Sulfate

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Author: Hans-Jürgen Schwarz

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Overview

The sulfate salts most frequently found on buildings and monuments are listed. The various salt phases are discussed in detail on separate pages, where their most likely origin and their potential deterioration action is discussed. Illustrations of their deterioration patterns are found in the Deterioration Pattern section and in some of the case studies.

Salt Formula Solubility at 20°C Deliquescence RH at 20°C Light refraction Birefringence Notes
Bassanite CaSO4•0.5H2O 3.0 g/l no = 1.558
ne = 1.586
Δ = 0.028 hardly soluble in water
Boussingaultite (NH4)2Mg(SO4)2•6H2O 159 g/l (0°C), 1300 g/l (100°C) nx = 1.472
ny = 1.473
nz = 1.479
Δ = 0.007
Epsomite MgSO4•7H2O 2.893 mol/kg 91.3 % nx = 1.432
ny = 1.453
nz = 1.4609
Δ = 0.0284 can be produced from an aqueous solution under 50°C
Ettringite Ca6Al2(SO4)3(OH)12•26H2O no = 1.491
ne = 1.470
Δ = 0.021 insoluble in water
Gypsum CaSO4•2H2O 2.14 g/l > 99% RH at 20°C α = 1.5207
β = 1.5230
γ = 1.5299
Δ = 0.0092
Mirabilite Na2SO4•10H2O 1.353 mol/kg 95.6 % nx = 1.394
ny = 1.396
nz = 1.398
Δ = 0.004 soluble in water and glycerin,
insoluble in pure alcohol
easily loses some water, converts to thenardite at 32°C
abnormal blue or brown interference colors
Pentahydrite MgSO4•5H2O nx = 1.482
ny = 1.492
nz = 1.493
Δ = 0.011 unstable in air
Schönite K2Mg(SO4)2•6H2O 250 g/l nx = 1.4607
ny = 1.4629
nz = 1.4755
Δ = 0.0148
Thaumasite Ca3Si(OH)6(CO3)(SO4)•
12H2O
no = 1.491
ne = 1.470
Δ = 0.039 insoluble in water
Thenardite Na2SO4 162 g/l 81.7% (25°C) nx = 1.468
ny = 1.473
nz = 1.483
Δ = 0.015 soluble in water and glycerin,
insoluble in pure alcohol
Hexahydrite MgSO4•6H2O 3.611 mol/kg α = 1.426
β = 1.453
γ = 1.456
Δ = 0.030 can be produced from an aqueous solution between 48-69 °C
Arcanite K2SO4 111.5 g/l 98.2% (10°C), 97.6% (20°C), 97% (30°C) nx = 1.494
ny = 1.495
nz = 1.497
Δ = 0.003
Aphthitalite K3Na(SO4)2
Na2SO4•3K2SO4
no = 1.491
ne = 1.499
Δ = 0.008
Astrakanite Na2Mg(SO4)2•4H2O nx = 1.483
ny = 1.486
nz = 1.487
Δ = 0.004
Darapskite Na3(SO4)(NO3)•H2O
NaNO3•Na2SO4•H2O
nx = 1.391
ny = 1.481
nz = 1.486
Δ = 0.095 can be produced from an aqueous solution of Na2SO4 and NaNO3 at 14-50°C
Glauberite Na2Ca(SO4)2 nx=1.515
ny=1.535
nz=1.536
Δ=0.021
Gorgeyite K2Ca5(SO4)6•H2O nx= 1.560
ny = 1.569
nz = 1.584
Δ = 0.024
Leonite K2Mg(SO4)2•4H2O nx=1.479
ny=1.483
nz=1.487
Δ=0.008
Syngenite K2Ca(SO4)2•H2O 2.5 g/l nx = 1.501
ny = 1.516
nz = 1.517
Δ = 0.016
Anhydrite CaSO4 nx=1.5698
ny=1.5754
nz=1.6136
Δ=0.0438
Starkeyite MgSO4•4H2O nx = 1.49
ny = 1.491
nz = 1.497
Δ = 0.007 unstable in air
Sodium sulfate heptahydrate Na2SO4•7H2O 3.143 mol/kg 89.1 %
Kieserite MgSO4•H2O 5.600 mol/kg nx = 1.520
ny = 1.533
nz = 1.584
Δ = 0.064 can be produced from an aqueous solution over 67°C
Magnesium sulfate
Meridianiite MgSO4•4H2O
Sodium sulfate
Sodium sulfate phase III Na2SO4 4.428 mol/kg 82.9 %