A natron source at Pikrolimni Lake in Greece? Geochemical evidence

E. Dotsika et al. / Journal of Geochemical Exploration 103 (2009) 133–143
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64% of all salts that precipitate, while halite only 10%. On the contrary,
Fig. 6 shows the results of PL-5/2003 sample when the initial chemical
composition of this water was modeled. It seems that when the model
stops halite is more abundant in the precipitates than trona: trona
represents only 0.5% of all salts that precipitate, while halite 60%.
These two models show the change which may have happened in the
water chemistry of the lake's springs and why trona is so hard to find
in Lake Pikrolimni when the models reaches their lowest point: trona
salts are hoarded by the deposition of large quantities of halite, as
shown by Shortland et al. (2006) in their study about Wadi Natrun.
The alimentation of the lake in the past had happened mainly from
deep springs with similar chemistry while today is arisen mainly from
meteoric waters and irrigation groundwaters (Pliny; Many public
testimonies vouch that there were a lot of springs in the middle of
the lake, but after the big earthquake of 1981 these springs have
disappeared).
The chemical composition of the residual water and of three samples
(S/1/2002; b/2/2002; PL-5/2003) after the simulation of evaporation
was similar with the chemical composition of the lake's water in
conditions of intense evaporation (Table 5).
Especially the chemical composition of the residual water, coded
S/1/2002, after the evaporation simulation, is similar to that of the
lake brine PL-9/2006 (Table 5). Furthermore, the model shows that
natron starts to precipitate when Cl − content is 1417 mmol/L [when
the concentration factor (CF) is about 395 times: 395⁎3.8 mmol/L
(initial Cl of s/1/2002)=1500 mmol/L] while trona starts to precipitate
when Cl − content is approximately 2500 mmol/L [when the
concentration factor (CF) is about 720 times: 700⁎3.8 mmol/L (initial Cl
of s/1/2002)=2651 mmol/L]. The lake water reaches similar concentrations
only during the summer, August (but not every summer), as
observed by sampling. According to Pliny, natron was collected in
Macedonia only in the rising of the Dog Star for nine days. This timing
places this event at the beginning of the hottest part of the summer,
when the lake is found at the peak of evaporation and trona starts to
precipitate. However, the lake does not reach such high concentrations
every year, a fact that is due to the weather (temperature,
humidity, precipitation). Therefore, the collection would have been
taking place only during particularly hot and dry summers.
7. Conceptual model
Fig. 5. Saturation Index (Log saturation ratio) versus ionic strength (logarithmic). White
triangles: nahcolite; x: gaylussite; –: pirssonite; white diamonds: natron; black
squares: trona; white circles: halite; crosses: mirabilite; white squares: thenardite;
crossed x: thermonatrite.
CaCO 3 þ 2Na þ CO 3 þ 2H 2 O↔Na 2 Ca⁎ðCO 3 Þ 2
⁎2H 2 O:
Also when the concentration factor (CF) is about 700 times of the
initial chemical composition, burkeite and trona precipitation initiates.
Much later, when the water is concentrated by about a factor of
1200, halite begins to precipitate. When the models stop, halite and
trona are the more abundant precipitates. Trona is more abundant in
the precipitates than halite: trona (50%) and natron (14%) represent
The Na/Cl ratio in Lake Pikrolimni supports evaporative concentrations
as the dominant process in the chemical evolution of these lake
waters that have high proportions of Na + and HCO 3 − . The derived high
Na + and HCO 3 − contents are due to silicate hydrolysis. The computing
program has shown that the dilute spring inflow was concentrated (by
the assumption of Cl − conservation in solution throughout evaporation)
about 12000 times to obtain the most saline residual actual lake water,
PL-8/2002; PL-9/2006). However, this evaporation budget calculation
(Table 5), if compared with the actual analysis of the most saline
Pikrolimni brine, shows that some constituents are not conservative,
due to different removal mechanisms. Therefore, it is believed that in the
past, the solute composition of such waters, which are controlled by
silicate hydrolysis and evaporative concentration, gave the natron salts
of Lake Pikrolimni. Namely, in the past, after acquisition of solutes from
atmospheric precipitation, hydrolysis of silicate and possibly CO 2 of
deep origin, the waters are subjected to evaporation (at the surface or by
capillarity). Initially, the concentrations of all constituents increase until
the mineral precipitation occurs. The precipitation may take the form of
efflorescent crusts on top of the surface or of intergranural caliche-type
films and cements. The efflorescent crusts are the product of complete
desiccation and they are subjected to complete or differential solution
byrain.ThisrecyclingofsaltsisobservedactuallyatLakePikrolimni.
Thus, during summer when the water level of the lake is low or there is
no water at all (total desiccation), the first rains dissolve the residual
brines (or salts) and get charged by solutes. This recycling of salts is also