Lacustrine carbonate minerals

by Mark Shapley

Carbonates are the most common of the chemically precipitated minerals occurring in lacustrine sediment.  In contrast with marine sediments, inorganically precipitated (though often biomediated) carbonates are widespread and sometimes dominant components of lacustrine sediment assemblages.  When seen in smear slides, they are even more prominent than their abundance would suggest, due to their characteristically high birefringence when viewed by transmitted, cross-polarized light. In silt size ranges, no other common mineral component of lacustrine sediment displays the bright interference colors characteristic of the carbonate minerals.  The mineral occurrence, form, size, and assemblages of associated constituents are all influenced by the chemistry and physical state of lake water from which they precipitate (as are other environmentally informative characteristics, such as isotopic composition and elemental substitution, not discernible by optical microscopy).  You will also see carbonate minerals occurring as detrital sediment components, sometimes in abundance.  

Calcite

Calcite (calcium carbonate, CaCO₃) is the predominant chemically precipitated carbonate mineral in most freshwater lakes.  Lacustrine calcite forms as a primary precipitate in the water column of lakes, and also as a diagenetic mineral at or below the sediment-water interface.  As a primary lacustrine precipitate (a process often biomediated through photosynthesis), calcite can take the form of regular rhombohedra following the crystallographic structure of the mineral, but often adopts forms that represent kinetically-controlled departures from the euhedral growth structure.  These commonly include ellipsoids, ‘footballs’, and twinned forms in which two or more grains grow in interpenetrating configurations. Although lacustrine calcite forms over a wide range of sizes, kinetics of crystal growth often result in a strong mode in grain size distribution, giving the calcite component a ‘well-sorted’ character.  A commonly observed upper limit in dimension, around 30 μm, relates to grain settling rates in the water column. 

Endogenically precipitated calcite from Lago Titicaca, Peru, in plane-polarized (top) and cross-polarized light (above). Note gradation in grain form from slightly rounded rhombs to axially symmetric ellipsoids.

One exception to this generalization is provided by charophytic algae.  Through CO₂ uptake, charophytes and other macrophytic plants stimulate localized CaCO₃ precipitation on and around plant structures; calcite crusts and scales formed in this fashion may be considerably larger than water-column precipitates.  Diagenetic calcite may appear as aggregates of fine-grained, poorly-formed grains intermingled with organic matter.  In smear slides you may see detrital calcite (or dolomite) occurring in any grain-size fraction up to sand, often in blocky rhombic forms if well-preserved, perhaps exhibiting pitting and other resorption textures if the lake environment is undersaturated with respect to the mineral phase.

Large calcite flakes, probable charophyte coatings from a shallow-water setting, Laguna Miscanti, Chile.  Macrophyte coatings may be crystallographic composites, with optical extinction occurring over local regions at different angles of stage rotation.

Anhedral, subangular detrital calcite of Mesozoic age, very fine sand size class, in plane-polarized (top) and cross-polarized (above) light.  Note the high-order pastels and lower-order interference colors along the thinner edges of the grain in the cross-polarized view.  Tahltan Lake, Canada.

The common feature of these diverse forms is the high birefringence imparted by the underlying crystal structure of the mineral.  The birefringence you observe for any given mineral in a smear slide scales by grain size, but calcite and other carbonate minerals viewed in cross-polarized light will always exhibit high-order interference colors relative to all other common constituents.  In small grains, this means carbonates will appear prominently bright with vibrant first-order interference colors.  In larger, thicker grains (>20 μm in dimension), you’ll see high optical birefringence expressed as higher-order interference colors with less vibrant, pastel appearance.  For very large grains this high-order interference can be initially difficult to recognize, however such grains will usually appear rimmed with brighter, lower-order colors where the grain thickness tapers.  

Corroded calcite with resorption accentuated along cleavage planes; plane-polarized (top) and cross-polarized (above) light.  Tahltan Lake, British Columbia, Canada.  

In plane-polarized light, lacustrine calcite may appear colorless, but it’s also common to see faint green or yellow-green hues.  Color and the details of other optical properties are influenced by the substitution of minor amounts of other doubly-charged cations (especially Mg²⁺, Fe²⁺, and Mn²⁺) for Ca²⁺ in the mineral structure.  These compositional details are best evaluated through x-ray diffraction.

Aragonite

Lacustrine calcium carbonate occurs as two distinct minerals (or polymorphs) of CaCO₃ having very different crystal structure: calcite and aragonite.  Calcite is the less soluble (more stable) mineral phase, but its formation is inhibited by elevated ion activities of Mg²⁺, SO₄^2-, and possibly other ions.  When calcite precipitation is inhibited in this manner, aragonite will form as a primary lacustrine mineral phase.   While the mechanism is complex and thresholds inexactly defined, generally lacustrine aragonite formation is recognized as an indication of elevated Mg²⁺:Ca²⁺ in lake waters, typically as a result of CaCO₃ formation and resulting solute evolution (Bischoff and Fyfe 1968; Eugster and Lawrence 1978; Kelts and Hsu 1978).  Also, partitioning of carbon and oxygen isotopes between carbonate mineral, solute, and water depends on which of the two calcium carbonate polymorphs precipitates.  Distinguishing CaCO₃ mineralogy is therefore an important step toward interpreting the depositional environment of lacustrine sediment, or inferring, for example, the isotopic composition of water.  You will need x-ray diffraction (xrd) to identify mineral phases with certainty, but smear slide analysis can provide you with valuable initial identification to be confirmed by xrd.

Aragonite shares the high birefringence and strong interference colors (relative to grain size) of calcite and other carbonate minerals, but when formed as an inorganic precipitate in natural waters, aragonite most often forms needles or elongated ellipsoids  <15 μm in length and often compared to rice grains in form.  While this describes a typical lacustrine aragonite occurrence, you may see overlap in grain form between some aragonite and ellipsoidal calcite.  Due to sediment mixing and temperature effects, you may also see aragonite and calcite co-occurring in a single smear slide, even though the favored polymorph is largely controlled by lake-water chemistry.

Aragonite grains in plane-polarized (top) and cross-polarized light (above) from the Black Sea.  Endogenic grain forms include acicular needles, rice-grain forms such as those shown here, and narrow ellipsoids.

Dolomite

Dolomite shares the extreme birefringence of other carbonate minerals, often shows rhombic habit in smear slides, and may be distinguished with difficulty from calcite of similar habit by higher relief and more equant habit.  Unambiguous distinction from other carbonate minerals is again an xrd task.  Often thought of as a secondary mineral replacing other carbonates, dolomite is known as a primary lacustrine precipitate in certain hypersaline, magnesian environments with favorable microbial ecology (Baker and Kastner 1981; Krause et al 2012), where you might expect to see it associated with other, less common Mg-carbonates such as magnesite and hydromagnesite. In the absence of hypersaline conditions, dolomite seen in smear slides is probably a detrital sediment component.

Siderite and rhodocrosite

Siderite (iron carbonate, FeCO₃) and rhodocrosite (manganese carbonate, MnCO₃) are minerals with widespread but generally less-abundant occurrence as lacustrine sediment components. Lacustrine siderite may be strongly colored in plane-polarized light, ranging from pale green to orange-brown.  Grain forms include square-ended laths, ellipsoids, and ‘footballs’ or lemon-shaped euhedra. Interpenetrating twins are common, and if you are seeing grains in the <15 μm range with a ‘bow-tie’ or ‘dog-bone’ appearance, there is a good chance you have siderite present.  Photographed at high resolution in a split core face, lamellae or nodules of siderite will sometimes appear a distinctive bronze color in a linescan image.  Rhodocrosite is greenish-brown in plane polarized light, may be strongly pleiochroic, and can display grain forms similar to siderite, including interpenetrating twins; distinguishing the two minerals reliably is a task for xrd analysis.

Siderite in plane-polarized (top) and cross-polarized light.  Grain forms include simple laths and twinned laths giving a 'bow-tie' form, accentuated under cross-polarized light.

Formation of siderite and Mn-carbonates requires the moderately negative redox potentials that occur in the near-absence of dissolved oxygen, and at commonly-encountered carbonate ion activities, also implies near-neutral pH.  These conditions normally prevail only in hypolimnetic regions of the water column or in pore-water environments, where dissolved oxygen has been consumed by microbial respiration and pH lowered by dissolution of respired CO₂. 

Simplified equilibrium phase diagram for a dilute aqueous system with
dissolved carbon dioxide, sulfur and phosphorous. The size and position of
mineral phase stability fields depends on the assumed solute concentrations,
but  crossing a lake oxycline from epilimetic (epi) to hypolimnetic (hypo)
pH/Eh conditions is commonly required for formation of siderite and
Mn-carbonates. Modified from Lemos et al 2012 and Hem 1977.  

Water-column stratification in lakes fosters spatial (position in the water column) and temporal (mixing state) gradients in the stability of carbonate minerals.  This means that the occurrence and state of preservation of different carbonate mineral phases can provide valuable clues about changes in lake mixing and other processes over time.  Calcite or aragonite precipitated in a lake’s epilimnion (mixed layer) may, upon sinking into the hypolimnion, encounter more acidic conditions resulting from the respiration of organic matter in poorly mixed conditions below the lake chemocline.  In a smear slide you may see partially dissolved (pitted) carbonate grains reflecting this water-column gradient, or you may see intervals lacking any carbonate minerals at all, perhaps reflecting complete hypolimnetic dissolution at times in lake history.   So the textural observations made may reflect the interplay of two very important components of lake carbon cycling, 1) transfer of fixed organic carbon into hypolimnetic water (by sinking and respiration of dead algal matter), and 2) transfer of inorganic carbon from mixed-layer water in exchange with the atmosphere into hypolimnetic environments isolated from the atmosphere, each involving subsequent partitioning between sediment and hypolimnetic water.  Carbon mass balance and isotopic composition depend on these interactions (Bade et al 2004; Myrbo and Shapley 2006).

Overgrowths and secondary precipitation

Through the processes of settling through the water column, changes in water column stratification, and burial below the sediment-water interface, carbonate minerals may pass from saturated to unsaturated conditions and back again over the course of sediment accumulation.  This can at times result in multiphase overgrowths, an indicator of oscillatory saturation state that may support useful environmental interpretations.  Look for secondary rims on carbonate grains, which may or may not be in crystallographic unity with the host grain (indicated by coinciding or disjunct optical extinction of the host grain and its overgrowths as you rotate the microscope stage).  In some interesting cases, a host carbonate grain may be overgrown by a different carbonate mineral reflecting equilibration in distinct parts of the lake water column (e.g., Stevens et al 2000).

References cited

Bade, D. L., Carpenter, S. R., Cole, J. J., Hanson, P. C., and Hesslein, R. H., 2004, Controls of d¹³C DIC in lakes: Geochemistry, lake metabolism and morphometry: Limnology and Oceanography, v. 49, no. 4, p. 1160-1172.
Baker, P. A., and Kastner, M., 1981, Constraints on the formation of sedimentary dolomite: Science, v. 213, p. 214-216.
Bischoff, J. L., and Fyfe, W. S., 1968, Catalysis, inhibition, and the calcite-aragonite problem: American Journal of Science, v. 266, p. 65-79.
Eugster, H. P., and Hardie, L. A., 1978, Chapter 8: Saline Lakes, in Lerman, A., ed., Physics and Chemistry of Lakes: New York, Springer-Verlag, p. 237-293.
Hem, J. D., 1977, Reactions of metal ions at surfaces of hydrous iron oxide: Geochimica et Cosmochimica Acta, v. 41, p. 527-538.
Kelts, K., and Hsu, K. J., 1978, Freshwater carbonate sedimentation, in Lerman, A., ed., Lakes: Chemistry, Geology, Physics: New York, Springer-Verlag.
Krause, S., Liebetrau, V., Gorb, S., Sanchez-Roman, M., McKenzie, J. A., and Treude, T., 2012, Microbial nucleation of Mg-rich dolomite in exopolymeric substances under anoxic modern seawater salinity: New insight into an old enigma: Geology, v. 40, no. 7, p. 587-590.
Lemos, V. P., Lima da Costa, M., Lemos, R. L., and Gomes de Faria, M. S., 2007, Vivianite and siderite in lateritic iron crust: and example of bioreduction: Quimica Nova, v. 30, no. 1, p. 36-40.
Myrbo, A., and Shapley, M. D., 2006, Seasonal water-column dynamics of dissolved inorganic carbon stable isotopic compositions (d¹³CDIC) in small hardwater lakes in Minnesota and Montana: Geochimica et Cosmochimica Acta, v. 70, p. 2699-2714.
Stevens, L. R., Ito, E., and Olson, D. E. L., 2000, Relationship of Mn-carbonates in varved lake sediments to catchment vegetation in Big Watab Lake, MN, USA: Journal of Paleolimnology, v. 24, p. 199-211.