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Abstract

Sulfate and chloride minerals occur as cements, displacive and replacive nodules, and interbedded strata in carbonate rocks. They precipitate from evaporatively concentrated waters in arid-region lakes, ponds and lagoons along marine shorelines and, more rarely, in deeper shelf and basinal settings with restricted marine inflow. Evaporite deposits are products of arid environments; however, evaporitic solutions are highly mobile due to their high density. Evaporative brines thus may migrate into adjacent or underlying strata and precipitate diagenetic sulfate or chloride minerals (generally as displacive crystals and nodules, or as carbonate replacements) in units that may otherwise be unrelated to arid settings. Even after deposition and substantial burial, evaporite minerals can be remobilized and reprecipitated in distant, stratigraphically unrelated units. Therefore, careful petrographic analysis is needed to determine both the conditions of primary deposition and the timing of diagenetic events in evaporite-bearing limestones and dolomites.

Barite, celestite and anhydrite also can occur as hydrothermal precipitates in carbonate rocks.

Calcite solution-fill replacement (calcitization) of gypsum and anhydrite results from the dissolution of evaporites by sulfate-poor pore fluids. These pore fluids become saturated to supersaturated with respect to Ca2+; if there is enough bicarbonate in the pore fluids, calcite may precipitate.

Anhydrite crystals have high birefringence (up to third order); in thin section, the other common sulfate and halide minerals have much lower birefringence. Anhydrite’s birefringence also can appear to “twinkle” like that of calcite, but the effect is less strongly developed than in calcite. Anhydrite crystals normally are colorless, but may contain inclusions of precursor phases. Anhydrite may form large tabular crystals or felted, fibrous crystal masses (generally as nodules). The larger crystals may exhibit pseudo-cubic cleavages.

Gypsum, celestite and barite can be extremely difficult to differentiate from each other in thin section. They all have low relief and birefringence (gray to white). Gypsum tends to form colorless, elongate, tabular to lenticular crystals or fibrous masses or aggregates of crystals. Gypsum also tends to form poikilotopic cements that encase numerous grains – siliciclastic or carbonate. Gypsumʼs cleavage is lozenge-shaped; therefore, if cleavage planes are visible, they are diagnostic for gypsum. Gypsum crystals may form rosettes and twins that are called swallow- or fish-tailed selenite. These larger crystals form displacively below the sediment/water interface in unconsolidated sediments; such crystals contain abundant inclusions of the sediment. Selenite crystals also can grow upward from the sediment-water interface into standing saline water bodies.

Celestite ranges from colorless to blue in thin section. Blue crystals of celestite can be pleochroic, which helps to differentiate it from gypsum and barite. Celestite forms fibrous to rounded aggregates of crystals. When it is found in fibrous masses, the crystals are normally more elongate than similar crystals of gypsum. Cleavage, when visible, is pseudo-cubic.

Barite normally is colorless and forms globular concretions, granular to earthy masses, fibrous or bladed crystals. Cleavage, when visible, is pseudo-cubic. Because barite and celestite form a solid solution series, they are extremely difficult to tell apart in thin section. Generally, other chemical techniques must be used to be confirm identifications. Like gypsum, barite also forms crystal rosettes.

Halite is difficult to see in thin section, because it is isotropic and highly soluble. Because halite is isotropic, it can easily be overlooked if the cleavages are not prominent or if it doesnʼt contain inclusions (i.e., it may be indistinguishable from the glass on which the section is mounted). Impregnating the sample with blue epoxy makes the halite stand out from the porosity. If the thin section is not properly prepared (cut and ground in oil, not water), however, halite is unlikely to be preserved. Halite crystals are normally colorless and exhibit low relief, but they may appear dusty due to the great abundance of solid and liquid inclusions. Halite can occur as a poikilotopic cement in either carbonate or siliciclastic strata.

Anhydrite – CaSO4, orthorhombic

Gypsum – CaSO4 • 2H2O, monoclinic

Celestite – SrSO4, orthorhombic, complete solid solution series exists with barite

Barite – BaSO4, orthorhombic, commonly contains up to 3% lead

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