In sandstones "cement" is somewhat rigidly defined as an authigenic precipitate that
forms within primary intergranular pores. Primary intragranular pores make up such a small
portion of the total porosity in sandstones that this scheme works well and is
quite useful for determining a parameter known as the intergranular volume
(IGV), a measure of compaction.In carbonate rocks it is reasonable to define cement more broadly as a precipitate from aqueous solution (an authigenic mineral) that occupies any primary pore space in the rock. A precipitate that occupies a secondary pore is either a replacement if it fills a secondary pore space that was originally a grain or a fracture-fill if it occupies a secondary space opened by fracturing. Some workers prefer to also include these types of precipitates within an even broader definition of "cement", in essence making "cement" synonymous with "authigenic". Filling of secondary pores necessarily post-dates the pore formation, but otherwise, there is little implication with regard to timing, based on petrographic relationships alone. Precipitation of sparry calcite within a hole opened by dissolution of a grain may proceed almost simultaneously with dissolution or occur at some time long afterwards. A given chemical event may emplace the same precipitate into all three types of pore spaces (primary, secondary dissolution, and fracture) but keeping a separate accounting of these varied authigenic components is essential to understanding the mass balance of chemical exchanges that take place during the diagenesis.
The following cartoons and images give a primer on the complex terminology applied to the description of cement in carbonate rocks. Some terms reference the size and shape of cement crystals whereas other terms address aspects of the cement's spatial distribution within the rock. In the cartoons pore space is aqua blue (a common color for the impregnation media used to highlight the pores in thin sections), grains are yellow, and cement is various shades of blue.The cartoons are fairly generic with respect to scale---in general, they represent fields of view between about 0.2 and 2 cm across. See Scholle & Ulmer (2003) for a summary of various cement classifications that have been devised.
Click the thumbnails to open a larger view of the image; once they are opened, some of the enlarged images can be toggled between plane-light and cross-polarized views with a mouse-over. Open image windows can be re-sized and scrolled.
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Crystal size - Size is perhaps the most fundamental parameter to include in cement description. Folk's Orange Book has a nice chart with terms to be applied to particular size ranges. Cement crystals span a very large range in size, from sub-micron (too small to resolve with a conventional light microscope) to the giant crystals that span many meters within caverns. A common pattern in crystal size is coarsening from the grain surface toward the pore center. |
| Terms relating to cement spatial distribution: | |
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Intergranular/Intragranular - Cement can fill any type of pore. As described in the introduction "primary intragranular" pores are abundant in some carbonate sediments and as a result, a significant volume of cement can be emplaced into these spaces. In this image fibrous cement fills the spaces within the gastropod fragment (intragranular pore) and also the intergranular pores. |
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Isopachous - A common spatial distribution of cement in carbonate rocks is a coating of uniform thickness that covers most grains without regard to their individual composition or texture. Such a cement pattern is called "isopachous" and can be displayed by cement crystals of many different sizes and shapes. |
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Syntaxial - In some cases nucleation sites are not created equal (as in the case of isopachous cements). Instead, specific grains or specific surfaces on grains are preferred sites of cement nucleation. Common examples of this are syntaxial "overgrowths" that may nucleate on the single crystals that make up the plates of echinoids, crinoids, and related groups. Syntaxial refers to the manner in which the nucleated cement crystal is simply an enlargement of the crystal lattice of the underlying nucleus crystal. Grain coatings can inhibit nucleation of syntaxial cements. Nonetheless, despite a substantial coating of fibrous cement, in the example image an overgrowth has gotten a toehold on an echinoderm fragment and grown well out into the surrounding pore. |
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Poikilotopic - If a single cement crystal is large enough to engulf several grains it is called poikilotopic. It is common for syntaxial crystals nucleated on echinoderm fragments to become poikilotopic, but the nucleation sites of large poikilotopes are not always apparent. The lower right-side area of this cartoon depicts a syntaxial overgrowth on an echinoderm fragment that has grown large enough to surround several of the adjacent grains, forming a poikilotope. |
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Meniscus - In the vadose zone, air fills a portion of the intergranular space and the water from which cement precipitates may be restricted to a meniscus film that coats grains. Cements precipitated under such conditions will be limited to those portions of pore throats where the fluid meniscus was more persistent. Wider parts of the pores remain open and may have distinct rounded shapes that reflect the shapes of the air bubbles that prevented more pervasive cementation. |
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Pendant - Another manifestation of vadose cementation is pendant (or pendulose) cements which has a prominently asymmetric distribution around grains, being thicker on the undersides. In tectonically deformed successions such geopetal cements are useful indicators of the original up direction. |
| Terms relating to cement crystal shape and extinction: | |
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Anhedral - Terms relating to the degree of euhedrality are usually applied to dolomites and other replacement fabrics. Anhedral crystals display few crystallographic terminations. |
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Subhedral - Subhedral is a description that is applied to crystal masses that display euhedral terminations on only a portion of the crystals. |
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Euhedral - Euhedral is a description applied to crystal masses in which most of the crystals display some crystallographically controlled terminations. |
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Fibrous (Acicular) - Crystals of long aspect ratio (generally > 6:1) |
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Bladed - Cement crystals in this image have aspect ratios in the range of 2:1 to 3:1 and thus they fall into the "bladed" category (6:1 to 1.5:1). |
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Equant/Spar - Crystals with equant shape (generally < 1.5:1); equant crystals that appear translucent in transmitted light are called "spar". In this image fibrous to bladed cement coats the grain surfaces and equant spar fills the pore center. Sparry crystals that display euhedral terminations within partially filled pores are sometimes referred to as "druse" or "drusy". |
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Twins - It is common for larger calcite crystals to display twins that appear as parallel stripes of contrasting birefringence as seen in cross-polarized light. |
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Saddle Dolomite - Highly undulose extinction is a common feature of Fe-rich dolomite; baroque dolomite is another term applied to these crystals, which occur as cements and grain replacements. Distortion of the crystal lattice is so extreme in some cases that the crystal form is visibly bent, with curving cleavage. |
| Non-carbonate cements: | |
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Not all authigenic minerals in carbonate rocks are carbonate minerals. There are silicates, oxides, sulfates, and sulfides to name a few. Non-carbonate minerals also appear as grains in limestones and examples of these are featured in a section of the Grains subtutorial.
Click the thumbnails below to open a larger view of the image; once they are opened, some of the enlarged images can be toggled between plane-light and cross- polarized views with a mouse-over. | |
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Anhydrite cement - Anhydrite, and its hydrated form gypsum, is acommon constituent in carbonate rocks deposited in peritidal environments under highly arid conditions. Anhydrite also forms by precipitation from highly concentrated brines in the subsurface. Both minerals tend to form poikilotopic cements and to have bladed fabrics. Anhydrite has birefringence much lower than carbonate minerals but still, very colorful. |
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Pyrite - The opaque patches within this bivalve shell are pyrite. These formed by replacement of the shell during diagenesis. |
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Pyrite - In this back-scattered electron image (see techniques section) the iron sulfide is white. Two bivalve shells have been extensively replaced by the sulfide, the lower one by big euhedral crystals that are almost certainly pyrite while the upper shell has been replaced by bladed crystals that may be pyrite or marcasite. |
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Kaolinite - In organic-rich marls (carbonate-rich mudstones) it is not unusual to see authigenic kaolinite filling primary intergranular pore spaces. Here, the kaolinite (low-birefringent microcrystals in cross-polars) fills the tests of planktonic foraminifers. |
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Kaolinite & pyrite - In this back-scattered electron image (see techniques section), a planktonic foraminifer (slightly crushed by compaction, is filled with kaolinite. The surrounding silty mud contains many of the spherical pyrite aggregates known as framboids (brightest white). |
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Quartz - Small euhedral crystals of quartz are a common authigenic component in limestones. Such quartz typically has a replacive habit, occupying space that once belonged to grains or cements. In this example, the quartz crystal (euhedral and low-birefringent) cuts through a small oyster shell and its surrounding cement and also extends into an adjacent grain dissolution pore. In the cross-polar image it is clear that the quartz still contains remnants the rock components it has replaced in the form of microcrystals of calcite. |
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Quartz - Nucleation of quartz is difficult, and as a consequence it tends to prefer nucleation on a pre-existing quartz substrate. Here, an authigenic quartz crystal (euhedral and low-birefringent) has taken advantage of a quartz-grain nucleus in an ooid to get started. Subsequent growth of the quartz crystal has partially replaced the ooid cortex. |
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Chalcedony - Silicification (replacement by quartz) is observed in limestones ranging in age from Precambrian to Cenozoic. Here you see two brachiopods (Mississippian age) that are partially replaced by masses of the fibrous (chalcedonic) quartz type known variously as lutecite, quartzine, or length-slow chalcedony. |
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Chalcedony - Fibrous (chalcedonic) quartz in many cases takes the form of radiating masses called spherules. In this image set, ghosts of bladed anhydrite (which the quartz is replacing) can be seen in the plane-light image; the cross-polar view shows the radial extinction pattern of the spherule. |