprepared in deionized water using NaCl, KCl, CaCl2, and

Table 1: Minerals discussed in this paper, their formulas, andabbreviations.

Mineral Formula Abbreviation

Halite NaCl Ha

Sylvite KCl Sy

Carnallite KCl·MgCl2·6H2O Car

Bischofite MgCl2·6H2O Bis

Tachyhydrite 2MgCl2·CaCl2·12H2O Tac

Chlorocalcite KCl·CaCl2 Chle

Antarcticite CaCl2·6H2O Ant

Calcium chloridetetrahydrate

αCaCl2·4H2O T4

Calcium chloride dihydrate CaCl2·2H2O Sin

2 Geofluids

MgCl2·6H2O salts based on the compositions of fluid inclu-sions in halite (see Table 2). Because of the temperaturedependence of the salt solubility, a small variation in temper-ature will change the saturated brine concentration. There-fore, saturated brines were stirred for 24 h to achievesaturation at room temperature. All saturated brines were fil-

tered using an 8μm membrane filter to remove fine salt par-ticles and other insoluble material prior to use.

2.3. Analytical Methods. Na+, K+, Mg2+, Cl−, and Ca2+ con-centrations in brine samples were determined at the QinghaiInstitute of Salt Lakes, Chinese Academy of Sciences. All

Table 2: Major-ion chemical composition and Jänecke coordinates of individual primary fluid inclusions in halite from the Khorat Plateau.

Sample Inclusion typeMg K Ca Na Cl 2KCl CaCl2 MgCl2

mmol/kg H2O Jänecke coordinates

Albian-Cenomanian: Sakon Nakhon Basin, Laos, Maha Sarakham Formation [14]

353m Chevron 930 350 340 3710 6590 12.11 23.53 64.36

353m Chevron 860 390 290 3890 6570 14.50 21.56 63.94

353m Chevron 780 400 370 3890 6590 14.82 27.41 57.78

353m Chevron 1180 420 390 3210 6760 11.80 21.91 66.29

353m Chevron 620 320 320 4270 6480 14.55 29.09 56.36

353m Chevron 690 310 330 4210 6650 13.19 28.09 58.72

353m Chevron 740 360 350 4000 6550 14.17 27.56 58.27

353m Chevron 800 380 340 3920 6570 14.29 25.56 60.15

353m Chevron 640 330 300 4270 6480 14.93 27.15 57.92

353m Chevron 740 320 310 4080 6510 13.22 25.62 61.16

381m Chevron 850 250 270 3990 6480 10.04 21.69 68.27

381m Chevron 570 250 290 4450 6410 12.69 29.44 57.87

381m Chevron 850 260 280 3980 6490 10.32 22.22 67.46

381m Chevron 610 210 280 4420 6400 10.55 28.14 61.31

381m Chevron 550 190 290 4510 6370 10.16 31.02 58.82

383.5m Chevron 620 270 330 4280 6450 12.44 30.42 57.14

383.5m Chevron 780 260 240 4160 6450 11.30 20.87 67.83

383.5m Chevron 570 230 280 4470 6400 11.92 29.02 59.07

425m Chevron 1190 230 250 3430 6540 7.40 16.08 76.53

425m Chevron 1150 220 280 3460 6550 7.14 18.18 74.67

426.5m Chevron 720 240 280 4200 6450 10.71 25.00 64.29

426.5m Chevron 680 290 300 4220 6460 12.89 26.67 60.44

428m Chevron 710 240 250 4250 6410 11.11 23.15 65.74

428m Chevron 880 200 250 4000 6460 8.13 20.33 71.54

428m Chevron 700 200 220 4340 6390 9.80 21.57 68.63

428m Chevron 740 190 240 4270 6410 8.84 22.33 68.84

430.5m Chevron 470 240 240 4710 6360 14.46 28.92 56.63

430.5m Chevron 1070 220 300 3570 6530 7.43 20.27 72.30

430.5m Chevron 1040 230 360 3540 6570 7.59 23.76 68.65

430.5m Chevron 800 200 360 3960 6480 7.94 28.57 63.49

430.5m Chevron 1110 250 360 3420 6620 7.84 22.57 69.59

430.5m Chevron 560 230 290 4480 6400 11.92 30.05 58.03

430.5m Chevron 580 220 240 4510 6380 11.83 25.81 62.36

430.5m Chevron 560 230 220 4590 6370 12.85 24.58 62.57

430.5m Chevron 620 210 280 4380 6390 10.45 27.86 61.69

430.5m Chevron 1060 220 260 3660 6520 7.69 18.18 74.13

430.5m Chevron 690 270 240 4320 6440 12.68 22.54 64.79

430.5m Chevron 820 210 290 4030 6460 8.64 23.87 67.49

430.5m Chevron 830 240 270 4030 6470 9.84 22.13 68.03

Average 778 264 292 4079 6484 11.13 24.69 64.18

3Geofluids

analyses for major in this study followed the procedures ofthe Qinghai Institute of Salt Lakes [15]. Mg2+ and Ca2+ ionconcentrations were determined by complexometric titra-tion with ethylenediaminetetraacetic acid (EDTA) standardsolution. K+ ion concentration was determined by gravimet-ric methods using sodium tetraphenylborate [KB(C6H5)4].Cl− ion concentration was determined by Hg(NO3)2 titra-tion. Na+ ion concentration was calculated by charge balanceNCl− − NK+ + NCa2+ + NMg2+ (N represents ionic equiv-

alent value). The analytical precision for cations and anionsis better than ±2%.

2.4. Jänecke Phase Diagram Analysis for the Quinary System,Na+, K+, Mg2+, Ca2+//Cl–-H2O. In the quinary system dia-gram, Na+, K+, Mg2+, Ca2+//Cl–-H2O, the Jänecke coordi-nates are expressed as

2KCl% = n 2KClD

× 100 ;

MgCl2% = n MgCl2D

× 100 ;

CaCl2% = n CaCl2D

× 100,

1

where “n” is the molar number and D = n 2KCl + n MgCl2+ n CaCl2 .

In the application of the Jänecke phase diagram, it isassumed that the solution is characterized by (1) halite satu-ration, which means that the solution will coprecipitate halitejointly with other salt phases that have separated duringevaporation, and (2) sulfates and carbonates that are consid-ered unimportant constituents because their concentrationin the saturated solution is very low [16, 17].

3. Results and Discussions

3.1. Crystallization Path during Evaporation. Ionic composi-tions and Jänecke coordinates of the studied brine sample,measured at different densities during experimental evapora-tion at 25°C, 35°C, and 50°C, are given in Table 3.

The crystallization path is the progress of chemical trans-formations by the loss or addition of a constituent through agiven solubility phase diagram. It can define the number,nature, composition, and relative quantity of different con-densed phases that precipitate or disappear during a system’sevolution. In our case, water leaves as steam at constant pres-sure and temperature. Such a graphic representation showsthe reactions that occur by changing an intensive variable,mainly composition. The crystallization path construction

Table 3: Ionic compositions and Jänecke coordinates of the studied brine samples, measured at different densities during experimentalevaporation at 25°C, 35°C, and 50°C.

Temperature(°C)

No.Masses(g)

Density(g/cm3)

Ionic compositions (mmol/kg H2O) Jänecke coordinatesSolid phases

Mg K Ca Na Cl 2KCl CaCl2 MgCl2

25

Initial studied brine 1000.00 1.2043 751 264 262 3800 6082 11.54 22.86 65.60 —

Ep-b-1 764.34 1.2187 1012 355 350 3614 6692 11.53 22.76 65.72 Ha

Ep-b-2 456.72 1.2382 1713 595 592 2023 7221 11.43 22.74 65.83 Ha

Ep-b-3 313.88 1.2745 2630 864 914 750 8700 10.87 22.99 66.14 Ha

Ep-b-4 275.43 1.2935 3072 704 1037 420 9334 7.89 23.25 68.86 Ha+Sy

Ep-b-5 194.16 1.3106 3583 164 1480 205 10198 1.59 28.78 69.63 Ha+Car

35

Initial studied brine 1000.00 1.2008 715 316 283 3830 6137 13.66 24.47 61.87 —

Ep-g-1 616.76 1.2249 1215 440 301 3444 6914 12.68 17.35 69.97 Ha

Ep-g-2 456.11 1.2345 1544 574 610 2495 7371 11.77 24.98 63.26 Ha

Ep-g-3 386.33 1.2451 1863 661 647 1896 7573 11.64 22.77 65.59 Ha

Ep-g-4 295.57 1.2787 2571 924 909 847 8724 11.72 23.06 65.22 Ha

Ep-g-5 226.73 1.3055 3103 752 1179 434 9741 8.08 25.31 66.61 Ha+Sy+Car

Ep-g-6 180.06 1.3247 3497 171 1592 321 10657 1.65 30.76 67.59 Ha+Sy+Car

Ep-g-7 128.85 1.4025 4181 41 2461 253 13562 0.31 36.94 62.75 Ha+Car+Bis

Ep-g-8 89.56 1.4153 3963 42 2571 156 13248 0.32 39.22 60.46 Ha+Bis

50

Initial studied brine 1000.00 1.2019 790 273 264 3900 6281 10.22 15.97 73.80 —

Ep-h-1 700.58 1.2156 1072 383 306 3829 6964 12.19 19.49 68.32 Ha

Ep-h-2 485.31 1.2284 1562 555 439 2766 7316 12.18 19.26 68.56 Ha

Ep-h-3 278.66 1.2876 2961 1057 828 958 9557 12.24 19.18 68.58 Ha+Sy

Ep-h-4 224.56 1.3438 3495 797 1092 447 10371 8.00 21.90 70.10 Ha+Sy+Car

Ep-h-5 185.91 1.3519 3525 401 1269 838 10790 4.02 25.40 70.58 Ha+Car

Ep-h-6 153.25 1.3594 3876 229 1447 421 11237 2.10 26.62 71.28 Ha+Car

Ep-h-7 102.59 1.4266 4656 72 2423 226 14359 0.50 34.05 65.44 Ha+Car+Bis

Ep-h-8 86.68 1.4404 4420 62 3149 225 14722 0.41 41.44 58.16 Ha+Bis+Tac

4 Geofluids

demonstrates the sequence of salt precipitation. In this study,the fluid inclusion composition showed that the studied brineis composed mainly of Na+, K+, Mg2+, Ca2+, and Cl– (Table 2).The crystallization path of such a studied brine can be pre-dicted using the quinary system diagram (Na+, K+, Mg2+,Ca2+//Cl–-H2O). The crystallization path during evaporationof the studied brine is based mainly on the location of Jäneckecoordinates (moleΣ 2KCl +MgCl2 + CaCl2 = 100) for thequinary system, Na+, K+, Mg2+, Ca2+//Cl–-H2O.

3.1.1. Predicted Crystallization Path. The predicted crystalli-zation path during evaporation of the studied brine is basedmainly on the location of the Jänecke coordinates of the ini-tial studied brine on the Jänecke phase diagram for the Na+,K+, Mg2+, Ca2+//Cl–-H2O system. Figures 1(a)–1(c) showedthat all the initial studied brines (point “O”) lie on the stabil-ity field of sylvite at different temperatures. If the solutionreaches the drying-up point, the predicted crystallizationpath (pink dotted line in Figure 1) is “O-E-F-G-H-R” at25°C (Figure 1(a)), “O-E-F-G-J-K” at 35°C (Figure 1(b)),and “O-E-F-G-J-K-M” at 50°C (Figure 1(c)).

In Figure 1(a), upon evaporation of the initial studiedbrine at 25°C, halite will be precipitated. When the brine issaturated with sylvite at point E, sylvite+halite will be precip-itated along O-E. At point E, sylvite will be consumed, andcarnallite+halite will be formed along E-F. At point F, bischo-fite will become stable and will be precipitated together withcarnallite and halite along F-G. At point G, bischofite disap-pears and tachyhydrite+carnallite+halite are precipitatedalong G-H. At point H, tachyhydrite will be consumed, andantarcticite+halite will be formed along H-R. Evaporatingto dryness leads to the final assemblage antarcticite+halite.Along the crystallization path “O-E-F-G-H-R,” further evap-oration should make the brine evolve from its initial locationat point “O” toward point “R.” Point “R” is a drying-up pointat which the crystallization process terminates. The predictedcrystallization path of 35°C and 50°C for the same processesas 25°C can be seen in Figures 1(b) and 1(c), respectively.We found that the predicted crystallization path become longwith increasing temperature indicating that more types ofcrystalline minerals would occur.

3.1.2. Experimental Crystallization Path. The ionic composi-tions (mmol/kg H2O) and Jänecke coordinates of the studiedbrine sample measured at different densities during experi-mental evaporation at 25°C, 35°C, and 50°C are given inTable 3. Brine evolution during evaporation is presentedgraphically on the Jänecke phase diagram for the Na+,K+, Mg2+, Ca2+//Cl–-H2O system (see Figure 1). Underthis experiment condition (relative humidity, 36-43%), theexperimental crystallization path (black solid line inFigure 1) is “O-e-f” at 25°C (Figure 1(a)), “O-e-f-m” at 35°C(Figure 1(b)), and “O-e-f-g-h” at 50°C (Figure 1(c)).

Table 4 shows the evaporation-crystallization path andmineral phase of predicted and experimental results. Wefound that the types of mineral phase increases with increas-ing temperature indicating that salt minerals are sensitive totemperature especially calcium-bearing mineral-like tachy-hydrite. Figure 1 shows that the experimental crystallization

path agrees well with the predicted crystallization path. How-ever, in this case, the evaporation process stopped near point“F” at 25°C, point “G” at 35°C, and point “J” at 50°C in theexperimental evaporation. This may be because of someextremely high salinity. Furthermore, relative humidity con-dition (in this study, 36-43%) prevents further evaporation. Itcan be seen that the crystallization path becomes long whichis more beneficial to the tachyhydrite formation with theincrease of evaporation temperature (from 25 to 50°C).

3.2. Evolution of Initial Studied Brine during Evaporation.The evolution of major ions during progressive evaporationis based on the principle of “chemical divide.” The basic ideaof the chemical divide rule is “whenever a binary salt is pre-cipitated during evaporation, and the initial molar propor-tion of the two ions forming this salt is not equal insolution, further evaporation will result in an increase inthe concentration of the ion present in greater relative con-centration in solution, and a decrease in the concentrationof the ion present in lower relative concentration” [18].

A plot of the major ion concentrations as a function ofdensity during evaporation at 25°C, 35°C, and 50°C is givenin Figure 2. In Figure 2(a), for evaporation process at 25°C,at the beginning of evaporation, and at a starting density of1.2043, the initial Cl– ion concentration is greater than thatof the Na+ and K+ ions. Therefore, during the evaporationof this brine, it is expected that halite precipitation shouldcause an increase in concentration of Cl– and a depletionin Na+ concentration. The same effect will occur when syl-vite and carnallite start precipitating, where Cl– should buildup in solution with decreasing K+ during progressive evapo-ration. The concentrations of Mg2+ and Ca2+ show a steadyincrease, for Ca2+ ions without any depletion. In Figure 2(b),for evaporation process at 35°C, the change in the concen-trations of Na+, K+, Mg2+, Ca2+, and Cl– is consistent withthe change of these ions at 25°C evaporation processbefore bischofite start precipitating. Due to the precipita-tion of bischofite, the Mg2+ and Cl– concentrations showa slightly decrease. In Figure 2(c), for evaporation processat 50°C, the change in the concentrations of Na+, K+, Mg2+,Ca2+, and Cl– is consistent with the change of these ions at25°C and 35°C evaporation process before tachyhydrite startsprecipitating. Tachyhydrite precipitation should cause adecrease in concentration of Mg2+ and a depletion in Na+

and K+ concentration. Nevertheless, the Ca2+ concentrationstill shows an increasing trend.

3.3. Crystallization Sequences and Evaporation Stages. Basedon the mineral types during experimental evaporation at25°C, 35°C, and 50°C (see Table 4), the experimental crystal-lization sequences (see Figure 2) can be divided into (1) threesequences, namely, the halite, sylvite, and carnallite stages, at25°C; (2) four sequences, namely, the halite, sylvite, carnall-ite, and bischofite stages, at 35°C; and (3) five sequences,namely, the halite, sylvite, carnallite, bischofite, and tachyhy-drite stages, at 50°C.

3.4. The Relationship between Major Concentration andDensity. In Figure 2(a), the first sequence was observed at

5Geofluids

0 10 20 30 40 50 60 70 80 90 100

1000

10

20

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100 0

10

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B

e

f

Salt point of carnalliteSalt point of tachyhydrite Experimental data

Predicted cystallization pathInitial studied brine Experimental cystallization path

R

H

F

E

F

E

O

65

75M

gCl 2 70

Car Sy

Bis

Tac

Ant

Sy

Car

CaCl2M

gCl 2

2KCl

Bis

O

G

(a)

0 10 20 30 40 50 60 70 80 90 100

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m

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e

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J

F

E

F

E

O

MgC

l 2

CarSy

Bis

Tac

T4

Sy

Car

CaCl2MgC

l 2

2KCl

Bis

O

G

Experimental dataPredicted cystallization pathExperimental cystallization pathInitial studied brine

Salt point of carnalliteSalt point of tachyhydrite

0

70

60

80

(b)

Figure 1: Continued.

6 Geofluids

densities from 1.2043 to 1.2745. In this sequence, the Na+

concentration decreases significantly once evaporation com-mences. This may occur because halite precipitation isaccompanied by the effect of the principal of chemical dividewhere the initial Cl– concentration is greater than the Na+

concentration. K+ increased steadily with evaporation untilthe K+ ion reached a maximum concentration at a densityof 1.2745. The second sequence in experimental evaporationstarted at densities from 1.2745 to 1.2935. In this sequence,the Na+ ion concentration continued to decrease becauseof halite precipitation whereas the Cl– ion concentrationcontinued to increase. The K+ concentration after it reacheda maximum at a density of 1.2745 started to decline at thenext densities, whereas the Cl– solution concentration con-tinued to increase. This may be attributed to sylvite precip-itation. The third sequence was defined at densities from1.2935 to 1.3106. In this sequence, the K+ concentrationdecreases significantly because carnallite starts precipitatingwhere Cl– should continue to increase. Although the Mg2+

ion coprecipitated with the K+ ion during carnallite precip-itation, the concentration of Mg2+ in solution continues toincrease with evaporation. The Ca2+ ion showed a steadyincrease during evaporation in the three sequences withoutany depletion.

In Figure 2(b), the first sequence was observed at densi-ties from 1.2008 to 1.2787. In this sequence, the Na+ concen-tration decreases significantly once evaporation commences.This may occur because halite precipitation is accompaniedby the effect of the principal of chemical divide where the ini-tial Cl– concentration is greater than the Na+ concentration.K+ increased steadily with evaporation until the K+ ionreached a maximum concentration at a density of 1.2787.The second sequence of the experimental evaporation startedat densities from 1.2787 to 1.3055. In this sequence, the Na+

ion concentration continued to decrease because of the haliteprecipitation whereas the Cl– ion concentration continued toincrease. After reaching its maximum value at a density of1.2787, the K+ concentration started to decline at the nextdensities, whereas the Cl– solution concentration continuedto increase. This may be attributed to sylvite precipitation.The third sequence was defined at densities from 1.3055 to1.4025. In this sequence, the K+ concentration decreases sig-nificantly because carnallite starts to precipitate where Cl–

should continue to increase. Although the Mg2+ ion copreci-pitated with the K+ ion during carnallite precipitation, theconcentration of Mg2+ in solution continues to increase withevaporation. The fourth sequence was defined at densitiesfrom 1.4025 to 1.4153. In this sequence, the Mg2+ and Cl–

0 10 20 30 40 50 60 70 80 90 100

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60

MgC

l 2

Car

SyBis

Tac

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SyCar

CaCl2MgC

l 2

2KCl

Bis

O

Experimental data

Experimental cystallization pathInitial studied brineSalt point of carnalliteSalt point of tachyhydrite

Predicted cystallization path

F

80

70

(c)

Figure 1: Graphical representations of Jänecke coordinates (moleΣ 2KCl +MgCl2 + CaCl2 = 100) on the Jänecke phase diagram at 25°C (a),35°C (b), and 50°C (c) for the Na+, K+, Mg2+, Ca2+//Cl–-H2O system with halite saturation throughout. The experimental crystallization pathand predicted evaporation–crystallization path are shown. Points B, R, K, and M denote a sylvite, antarcticite, calcium chloride tetrahydrate,and calcium chloride dihydrate salt point, respectively.

7Geofluids

concentrations decrease because bischofite starts to precipi-tate where Na+ and K+ should remain unchanged. The Ca2+

ion concentration showed a steady increase during evapora-tion in the four sequences without any depletion.

In Figure 2(c), the first sequence was observed at densitiesfrom 1.2019 to 1.2876. In this sequence, the Na+ concentra-tion decreases significantly once evaporation commences.This may occur because halite precipitation is accompaniedby the effect of the principal of chemical divide where the ini-tial Cl– concentration is greater than the Na+ concentration.K+ increased steadily with evaporation until the K+ ionreached its maximum concentration at a density of 1.2876.The second sequence of experimental evaporation started atdensities from 1.2876 to 1.3438. In this sequence, the Na+

ion concentration continued to decrease because of the haliteprecipitation whereas the Cl– ion concentration continued toincrease. After reaching its maximum value at a density of1.2787, the K+ concentration started to decline at the nextdensities, whereas the Cl– solution concentration continuedto increase. This may be attributed to sylvite precipitation.The third sequence was defined at densities from 1.3438 to1.3594. In this sequence, the K+ concentration decreases sig-nificantly because carnallite starts to precipitate where Cl–

should continue to increase. Although the Mg2+ ion copreci-pitated with the K+ ion during carnallite precipitation, theconcentration of Mg2+ in solution continues to increase withevaporation. The fourth sequence was defined at densities

from 1.3594 to 1.4266. In this sequence, the Mg2+ and Cl–

concentrations increase because bischofite starts to precipi-tate where Na+ and K+ should remain unchanged. The fifthsequence was defined at densities from 1.4266 to 1.4404. Inthis sequence, the Mg2+ concentration decreases becausetachyhydrite starts to precipitate where Cl– and Ca2+ shouldcontinue to increase. The Ca2+ ion concentration increasedsteadily during evaporation in the five sequences withoutany depletion.

3.5. Composition of Halite Inclusion from CretaceousEvaporites on Jänecke Phase Diagram at 25°C. To obtain thelocation of halite inclusion from Cretaceous evaporites onthe Jänecke phase diagram at 25°C for the quinary system,Na+, K+, Mg2+, Ca2+//Cl–-H2O, the concentrations of Na+,Mg2+, Ca2+, K+, and Cl– in fluid inclusions from the Sergipebasin (13 fluid inclusions) and the Congo basin (16 fluidinclusions) were also used as shown in Table 5 [14].Figure 3 shows that the Khorat Plateau brines lie on the sta-bility field of sylvite, the Sergipe basin brines lie on the phaseboundary of sylvite and carnallite, and the Congo basinbrines lie on the stability field of carnallite. We found that(1) the concentration of Ca2+ increases with time (from LateCretaceous to Early Cretaceous), in this case, MgCl2- andCaCl2-bearing salts such as bischofite and tachyhydrite areformed easily; (2) the concentration of K+ decreases slightlywith an increase in time, in this case, sylvite does not occur;

Table 4: Evaporation-crystallization path and mineral phase of predicted and experimental results.

Condition 25°C 35°C 50°C

Evaporation-crystallization path

Predicted “O-E-F-G-H-R” “O-E-F-G-J-K” “O-E-F-G-J-K-M”

Experimental “O-e-f” “O-e-f-m” “O-e-f-g-h”

Mineral phase

Predicted

“O”: halite “O”: halite “O”: halite

“O-E”: halite+sylvite “O-E”: halite+sylvite “O-E”: halite+sylvite

“E-F”: halite+carnallite “E-F”: halite+carnallite “E-F”: halite+carnallite

“F-G”: halite+carnallite+bischofite

“F-G”: halite+carnallite+bischofite

“F-G”: halite+carnallite+bischofite

“G-H”: halite+carnallite+tachyhydrite

“G-J”: halite+carnallite+tachyhydrite

“G-J”: halite+carnallite+tachyhydrite

“H-R”: halite+antarcticite“J-K”: halite+calciumchloride tetrahydrate

“J”: halite+carnallite+tachyhydrite+chlorocalcite

“J-K”: halite+tachyhydrite+chlorocalcite

“K”: halite+tachyhydrite+chlorocalcite+

calcium chloride dihydrate

“K-M”: halite+calcium chloridedihydrate

Experimental

“O”: halite “O”: halite “O”: halite

“O-e”: halite+sylvite “O-e”: halite+sylvite “O-e”: halite+sylvite

“e-f”: halite+carnallite “e-f”: halite+carnallite “e-f”: halite+carnallite

“f-m”: halite+carnallite+bischofite

“f-g”: halite+carnallite+bischofite

“g-m”: halite+carnallite+tachyhydrite

8 Geofluids

and (3) the MgCl2/CaCl2 ratio decreases with an increase intime. This result was consistent with the occurrence of theextremely soluble bischofite and tachyhydrite in a thicknessof several tens of meters in the evaporite deposits of Congobasin [19].

3.6. Prediction of Mineral Precipitation Sequences in HaliteInclusion from Cretaceous Evaporites on Jänecke PhaseDiagram at 50°C. There are three evaporite deposits in Creta-ceous: the Early Cretaceous of the Sergipe basin, Brazil, theCongo basin, Republic of the Congo, and the Early to LateCretaceous of the Khorat Plateau, Laos, and Thailand. Thepotash deposits of the Khorat Plateau include only two potas-sium minerals, sylvite (Sy.) and carnallite (Car.), and the lat-ter is by far the most abundant. A common constituent of

carnallite deposits is the mineral tachyhydrite. Althoughtachyhydrite generally is in amounts of less than 30 percentof the total carnallite deposits, it locally forms nearly purelayers as much as 16m thick [20]. Tachyhydrite is presentwith halite and carnallite and is concentrated mostly in thebasin centers [7]. The vertical evaporite mineral successionin the Ibura member of the Muribeca Formation in theSergipe basin is anhydrite, halite, carnallite, and tachyhy-drite, with beds of tachyhydrite as thick as 100m [1].Tachyhydrite is located within the central and deepestportions of the Sergipe basins [1, 21]. The basic salt cycleof the Congo basin is made up from bottom to top by (1)a thin black shale, (2) a layer of halite, (3) a mixture ofhalite and carnallite, and (4) bischofite and/or tachyhydrite[19]. In general, mineral succession in the Khorat Plateau,

0100020003000400050006000700080009000

1000011000

1.2 1.22 1.24 1.26 1.28 1.3 1.32 1.34

Ion

conc

entr

atio

ns (m

mol

/kg

H2O

)

Density

MgKCa

NaCl

HaSy Car

(a)

0100020003000400050006000700080009000

1000011000120001300014000

1.2 1.24 1.28 1.32 1.36 1.4 1.44

Ion

conc

entr

atio

ns (m

mol

/kg

H2O

)

DensityMgKCa

NaCl

HaSy Car Bis

(b)

0100020003000400050006000700080009000

100001100012000130001400015000

1.2 1.24 1.28 1.32 1.36 1.4 1.44 1.48

Ion

conc

entr

atio

ns (m

mol

/kg

H2O

)

Density

MgKCa

NaCl

HaSy Car Bis Tac

(c)

Figure 2: A plot of the major ion concentrations as a function of density during evaporation at 25°C (a), 35°C (b), and 50°C (c).

9Geofluids

Sergipe, and Congo basin are “Ha, Ha+Sy,” “Ha, Ha+Car,Ha+Car+Tac,” and “Ha, Ha+Car, Ha+Bis+Tac,” respec-tively (see Figures 4(d)–4(f)). The mineral precipitationsequences predicted from their own fluid inclusion inhalite, depending on the quinary system, Na+, K+, Mg2+,Ca2+//Cl–-H2O at 50°C, are “Ha, Ha+Sy, Ha+Car, Ha+Bis+Car,” “Ha, Ha+Car, Ha+Car+Tac,” and “Ha, Ha+Car,Ha+Car+Tac,” respectively (see Figures 4(a)–4(c)). Theevaporite mineral succession in the Khorat Plateau, Sergipe,and Congo basins agrees well with the mineral precipitationsequences predicted from their own fluid inclusion inhalite, depending on the quinary system, Na+, K+, Mg2+,Ca2+//Cl–-H2O, at 50°C. These simulated evolution pathsby the application of Jänecke phase diagram for fluidinclusion brines indicated that a hot state is favorable fortachyhydrite formation.

3.7. Formation Conditions of Tachyhydrite

3.7.1. Calcium Source. CaCO3 and CaSO4 are relativelyinsoluble salts, and because of an excess of sulfate in normalseawater relative to calcium, the small amount of calciumpresent tends to be removed completely from the solutionat an early evaporation stage. Calcium is no longer availablefor calcium salt formation, such as tachyhydrite formation,during the later stages of brine concentration. Tachyhydrite,a rare mineral, occurs in Cretaceous evaporites such asmarine evaporites from the Khorat Plateau, Sergipe, andCongo evaporites. At an advanced stage of evaporation,brines must have been enriched significantly in CaCl2 anddepleted in SO4, and the question arises as to how thesebrines become enriched in calcium and depleted in sulfate.Possible processes are (1) a release of calcium during

Table 5: Major-ion chemical compositions and Jänecke coordinates of primary fluid inclusions from the Sergipe basin and the Congo basin.

Sample Inclusion typeMg K Ca Na Cl 2KCl CaCl2 MgCl2

mmol/kg H2O Jänecke coordinates

Aptian: Sergipe basin, Brazil, Muribeca Formation [14]

792.2m Chevron 540 180 420 4300 6410 8.57 40.00 51.43

792.2m Chevron 970 250 570 3300 6630 7.51 34.23 58.26

792.2m Chevron 1090 260 570 3130 6730 7.26 31.84 60.89

792.2m Chevron 1100 240 630 3040 6750 6.49 34.05 59.46

792.2m Chevron 710 220 560 3780 6540 7.97 40.58 51.45

792.2m Chevron 910 240 620 3340 6660 7.27 37.58 55.15

792.2m Chevron 1010 240 640 3170 6710 6.78 36.16 57.06

792.2m Chevron 1130 250 640 2980 6780 6.60 33.77 59.63

792.2m Chevron 360 190 500 4480 6400 9.95 52.36 37.70

792.2m Chevron 900 250 600 3400 6650 7.69 36.92 55.38

792.2m Chevron 860 230 600 3460 6620 7.30 38.10 54.60

792.2m Chevron 1120 290 640 2990 6800 7.61 33.60 58.79

792.2m Chevron 1460 310 680 2430 7030 6.75 29.63 63.62

Average 935 242 590 3369 6670 7.52 36.83 55.65

Aptian: Congo basin, Congo, Loeme Formation [14]

746m Chevron 1580 220 1060 1830 7330 4.00 38.55 57.45

746m Chevron 1020 200 930 2760 6850 4.88 45.37 49.76

746m Chevron 910 180 870 3000 6750 4.81 46.52 48.66

746m Chevron 800 170 830 3240 6680 4.96 48.40 46.65

780m Chevron 2140 160 1320 1020 8100 2.26 37.29 60.45

780m Chevron 1180 150 1310 2060 7200 2.92 51.07 46.00

780m Chevron 1980 160 1600 950 8270 2.19 43.72 54.10

869m Chevron 2520 140 2610 240 10640 1.35 50.19 48.46

869m Chevron 3010 130 2780 130 11840 1.11 47.48 51.41

869m Chevron 2750 150 2700 180 11240 1.36 48.87 49.77

869m Chevron 2770 140 2640 190 11140 1.28 48.18 50.55

869m Chevron 2120 110 2250 500 9340 1.24 50.85 47.91

869m Chevron 1220 90 1970 1330 7800 1.39 60.90 37.71

869m Chevron 2840 130 2590 180 11170 1.18 47.13 51.68

869m Chevron 2170 100 2350 430 9580 1.09 51.42 47.48

869m Chevron 2640 120 2460 250 10570 1.16 47.68 51.16

Average 1978 147 1892 1143 9031 2.32 47.73 49.95

10 Geofluids

dolomitization [22–25], (2) a liberation of calcium throughion exchange with clays [26], (3) an addition of groundwaterthat may be meteoric, connate, or juvenile in origin, butwhich has become enriched in calcium [27], and (4) a reduc-tion in sulfate by bacteria or by other processes [28–30].These alternatives are difficult to evaluate because evaporitebasins tend to be open systems with in- and out-flowingbrine steams [1]. Deposition can occur from brine which isflowing rather than static. With pronounced lateral fraction-ation, it would be possible for basal carbonates and sulfates toexist in one portion of an evaporite basin but not in another.Incomplete information on the lateral distribution of evapo-rite facies in connected subbasins and a lack of knowledge onhow brines flowed or were decanted from one basin toanother makes it difficult to reconstruct the factors responsi-ble for the progressive change in brine composition. In previ-ous work, the source of calcium from brines enriched inCaCl2 is still being debated. Wardlaw [1] argued that brinesenriched in CaCl2 and depleted in sulfate may have resultedin a modification of marine brines by CaCl2-rich groundwa-ter discharge. Brines with these characteristics exist in theRed and Dead seas today and provide an interesting analog.Vysotskiy [8] also argued that chloride brines becameenriched in calcium in late stages mainly through the out-flow of CaCl2-rich brines from underlying rocks along

faults and weakened zones. Hardie [31] proposed that mid-ocean ridge hydrothermal brines became enriched in CaCl2and impoverished in MgSO4 compared with parent seawa-ter. Particularly extensive carbonate platforms existed dur-ing periods of high sea level during the Cretaceous, andthese platforms were conducive to widespread dolomitiza-tion during seawater evaporation, which would take up Mgand release calcium [32–35]. Some evidence of hydrothermalactivates and a thermal event during the Cretaceous exists inthe Khorat Plateau, which may have supplied CaCl2-richwaters into the basin [9]. Hite and Japakasetr [20] suggestedthat a Khorat sea that could produce tachyhydrite deposits iscompatible with a lack of magnesium sulfate minerals becausethe necessary concentration of Ca2+ should have kept the brinepurged of SO4

2- because of the reaction, Ca2+aq + SO2−4 S →

CaSO4 S . EI Tabakh et al. [7] argued that CaCl2-rich brinesenter the waters of the basin and modify the chemistry ofthe surface waters and of the associated groundwater becauseof early halite replacement of primary gypsum. Variations inmajor-ion chemistry of Cretaceous seawater may explain the“anomalous” evaporites from the Khorat Plateau [1, 14, 20,36–39]. This result is consistent with our observation thattachyhydrite will be precipitated by a simulation of evapora-tion of fluid inclusion in halite from the Khorat Plateau.Therefore, to form tachyhydrite, the calcium source may be

0 10 20 30 40 50 60 70 80 90 100

1000

10

20

30

40

50

60

70

80

90

100 0

10

20

30

40

50

60

70

80

90

MgCl2/CaCl2 ratio decreases with an increase in time

Loeme Formation, Congo basin

Aptian, 121.0-112.2 Ma

Muribeca Formation, Sergipe basin

Maha Sarakham Formation, Khorat PlateauAlbian to Cenomanian, 112.2-93.5 Ma

Bis

Tac

Ant

Sy

Car

CaCl2M

gCl 2

2KClCongo basinSergipe basinKhorat Plateau

Figure 3: Compositions of halite inclusions from Cretaceous evaporites on the Jänecke phase diagram at 25°C for the Na+, K+, Mg2+,Ca2+//Cl–-H2O system.

11Geofluids

mainly from CaCl2-type hydrothermal brines because of thelink between seawater chemistry and the midocean ridgehydrothermal brine flux.

3.7.2. Climate and Extreme Temperature. Ancient marineevaporite deposition required particular climate, eustatic, ortectonic juxtapositions. When megaevaporite settings wereactive within appropriate arid climatic and hydrological set-tings, huge volumes of seawater were drawn into the sub-sea-level evaporitic depressions. These systems were typicalof a region where the evaporation rates of ocean waterswere at a maximum and thus were centered on past latitu-dinal equivalents of today’s horse latitudes [40]. The term

horse latitudes encompass the regions beneath the northand south subtropical high atmospheric pressure belts[41]. The subtropical high atmospheric pressure belts aremarked by dry sinking air masses that form part of theworld’s Hadley cells. They are centered around 35°N and30°S and extend to ~25°N to 40°S, respectively [40]. Thepaleolatitudes of ~21.1–21.3°N for the Cretaceous IndochinaBlock (Vientiane) and ~20.9–27.6°N for the Cretaceous-Paleocene Lanping-Simao Blocks (Jiangcheng) indicate thatthese regions are situated in the subtropical high atmosphericpressure belts [42]. The estimated palaeoposition of theKhorat Plateau at ~21–26°N during the Jurassic to Cretaceousalso supports that these regions are situated in the subtropical

Ha

Ha+Sy

400 m

150 m

100 m

1000 m

900 m

1200 m

1100 m

Ha

Ha

Ha+Car

Ha+Car+Tac

Ha+Car

Ha+Bis+Tac

Sy

80

CaCl2MgC

l 2

MgC

l 260

2KCl

40

20

40

(a) (b) (c)

(d) (e) (f)

60

80

40 60 8020

20

60

80

SinChle

Tac

Car

Car

Sy

Tac

Bis Bis

Sy

80

CaCl2

60

2KCl

40

20

40

60

80

40 60 8020

20

60

SinChle

Tac

Car

Car

Sy

Tac

BisBis

Sy

80

CaCl2MgC

l 2 60

2KCl

40

20

40

60

80

40 60 8020

20

40

SinChle

Tac

Car

Car

Sy

Tac

Bis

Average concentrations in fluid inclusion from (A) Khorat Plateau, (B) Sergipe basin, (C) Congo basin (Tables 2 and 3)

Predicted evaporation-crystallization path on the Jänecke diagram at 50 °C for the Na+ K+, Mg2+, Ca2+//Cl− −H2O system

Predicted mineral precipitationHa, Ha+Sy, Ha+Car, Ha+Bis+Car

Predicted mineral precipitationHa, Ha+Car, Ha+Car+Tac

Predicted mineral precipitationHa, Ha+Car, Ha+Car+Tac

Core L-1 Core-9-GTP-8 Core Kou-11

Mineral succession in evaporitesHa, Ha+Sy

Mineral succession in evaporiteHa, Ha+Car, Ha+Car+Tac

Mineral succession in evaporitesHa, Ha+Car, Ha+Bis+Tac

Sergipe basin Congo basinKhorat Plateau

Figure 4: A simulated evaporation-crystallization path on the Jänecke phase diagram at 50°C. (a) Khorat Plateau; (b) Sergipe basin; (c) Congobasin. The evaporite mineral succession. (d) Khorat Plateau; (e) Sergipe basin; (f) Congo basin.

12 Geofluids

high atmospheric pressure belts [43]. Accordingly, theextreme arid state of the subtropical high atmospheric pres-sure belts is also favorable for tachyhydrite formation.

The abovementioned discussion indicates that the forma-tion of tachyhydrite is sensitive to temperature. The Creta-ceous was a time of hothouse climate, elevated atmosphericCO2, relatively warm surface and deep ocean waters, andhigh sea levels [44–56]. Apparently, the mid-Cretaceousexperienced the highest temperatures in the past 100Ma[57–60]. Evidence from reconstructed temperatures for themid-Cretaceous Khorat Plateau [61] (62.1°C), the SilurianMichigan Basin [62] (59°C), and the Quaternary Lop Nur[63] (35.6-43°C) showed that hot conditions were probablya prerequisite for the formation of potash deposits. Li et al.[64] also proposed that hydrothermal fluids may have playedan active role in the formation of the Mengye potash depositsin the Lamping-Simao Basin. Accordingly, a hot state isfavorable for evaporite formation.

3.7.3. Thermal Resource. A salinity gradient solar pond(SGSP) is a simple and effective way to capture and storesolar energy. A SGSP consists of three distinct zones, namely,an upper convecting, lower convecting, and nonconvectingzone (see Figure 5). Lake brines typically absorb solar energyas heat. Normally, this heat is lost as warm water rises to thesurface of the lake and cools by radiation, convection, andevaporation. Water, however, is a poor conductor of heat,and therefore, if this natural circulation is stopped by thepresence of a salinity gradient that causes the fluid densityto increase with depth, then heat can become trapped at thebottom of the lake. The temperature profiles are correlatedpositively with SGSP salinity [65]. The temperature in thestorage zone can exceed 90°C during the summer whereas itcan exceed 50°C in the winter for SGSPs in the Middle East[66]. In Cretaceous, therefore, a SGSP will occur in theKhorat Plateau, because a paleolatitude of the CretaceousIndochina Block lies on the subtropical high atmosphericpressure belts and tachyhydrite-saturated brine has a highdensity (1.4266-1.4404 in this study). Tachyhydrite crystalli-zation occurs under extreme dryness and high temperature,

and the thermal resource may result from the temperatureprofile in a SGSP.

3.7.4. Preservation. The hygroscopic and extremely deliques-cent nature of tachyhydrite means that tachyhydrite couldnot have been exposed to the atmosphere during or afterdeposition because, under such conditions, it alters rapidlyto a bischofite residue. Wardlaw [1] proposed that there mustalways have been a covering of brine, and a “dry-lake” stagecould not have been reached during tachyhydrite deposition,even though extreme evaporation and volume reduction arerequired for its formation. Hardie [6] also argued that tachy-hydrite must always precipitate subaqueously, and thus, itsrarity in nature may be a factor in preservation potentialand not a lack of precipitation. Tachyhydrite is located withinthe central and deepest portion of the Sergipe basins [1, 21].The very regular upward increase in bromine (from 3000 to3700 ppm) and the constancy of strontium (average valueof 1800 ppm) through the upper tachyhydrite zone couldonly have been achieved where crystallization was uninter-rupted by sudden influxes of marine water and where crystalswere not subjected to later solution or diagenetic alteration[1]. At this deposition stage, the basin may have been closedcompletely to marine influxes and tachyhydrite could havecrystallized from tachyhydrite-saturated brine, although thebrine depth may have been considerably greater [1]. A deepbrine circumstance, therefore, is probably a prerequisite fortachyhydrite preservation.

4. Conclusion

Tachyhydrite occurs in Cretaceous graben and half-grabenbasin evaporite deposits in Brazil, western Africa, and Thai-land. At an advanced stage of evaporation, the brines whichproduced these deposits must have abnormally enrichedCaCl2. Brines with these characteristics have been verifiedby the available data for the composition of fluid inclusionin halite from these evaporite deposits [14, 37, 39, 67–69].However, the mechanism of tachyhydrite formation in thoseevaporites is still being debated. The present work dealt with

Salt gradient

UCZ

NCZ

LCZ

Temperature gradient

Figure 5: The most common shape of solar pond (modified from Leblanc et al. [65]). UCZ, NCZ, and LCZ denote upper convecting, lowerconvecting, and nonconvecting zone, respectively.

13Geofluids

studying the mineral crystallization sequences during theexperimental evaporation of an average concentration fluidinclusion involving the system Na+, K+, Mg2+, Ca2+//Cl–-H2O employed from the Khorat Plateau evaporite primaryhalite. The results showed (1) a halite, sylvite, and carnallitestage at 25°C; (2) a halite, sylvite, carnallite, and bischofitestage at 35°C; and (3) a halite, sylvite, carnallite, bischofite,and tachyhydrite stage at 50°C. The crystallization paths weresimulated by the application of the Jänecke phase diagram at25°C, 35°C, and 50°C involving the system Na+, K+, Mg2+,Ca2+//Cl–-H2O. The crystallization sequence predicted onthe Jänecke phase diagram showed a good agreement withthe experimental crystallization sequences and paths. Theseresults indicated that the precipitation of tachyhydrite is sen-sitive to the temperature.

A simulated evaporation-crystallization path on theJänecke phase diagram at 50°C involving the system Na+,K+, Mg2+, Ca2+//Cl–-H2O model was carried out to predictthe mineral precipitation sequences for fluid inclusion inhalite from the Khorat Plateau, Sergipe, and Congo basins.The model showed that the evaporite mineral succession inthe Khorat Plateau, Sergipe, and Congo basins agrees wellwith the mineral precipitation sequences predicted fromtheir own fluid inclusion in halite. We found that a hot stateis favorable for tachyhydrite formation.

Because of the occurrence of tachyhydrite precipitationin the late evaporation stage and its saturated brine with ahigh density, we proposed that the temperature profile inthe solar pond may have played an active role in the forma-tion of tachyhydrite. In addition, tachyhydrite could not havebeen exposed to the atmosphere during or after deposition.The surface layer of highly concentrated brine acted as ascreen protecting the tachyhydrite from the atmosphere. So,tachyhydrite could have crystallized from tachyhydrite satu-rated brine and been preserved in a deep brine.

Data Availability

The data used to support the findings of this study areincluded within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the applied Basic ResearchProject of Qinghai Province (Grant No. 2016-ZJ-781)and the National Program on Key Basic Research Projectof China (973 Program) (Grant No. 2011CB403004) forfinancial support.

References

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