Jökull - 01.01.2010, Page 169

Low-temperature alteration of basalts
Table 3. Mineral reactions used in the geochemical calculations. For solubility constants, see Gysi and Stefáns-
son (2010). – Efnahvörf milli steinda og vatns sem notast var við í líkanreikningunum. Fyrir leysnifasta, sjá
Gysi and Stefánsson (2010).
Mineral Reaction
Primary minerals
Stapafell glass Si1.000Ti0.024Al0.358Fe0.188Mg0.281Ca0.264Na0.079K0.008O3.370+2.644H++0.726H2O
= 0.358Al3++0.264Ca2++0.171Fe2++0.017Fe3++H4SiO4+0.008K++0.281Mg2++0.079Na++0.024Ti(OH)4
Secondary minerals
Allophane Al2O3(SiO2)1.22(H2O)2.5+6H+ = 2Al3++1.22H4SiO4+3.06H2O
Imogolite Al2SiO3(OH)4+6H+ = 2Al3++H4SiO4+3H2O
Kaolinite Al2Si2O5(OH)4+6H+ = 2Al3++2H4SiO4+H2O
Iron hydroxide Fe(OH)3+3H+ = Fe3++3H2O
Aluminium hydroxide Al(OH)3+3H+ = Al3++3H2O
Magnesite MgCO3+H+ = Mg2++HCO−3
Calcite CaCO3+ H+ = Ca2++HCO−3
Dolomite CaMg(CO3)2+2H+ = Mg2++Ca2++2HCO−3
Siderite FeCO3+H+ = Fe2++HCO−3
Ca-Mg carbonate FeMg(CO3)2+2H+ = Mg2++Fe2++2HCO−3
Chlorite Fe3.15Mg1.85Al2Si3O10(OH)8+16H+ = 1.85Mg2++3.15Fe2++2Al3++3H4SiO4+6H2O
Celadonite KMgAlSi4O10(OH)2+6H++4H2O = K++Mg2++Al3++4H4SiO4
Fe-Celadonite KFeAlSi4O10(OH)2+ 6H++4H2O = K++Fe2++Al3++4H4SiO4
Mg-Fe smectite Na0.04K0.10Ca0.21Mg1.44Fe1.78Al1.00Si3.00O10(OH)2+10H+ =
0.04Na++0.10K++1.44Mg2++0.21Ca2++1.78Fe2++Al3++3H4SiO4
Ca-Mg-Fe smectite Ca0.50Mg1.05Fe1.30Al1.00Si3.00O10(OH)2+10H+ = 0.5Ca2++1.05Mg2++1.3Fe3++Al3++3H4SiO4
Calcedony SiO2+2H2O = H4SiO4
Analcime NaAlSi2O6·H2O+H2O+4H+ = Na++Al3++2H4SiO4
Ca-mordenite Ca0.5AlSi5O12·4H2O+4H2O+4H+ = 0.5Ca2++Al3++5H4SiO4
Ca-stilbite CaAl2Si7O18·7H2O+3H2O+8H+ = Ca2++2Al3++7H4SiO4
Ca-Chabacite CaAl2Si4O12·6H2O+8H+ = Ca2++2Al3++4H4SiO4+2H2O
Heulandite CaAl2Si7O18·6H2O+4H2O+8H+ = Ca2++2Al3++7H4SiO4
Laumondite CaAl2Si4O12·4.5H2O+8H+ = Ca2++2Al3++4H4SiO4+0.5H2O
Mesolite Ca0.667Na0.666Al2Si3O10·2.667H2O+8H+ = 0.667Ca2++0.666Na++2Al3++3H4SiO4+0.667H2O
Scolecite CaAl2Si3O10·3H2O+8H+ = Ca2++2Al3++3H4SiO4+H2O
Thomsonite Ca2NaAl5Si5O20·6H2O+20H+ = 2Ca2++Na++5Al3++5H4SiO4+6H2O
losilicates, taking into account appropriate elemental
substitutions in common crystallographic sites. The
thermodynamic properties of the end-member phyl-
losilicates were taken from Fritz (1981), Tardy and
Fritz (1981), Kam (1986) and Holland and Powell
(1998). The approach adopted in this study was to
calculate the solubility of solid solutions of fixed com-
position and apply these in the simulation calcula-
tions. The composition of phyllosilicates and carbon-
ates selected for the calculations was taken from natu-
ral analogues of both CO2-rich and depleted systems
(Neuhoff et al., 1999, 2006; Rogers et al. 2006).
The redox conditions were not fixed during the re-
action calculations. Instead, redox potential was con-
trolled by the balance between the supply of Fe(II)
and Fe(III) through basalt dissolution and the removal
of Fe(II) and Fe(III) by incorporation in precipitating
secondary minerals. This may have resulted in some
uncertainties in the mass of various Fe-containing
phases. Data on natural water composition in Iceland
were selected from the database published by Stef-
ánsson and Arnórsson (1999). Only low-temperature
geothermal waters from Iceland were selected with
temperatures in the range of 20 to 150◦C. In the case
of >100◦C, the chalcedony geothermometry temper-
ature was selected. The aqueous speciation of the
aquifer waters was calculated with the aid of the
WATCH program (Bjarnason, 1994).
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