14.9: Sulfate Minerals
- Page ID
- 18680
Anhydrous Sulfates
anhydrite CaSO4
barite BaSO4
celestite SrSO4
anglesite PbSO4
Hydrous Sulfates
gypsum CaSO4•2H2O
chalcanthite CuSO4•5H2O
epsomite MgSO4•7H2O
antlerite Cu3SO4•(OH)4
alunite KAl3(SO4)2(OH)6
Mineralogists divide sulfate minerals into two groups: the anhydrous sulfates and the hydrous sulfates. Chemistries and structures of the anhydrous sulfates are related to the carbonates with SO4 replacing CO3. More than 100 sulfate minerals are known, and most are rare. Gypsum and anhydrite are the only rock-forming sulfates.
Sulfates, like borates and nitrates, are common minerals in evaporite deposits. They are also, however, found as secondary minerals in many different kinds of rocks.
For more general information about sulfates, see Section 7.4.4 in Chapter 7.
Anhydrite CaSO4
Origin of Name
From the Greek word anhydros, because it lacks water (compared with gypsum).
Hand Specimen Identification
Anhydrite is an orthorhombic mineral with three well-developed cleavages at 90° to each other, but the most common occurrences are massive and fine-grained like the sample seen in Figure 14.404. Anhydrite may look like calcite or gypsum, but is easily distinguished from calcite by its higher specific gravity and from gypsum by its hardness.
When crystals are visible, identification is simplified. Figure 14.405 shows anhydrite crystals displaying orthorhombic symmetry and cleavage. Many coarsely crystalline anhydrite specimens, like the one seen in Figure 14.406, however, are not as clearly orthorhombic.
Physical Properties
| hardness | 3 to 3.5 |
| specific gravity | 2.98 |
| cleavage/fracture | perfect cubic (010), good (100) and (001)/uneven, splintery |
| luster/transparency | pearly, vitreous/translucent |
| color | colorless |
| streak | white |
Properties in Thin Section
Anhydrite is colorless in thin section, displays up to third-order green interference colors, has pseudocubic cleavage, and displays parallel extinction. Gypsum has lower relief and birefringence; barite and celestite have higher indices of refraction and low birefringence. Biaxial (+), a = 1.570 , β = 1.575, γ = 1.614, δ = 0.044, 2V = 44°.
Crystallography
Anhydrite is orthorhombic, a = 6.22, b = 6.97, c = 6.96, Z = 4; space group \(C\dfrac{2}{c}\dfrac{2}{m}\dfrac{2_1}{m}\); point group \(\dfrac{2}{m}\dfrac{2}{m}\dfrac{2}{m}\).
Habit
Anhydrite is usually massive, granular, or fibrous; individual crystals, typically tabular or prismatic, are not typical.
Structure and Composition
Anhydrite‘s structure is similar to zircon’s; (SO4)2- tetrahedra share edges and are linked by (CaO8) polyhedra. Anhydrite is generally close to CaSO4 in composition but may be partially hydrated (tending toward gypsum).
Occurrence and Associations
Anhydrite is typically an evaporite mineral associated with gypsum, sulfur, halite, calcite, or dolomite. Thick anhydrite beds are well known. It is also found in amygdules or cracks in basalt, as a gangue mineral in hydrothermal ore deposits, as a component of soils, or as a hot spring deposit.
Related Minerals
Anhydrite is chemically related to other anhydrous sulfates, including barite (BaSO4); celestite (SrSO4); and anglesite (PbSO4), but has a different structure. A polymorph of anhydrite, γ-CaSO4, forms when gypsum (CaSO4•2H2O) is dehydrated.
Barite BaSO4
Origin of Name
From the Greek word barys, meaning “heavy.”
Hand Specimen Identification
High specific gravity, white or light color, distinctive crystal habit, and two principle cleavages at 90° all help identify barite.
Figure 14.407 shows nondescript barite from Michigan’s Upper Peninsula. Most barite has a similar appearance. However, sometimes this mineral forms spectacular reticulated masses such as those seen in Figures 14.408 and 14.409. See also Figures 3.74 and 7.48. When blue, as in Figure 14.408, barite appears similar to celestite, but barite has greater density that is easily discerned by the heft test.
Figure 10.1, from a previous chapter, is a photo of spectacular blue orthorhombic barite crystals up to 50 mm tall. It is from the Huanuco Department, Peru. Figure 10.59 shows similarly shaped, but clear, crystals.
Physical Properties
| hardness | 3 to 3.5 |
| specific gravity | 4.5 |
| cleavage/fracture | perfect (001), good (010) and {210}/uneven |
| luster/transparency | vitreous, pearly/transparent to translucent |
| color | white, gray, or colorless |
| streak | white |
Properties in Thin Section
Barite is colorless in thin section and displays up to second-order yellow interference colors. Biaxial (+), a = 1.636 , β = 1.637,g = 1.648, δ = 0.012, 2V = 37°.
Crystallography
Barite is orthorhombic, a = 8.87, b = 5.45, c = 7.14, Z = 4; space group \(P\dfrac{2_1}{n}\dfrac{2_1}{m}\dfrac{2_1}{a}\); point group \(\dfrac{2}{m}\dfrac{2}{m}\dfrac{2}{m}\).
Habit
Barite’s crystal habit is complex and variable. Tabular crystals may combine to form cockscomb aggregates called barite roses (Figure 14.410) or crested barite (as seen in Figures 14.408 and 14.409, above). Individual rosettes are common. Barite roses appear somewhat like gypsum roses such as those in Figure 14.346. Barite is also common as massive concretions, veins, or beds.
Structure and Composition
Barite contains (SO4)2- tetrahedra linked by (BaO12) polyhedra. Each (BaO12) group is bonded to seven individual (SO4)2- tetrahedra. The structure differs from anhydrite’s.
Occurrence and Associations
Barite is a common gangue mineral in hydrothermal veins, associated with fluorite, galena, quartz, calcite, or dolomite. It is also found in veins in limestone, and as residual masses in clays.
Related Minerals
Barite is chemically and structurally similar to celestite, SrSO4, and anglesite, PbSO4, although solid solutions between the three are limited in nature.
Celestite SrSO4
Origin of Name
From the Latin word caelestis, meaning “celestial,” in reference to the sky-blue color of some celestite.
Hand Specimen Identification
Celestite is commonly light blue, and the blue color can be diagnostic (Figure 14.411), although it is quite similar to the color of blue calcite. Figure 7.50 shows another example of blue celestite. Celestite is sometimes associated with native sulfur (Figure 14.432). Orthorhombic crystal shape and 90° cleavage angle aid identification.
When clear, like the celestite in Figure 14.412, this mineral may resemble strontianite, another orthorhombic mineral with similar density and hardness. Some celestite resembles barite, but barite is much heavier.
Physical Properties
| hardness | 3 to 3.5 |
| specific gravity | 3.97 |
| cleavage/fracture | perfect basal (001), good prismatic {210}, poor (011)/uneven |
| luster/transparency | vitreous, pearly/transparent to translucent |
| color | colorless, blue, rarely light red |
| streak | white |
Properties in Thin Section
Celestite is similar to barite in thin section but may have a light blue color. Biaxial (+), α = 1.622 , β = 1.624, γ = 1.631, δ = 0.009, 2V = 51°.
Crystallography
Celestite is orthorhombic, a = 8.38, b = 5.37, c = 6.85, Z = 4; space group \(P\dfrac{2_1}{n}\dfrac{2_1}{m}\dfrac{2_1}{a}\); point group \(\dfrac{2}{m}\dfrac{2}{m}\dfrac{2}{m}.
Habit
Tabular crystals, similar to those of barite, are typical for celestite. Acicular, fibrous, reniform, or granular crystals are also common.
Structure and Composition
Although in principal a complete solid solution exists between celestite (SrSO4) and barite (BaSO4), most celestite is close to end-member SrSO4. Small amounts of Pb may substitute for Sr.
Occurrence and Associations
Celestite is a rare mineral found in sedimentary rocks and in veins. Associated minerals often include dolomite, gypsum, halite, calcite, fluorite, or barite.
Related Minerals
Celestite is isostructural with barite, BaSO4, and anglesite, PbSO4. It may alter to strontianite, SrCO3, the only other Sr mineral of significance.
Anglesite PbSO4
Origin of Name
Named after the Welsh island of Anglesey, where it was discovered.
Hand Specimen Identification
Colorless to lightly colored translucent crystals, an adamantine luster, high density (easily discerned by the heft test), and association with galena distinguish anglesite.
Physical Properties
| hardness | 2.5 to 3 |
| specific gravity | 6.38 |
| cleavage/fracture | perfect (001), good {210}/conchoidal |
| luster/transparency | adamantine/translucent |
| color | white |
| streak | white |
Properties in Thin Section
Anglesite is biaxial (+), α = 1.877 , β = 1.883, γ = 1.894, δ = 0.017, 2V = 75°.
Crystallography
Anglesite is orthorhombic, a = 8.47, b = 5.39, c = 6.94, Z = 4; space group \(P\dfrac{2_1}{n}\dfrac{2_1}{m}\dfrac{2_1}{a}\); point group \(\dfrac{2}{m}\dfrac{2}{m}\dfrac{2}{m}.
Habit
Anglesite is normally blocky massive or in granular aggregates; individual crystals may be tabular, prismatic, bipyramidal, or nearly equant.
Structure and Composition
Anglesite, being isostructural with barite and celestite, may contain significant amounts of Ba or Sr.
Occurrence and Associations
Anglesite, a common alteration product of galena, PbS, is found in oxidized portions of Pb deposits. Associated minerals include cerussite, SrSO4; wulfenite, (PbMoO4); smithsonite, ZnCO3; hemimorphite, Zn4(Si2O7)(OH)2•H2O; and pyromorphite, Pb5(PO4)3Cl.
Related Minerals
There are few common Pb-minerals; galena is really the only one. Anglesite may be the second most common.
Gypsum CaSO4•2H2O
Origin of Name
From the Arabic word jibs, meaning “plaster.”
Hand Specimen Identification
Softness (H = 2) and three cleavages, tabular or platy crystals (like the crystals seen in Figure 14.414), and gray or white color distinguish gypsum. This mineral commonly twins; penetration and swallowtail twins (Figure 14.415) are particularly common and diagnostic. Gypsum is sometimes confused with anhydrite, a similar light-colored mineral that forms in the same environments.
The two photos above show classic gypsum; the sample in Figure 14.415 is twinned. Figure 3.17 shows a spectacular example of prismatic stellate gypsum crystals. They are quite small, but at the other end of the size scale, Figure 7.46 shows some of the largest crystals known. They are gypsum crystals that formed from hydrothermal waters in a cave in Mexico. The photo includes a person for scale.
Physical Properties
| hardness | 2 |
| specific gravity | 2.32 |
| cleavage/fracture | perfect basal (010), good (100) and {011}/conchoidal |
| luster/transparency | vitreous, pearly/transparent to translucent |
| color | colorless, white, variable |
| streak | white |
Properties in Thin Section
Gypsum is colorless in thin section and displays up to first-order yellow interference colors. It may be confused with anhydrite or barite, but anhydrite has higher relief and birefringence, and barite has higher relief and parallel extinction. Biaxial (+), α = 1.520 , β = 1.523, γ = 1.529, δ = 0.009, 2V = 58°.
Crystallography
Gypsum is monoclinic, a = 5.68, b = 15.518, c = 6.29, β = 113.83°, Z = 4; space group \(A\dfrac{2}{n}\); point group \(\dfrac{2}{m}\).
Habit
Gypsum is often massive but may form granular, acicular, or prismatic crystals. Typical gypsum crystals are tabular (like the ones seen in Figure 14.414, above), thick to thin, often forming as elongated sheaves or masses. When gypsum precipitates in wet sand, desert roses such as the ones seen in Figure 14.416 may develop. Twinning and crystal intergrowths are common.
Structure and Composition
In gypsum, layers of H2O alternate with layers containing Ca2+ and (SO4)2-. Ca2+ is bonded to six O and two H2O. The Ca2+ polyhedra link isolated (SO4)2- tetrahedra. Gypsum rarely contains significant impurities.
Occurrence and Associations
Gypsum, the most common sulfate mineral, is a rock-forming mineral of many evaporite deposits where it may be associated with other bedded salts. It is commonly found in sedimentary deposits of playa lakes (Figure 14.417). Gypsum is also found interlayered with limestones or shales and may be found in fractures or cracks in a variety of sedimentary rocks (Figure 14.418). Gypsum is a gangue mineral or alteration product in some ore deposits and is occasionally found around fumaroles.
Varieties
Well-known varieties of gypsum include selenite (clear, often needle-like, crystals), alabaster (compact white masses), and satin spar (fibrous deposits in veins) (Figure 14.419).
Related Minerals
Other hydrous sulfates include chalcanthite, CuSO4•5H2O; epsomite, MgSO4•7H2O; antlerite, Cu3SO4(OH)4; and alunite, KAl3(SO4)2(OH)6. Gypsum commonly forms from, or alters to, anhydrite, CaSO4.
Epsomite MgSO4•7H2O
Origin of Name
Derived from Epsom, England, where epsom salts were first precipitated from mineral waters.
Hand Specimen Identification
Typical crusty habit, low specific gravity, and salty taste are characteristic of epsomite. Figure 14.420 shows a specimen containing very fine needles of epsomite. Figure 14.421 also contain needles, but they are coarse.
Physical Properties
| hardness | 2 to 2.5 |
| specific gravity | 1.68 |
| cleavage/fracture | perfect (010), good {011}/conchoidal |
| luster/transparency | vitreous/transparent to translucent |
| color | colorless |
| streak | white |
Properties in Thin Section
Epsomite is biaxial (+), α = 1.433 , β = 1.455, γ = 1.461, δ = 0.028, 2V = 52°.
Crystallography
Epsomite is orthorhombic, a = 11.96, b = 12.05, c = 6.88, Z = 4; space group \(P2_{1}2_{1}2_{1\); point group 222.
Habit
Epsomite may be botryoidal, fibrous, or colloform, and typically forms as crusts. Large crystals are rare.
Structure and Composition
Epsomite contains two types of H2O molecules; some are coordinated with Mg and some are not.
Occurrence and Associations
Epsomite is uncommon but occurs in caves or mine adits as encrustations, as precipitates on carbonate or mafic igneous rocks, as an evaporite mineral, or as gangue in ore deposits. It is usually associated with other sulfates.