Sulfate

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 (°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 %

relative humidity