Tag Archives: calcite

Mineralogy of Carbonates

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Tetiaroa coral atoll, French Polynesia, atop an extinct submarine volcanoe, or seamount. The fore reef drops precipitously to the deep ocean floor.

This is the first of a How To…series…  on carbonate rocks – the mineralogy of calcite, aragonite and dolomite.

If the arithmetic is correct, limestones and dolostones make up about 5-10% of sedimentary rocks in Earth’s crust.  They host freshwater aquifers, mineral deposits, hydrocarbons, and some pretty spectacular landscapes (like caves and karst landforms). We treat carbonate rocks separately from sandstones for several reasons:

  • Their framework components are primarily of biological origin, either directly as skeletal material or indirectly where organisms like algae and bacteria mediate carbonate mineral precipitation,
  • All these critters inhabit the photic zone, The depth to which light penetrates is highly dependent on water turbidity. In clear water 50% of light is absorbed at one metre depth and 90% at 20m. There is little significant light below 200m depth. The photic zone is of critical importance for algae photosynthesis – algae are a primary food source for many invertebrates or join with them in symbiotic relationships.
  • Carbonate chemistry is fundamentally different to the siliciclastics, and dominated by three minerals – calcite, aragonite, and dolomite,
  • They tend to be chemically reactive at Earth’s surface and during all stages of burial,
  • Limestones form at all latitudes, but the biomass responsible for their accumulations is most prolific in tropical and subtropical environs.
  • Cementation of limestones begins at the sea floor and continues during burial. Original components like aragonite and high-magnesian calcite are usually replaced by calcite or dolomite during burial. In contrast, cementation of arenites does not begin until after burial.
  • Oceanic limestones (like foraminiferal ooze) tend not to accumulate at depths greater than about 4000-5000m – the calcite-aragonite compensation depth.

Carbonate mineralogy: optical properties

Calcite:

  1. CaCO3
  2. Hexagonal crystal system.
  3. Crystals shapes are varied: commonly as rhomohedra and scalenohedra but can occur as clustered prismatic needles.
  4. Under Polarized light it has high birefringence and appears to twinkle as the microscope stage is rotated. Relief changes from high to low during stage rotation.
  5. Excellent rhombohedral cleavage.
  6. Twin lamellae are common and generally parallel or oblique to the long rhombohedral diagonal.
  7. Uniaxial negative.
  8. Sedimentary calcite can incorporate up to 19 mole% magnesium in its crystal lattice. High magnesian calcite has >5 mol% Mg; low magnesian calcite <5%. The magnesium content of skeletal calcite seems to fall into two groups; <5 mole %, and 11-19 mole %. Ferroan calcite has up to 2-3 mole% Fe2+. Common methods used to distinguish these various forms are noted below.

Calcite twinning in thin section, plain polarised light.

Aragonite:

  1. CaCO3. Can have up to 10,000 ppm strontium (high Sr is common in many recent corals, ooids and calcareous algae.
  2. Aragonite is a polymorph of calcite but belongs to the orthorhombic crystal system.
  3. Mostly commonly as acicular and fibrous cements in limestones, and a common component of invertebrate and protist skeletal material.
  4. High birefringence.
  5. Biaxial negative.

SEM micrograph of radial aragonite clusters in intertidal sands, Auckland, NZ

 

Dolomite:

  1. CaMg(CO3)2. The crystal lattice contains alternating layers of CaCO3 and MgCO3. Highly ordered dolomite contains equal amounts of Ca and Mg. Protodolomites, found in modern hypersaline lagoons and sabkhas, and recently discovered in some calcareous algae, have disordered crystal lattices (on XRD diffractograms) and less Mg.
  2. Hexagonal crystal system, usually as rhomobedra. Commonly seen as a replacement of calcite and aragonite, in some cases the original textures are preserved, and in others they are completely obliterated.
  3. Twinning less common. Twins parallel the short and long rhombohedral diagonals.
  4. Very high birefringence and usually very high relief.
  5. Uniaxial negative

Some popular methods for detecting and distinguishing dolomite and the polymorphs of calcite include:

Staining

Chemical staining of thin sections and rock slabs may seem a bit unsophisticated in an age where the technology for detailed analysis of mineral assemblages is readily available. But for the common carbonate minerals the method can be a very useful first pass at identification. The two stains in common use, Alizarin red-s (ARS) and Potassium ferricyanide (PF), are cheap and easy to use, even in the field where they may give an indication of stratigraphic trends in mineralogy, for example calcite/dolomite ratio, or changes in ferroan calcite. ARS is used to distinguish between calcite (stains pink-red) and dolomite (no stain). The intensity of blue PF stains in ferroan calcite increases with increasing Fe content; ferroan dolomite stains in green hues.

Staining of bivalves reveals replacement of aragonite by ferroan calcite (blue) and non-ferroan calcite (pink)

XRD

Diffraction peaks in an X-ray diffractogram measure the spacings between planes of atoms in a crystal lattice; they are referred to as d-spacings. A characteristic peak in Ca-Mg carbonates is the ‘104 peak’ that increases or decreases according to crystal chemistry. XRD measurement of the 104 peak is one of the more useful methods for identifying the magnesium content of calcites and identifying basic mineralogy in fine-grained deposits.

 

Cathodoluminescence

Pure calcite and dolomite do not luminesce when exposed to a high energy electron beam. However, certain impurities in the crystal lattice may become excited enough to luminesce, emitting light at visible wave lengths. The most common luminescence activator in carbonates is manganese (Mn2+), which emits orange-red to orange-yellow light. Manganese replaces calcium in the calcite lattice during precipitation. As little as 10-20 ppm Mn will activate luminescence. Note that iron (as Fe2+) will quench Mn luminescence if it is present in concentrations greater than a few 10s ppm. Iron as Fe3+, cobalt (Co2+) and nickel (Ni2+) will also quench Mn luminescence.

The amount of Mn2+ incorporated into the lattice will vary during diagenesis in concert with changes in the Mn2+ concentration of the interstitial fluids. This is commonly manifested as zoning in crystals, where the emission colours vary from one zone to another. Analysis of luminescence patterns in carbonate crystals can provide valuable clues about diagenetic history, particularly the evolution of cements, evolving fluid composition, and changes to rock porosity and permeability.

Calcite spar cement stratigraphy revealed by cathodoluminescence. The changes in cement composition (presence of iron and manganese) reflects changes in fluid composition.

Changes in limestone composition through time

We tend to think of limestones as a Phanerozoic phenomenon – those fabulous Devonian, Carboniferous and Permian reefs and platforms. In fact, sedimentary carbonates accumulated across the last 3.5, perhaps even 4 billion years of Earth history. The Precambrian geological record is littered with extensive platform carbonates associated with stromatolite buildups. Most have been dolomitized but there is textural and geochemical evidence of them having been originally calcitic and aragonitic.

Production of skeletal and non-skeletal limestone during the Phanerozoic has not been steady and continuous but has fluctuated on a scale that reflects major changes in plate tectonics. Such grand scale cyclicity was recognised by Fred Mackenzie and John Morse (1992)  who noted a tenuous coincidence between high sea levels and high rates of addition of new crust at mid-ocean spreading ridges, and low sea levels with low accretion rates. A burgeoning database now allows us to be more confident about this grand-scale cyclicity. We now know that the coupling of plate tectonics and sea level also produced changes in sea water composition, specifically the Mg/Ca ratio. High rates of accretion result in a lowering of the Mg/Ca ratio because Mg is removed from seawater and Ca is added during hydrothermal alteration of hot new crust. Geochemical experiments and theoretical analyses also tell us that seawater at low Mg/Ca ratios favours precipitation of calcite cements with low Mg content; many molluscs and modern corals also enjoy these conditions. The opposite, high Mg/Ca ratios, are more likely during low sea floor accretion rates (and low sea levels) and promote precipitation of high magnesian calcite and aragonite cements. High Mg/Ca ratios also favour critters like brachiopods, some extinct corals, modern echinoderms and calcareous algae. Thus, 1st-order low sea level cycles coupled with high Mg/Ca ratios tend to favour aragonite seas and high sea levels with low Mg/Ca ratios calcite seas.

Phanerozoic history of seawater compositions with respect to aragonite versus calcite prone ocean waters

The posts that follow in this carbonate series will deal with: the skeletal and non-skeletal frameworks of limestones, lime mud or micrite, limestone classification, reefs and platforms, cool water limestones, cementation, recrystallization and neomorphism, porosity and permeability – this will do for a start.

links to other posts in this series

Mineralogy of carbonates; skeletal grains

Mineralogy of carbonates; non-skeletal grains

Mineralogy of carbonates; lime mud

Mineralogy of carbonates; classification

Mineralogy of carbonates; carbonate factories

Mineralogy of carbonates; basic geochemistry

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Atlas of cool-water carbonate petrology

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Thin section of the Oligocene Potikohua Limestone, New Zealand. Dominated by bryozoa (b) and a few forams (f), cemented by coarse calcite. Image donated by Cam Nelson

 

This is the companion Atlas to the Cool-water carbonates – outcrop images.

New Zealand cool-water carbonates are predominantly bioclastic, consisting of fauna like bivalves, calcareous bryozoa, barnacles, echinoderms, and flora such as rhodoliths (calcareous algae that encrust rock fragments and shells).  This is particularly the case on shelves with little terrigenous sediment input, such that there is a diverse epifauna. Good examples of this setting occur around New Zealand. There is no evidence in any of the Oligocene through Pleistocene stratigraphy for aragonite-producing algae like Halimeda and Penicillus.

However there is a range of bioclast compositions, ranging from low- to high magnesian calcites and aragonite, The bioclast compositional variation has a significant impact on cement types (micrite and rhomohedral calcite envelopes) calcite spar, and neomorphic replacement by calcite of original bioclast aragonite. Like cements in more tropical realms, the cement paragenesis in cool-water carbonates reflects complex histories of fluid flow and evolving fluid chemistry through sea floor cementation, burial, uplift and ingress of meteoric water. Some useful references describing the paragenesis of Pliocene cool-water carbonates from the east coast of North Island (Te Aute Group) are given below.

 

Contributors:

Vincent Caron is a lecturer in geology and researcher in carbonates based at the Université de Picardie Jules Verne in Amiens, France. He is also a member of the Basins Resources Reservoirs research group. Most of the images presented here on Te Aute limestones formed part of his PhD research at Waikato University. A short list of his publications on the Te Aute is shown below.

Cam Nelson, is one of the original adherents of the Cool Water Carbonate paradigm, who set the scene with his studies of the Oligocene Te Kuiti Group. Cam is an Emeritus Professor at Waikato University.

CS Nelson, PR Winefield, SD Hood, V. Caron, A Pallentine, and PJJ Kamp. 2003. Pliocene Te Aute limestones: Expanding concepts for cool-water shelf carbonates. New Zealand Journal of Geology and Geophysics, 46, 407-424.

V Caron, CS Nelson, and PJJ Kamp. 2004. Contrasting carbonate depositional systems for Pliocene cool-water limestones cropping out in central Hawke’s Bay, New Zealand. New Zealand Journal of Geology and Geophysics, 47, 697-717.

B.D. Ricketts , V Caron & C.S. Campbell 2004. A fluid flow perspective on the diagenesis of Te
Aute limestones. New Zealand Journal of Geology and Geophysics, 47:4, 823-838

V Caron and CS Nelson. 2009. Diversity of neomorphic fabrics in New Zealand Plio-Pleistocene limestones: Insights into aragonite alteration pathways and controls. Journal of Sedimentary Research, v. 79, p. 226-246.

 

The images:

Pliocene Te Aute Group, Hawkes Bay, New Zealand

neomorphic calcite cement

 

neomorphic calcite cement

 

partial neomorphism                          neomorphic calcite

 

 

calcite replacing aragonite                          calcite filling aragonite holes

 

 

micrite and spar cements                             coarse spar cement

 

 

aragonite biomolds                           acicular calcite

 

 

acicular calcite                           needle caclite

 

 

Pleistocene Pukenui Limestone, southern North Island, in plain polarized light and cathodoluminescence.

                          cathodoluminescence zoned cement

 

 

                          cathodoluminescence zoned cement

 

Oligocene Potikohua Limestone near Greymouth, South Island

bryozoan limestone

 

Oligocene Orahiri and Otorohanga limestones, Te Kuiti Group. Courtesy of Cam Nelson

 

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Geological Trappings; Carbon Capture and Storage

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CCS – what is it?

Carbon dioxide is a significant by-product of oil and natural gas production at the well-head, hydrocarbon and coal combustion (especially in power generation) and several manufacturing industries (e.g. cement).  Carbon Capture and Storage (CCS) involves technology that captures CO2 produced by these industrial processes and stores it underground.  In doing so, CCS technology attempts to prevent the CO2 from being released into the atmosphere.  From the point of view of potential climate change it seems like a sensible thing to do.  However, CCS does have its detractors who argue primarily that either it doesn’t matter how much CO2 enters the atmosphere, or that the costs far outweigh the benefits.  Indeed, the cost of CCS programs is high.  Regardless, the science of CCS is fascinating. Continue reading

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The Ancient Earth 7. The Art of the Stromatolite

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Algae, Fossil Slime and Organic Precambrian Art

Stromatolites are the earliest physical life forms on earth; they were the precursors to pretty well everything you see living today. There may be indications of earlier life forms preserved as chemical signatures, but as fossils go, something you can see and touch, stromatolites are it. The oldest stromatolites known are from Western Australia – about 3400 million years old. These ancient structures were built by primitive algae and bacteria, aka cyanobacteria, sometimes referred to as blue-green algae. Clearly life had already evolved to something quite complex by 3400 million years ago.

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From Sand to Stone = Sandstone; A Remarkable Transformation

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This sand will, over a few million years, become sandstone

You are at the beach, by the river, in the garden; you walk through soft sand, squish through mud, pull weeds and sow seed in soil. They’re all soft, squishable, digable.   But throughout the 3400 million years of our Earth’s known sedimentary record, countless millions of times, these same deposits have hardened to rock. Continue reading

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