10.1 Anhydrite
Occurrence—Anhydrite is typically found in evaporite or metaevaporite deposits. It may also be found in amygdules or cracks in basalt, as a minor mineral in hydrothermal deposits, and in hot spring deposits.
Distinguishing Features—Anhydrite is generally colorless with anhedral to subhedral crystals in fine- to medium-grained aggregates. It has moderate relief which varies a bit with stage rotation. Anhydrite is orthorhombic and has three cleavages that intersect at 90°; two are visible in most thin-section views.
Anhydrite has high birefringence; maximum interference colors are 3rd-order green. Polysynthetic twinning is commonly seen under crossed polars (Fig 10.1.2).
Similar Minerals—Anhydrite is found in some of the same kind of rocks as gypsum, but has higher relief and stronger birefringence.
Optical Properties
■■ Orthorhombic; biaxial (+)
■■ 2V = 42°
■■ α = 1.570, β = 1.576, γ = 1.614
■■ δ = 0.044; interference colors range up to 3rd-order
■■ Extinction is parallel to cleavages
■■ Twinning is common
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| Fig 10.1.5 Anhydrite with Calcite
This rock contains anhydrite and calcite. The anhydrite is colorless; the calcite is grayish in the PP view. Under crossed polars, the anhydrite displays up to 3rd-order interference colors. Calcite’s colors are higher-order but it is unclear just how high. FOV = 7 mm. Photos from alexstrekeisen.it. |
Fig 10.1.6 Anhydrite
This view is entirely anhydrite; the rock is from near Balmat, New York. Some bubbles left over from poor thin-sectioning are also present. The anhydrite displays poorly-developed pseudocubic cleavage and shows little color in the PP view. In XP, it shows up to low 3rd-order interference colors and displays parallel extinction. Twins can be seen in both the PP and XP view, which is unusual for most minerals. FOV = 3.5 mm. |
10.2 Barite
Occurrence—Barite occurs most commonly as a minor mineral in hydrothermal veins. It is also found in veins in limestone, and as residual masses in clays.
Distinguishing Features—Barite is colorless, and has high relief. Granular aggregates are typical; rare individual crystals are elongate orthorhombic prisms. It has two perfect cleavages that intersect at 90o. A third, imperfect, cleavage is sometimes visible. Polysynthetic twinning is possible but rare.
Barite has low birefringence and interference colors.
Similar Minerals—Barite is hard to distinguish from celestite, another sulfate, although celestite has a greater 2V.
Optical Properties
■■ Orthorhombic; biaxial (+)
■■ 2V = 26° to 38°
■■ α = 1.636, β = 1.637, γ = 1.648
■■ δ = 0.012; interference colors are typically 1st-order gray or white, occasionally yellow or orange
■■ Extinction is parallel to the best cleavage, but not to all cleavages
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| Fig 10.2.3 Barite Quartz Vein
Barite occurs as prismatic (elongated) crystals in this rock from a hydrothermal vein deposit near Leverett, Massachusetts. The rest of the vein consists of quartz. The quartz has lower relief and is generally clear compared with the grayish barite. It is clouded in places with fluid and other inclusions. In XP, the barite, like quartz, has low 1st-order interference colors. Some barite crystals show simple twins. This vein formed at temperatures around 200°C or below from fluids associated with the adjacent Mesozoic rift valley. FOV = 2.5 mm. |
Fig 10.2.4 Barite with Chabazite
This rock from central Nova Scotia, contains moderate-relief, colorless barite (BaSO4), moderate-relief, slightly brownish dolomite, and low-relief, colorless chabazite (chabazite group zeolite), developed as a crust on altered dark brown to black basalt (PP). In XP, the barite shows 1st-order gray to white intereference colors, dolomite is characteristically pearly gray to pastel. Chabazite’s interference colors are so low that the mineral appears almost isotropic. FOV = 8 mm. |
10.3 Gypsum
Occurrence—Massive gypsum is associated with evaporites. It is also found interlayered in limestones and shales, as fillings in fractures or holes in many sedimentary rocks, and as a gangue mineral or alteration product in some ore deposits.
Distinguishing Features—Gypsum is colorless and has low relief. It typically occurs as anhedral to subhedral aggregates, often with variable grain size. Less commonly, it is fibrous. It has one good cleavage and coarse crystals commonly cleave into parallel-sided plates or rhombs. Two other (poor) cleavages are sometimes seen. Polysynthetic twinning is common, sometimes created during thin section grinding.
Gypsum has low birefringence and display only 1st-order interference colors.
Similar Minerals—Gypsum is similar in some ways to anhydrite but has lower relief and birefringence. Brucite has higher relief.
Optical Properties
■■ Monoclinic; biaxial (+)
■■ 2V = 58°
■■ α = 1.520, β = 1.523, γ = 1.530
■■ δ = 0.010; maximum colors are first-order weak yellow
■■ Extinction is parallel to the best cleavage but not to all cleavages
■■ Cleavage angle is about 65o when two cleavages are present
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| Fig 10.3.5 Gypsum with Anhydrite
This sample from Roccastrada, Italy, contains high-relief anhydrite and lower-relief gypsum. Both are colorless in the PP view and show twins in XP. In XP, the gypsum shows up to 1st-order gray and white interference colors; the anhydrite displays up to 2nd-order violet colors. FOV = 7 mm. Photos from alexstrekeisen.it. |
Fig 10.3.6 Gypsum with Calcite
Gypsum (colorless somewhat prismatic crystals with high relief) with calcite (gray to black) from the Presolana gypsum mine near Bergamo, Italy. Gypsum crystals appear zoned in both the PP and XP views with gray to black cores and in XP shows up to 1st-order gray interference colors. Coarse calcite in the upper right shows very high order interference colors; fine-grained calcite surrounds gypsum crystals. FOV = 7 mm. Photos from alexstrekeisen.it. |
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| Fig 10.3.7 Alabaster
This is alabaster, a rock composed entirely of fine-grained gypsum. Alabaster has historically been used for carving because gypsum is very soft. In the PP view, the gypsum grains are clear; in the XP view they show up to 1st-order yellow interference colors. Note the many bubbles that are present (due to poor thin section making). The rock comes from a quarry in Pomaia, Italy. FOV = 3.5 mm. |
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