Aragonite An orthorhombic mineral consisting of CaCO3 in which each Ca2+ ion is bonded to nine oxygens. The resulting large cation site allows Sr2+, Ba2+, and Pb2+ to substitute for Ca2+ in measurable quantities. A common mineral in speleothems. Cf. calcite.
Birefringence The color or colors generated by a mineral when viewed in cross-polarized light using a petrographic microscope. See Petrographic Microscope.
Bladed Elongate, defined by some as 3 to 6 times as long as wide and by others as 2 to 6 times as long as wide. González et al. (1993) defined "bladed" as having a length/width ratio of 1.5 to 6. Cf. fibrous and equant.
Botryoid A mass of elongate or acicular crystals radiating from one point into a roughly hemispherical space. The term is derived from the Greek botrys (bunch of grapes) because several botryoids grown on one surface produce an exterior resembling that of a bunch of grapes. Cf. spherulite.
Calcite A trigonal or rhombohedral mineral consisting of CaCO3 in which each Ca2+ ion is bonded to six oxygens. The resulting small cation site allows Mg2+, Fe2+, Mn2+, Zn2+ and Sr2+ to substitute for Ca2+ in measurable quantitities. The most common mineral in speleothems. Cf. aragonite.
Cave deposits A term encompassing speleothems, detrital sediments (such as sand, silt, and clay), biological deposits (such as guano or organic remains), and anthropogenic artifacts.
Cave pearl A roughly spherical concretionary speleothem formed by chemical precipitation on a pre-existing nucleus in a pool of water in a cave. "Cave ooid" is a colloquial synonym, although most cave pearls have a diameter greater than the 2 mm upper limit conventionally used to define ooids. "Cave pisoid" is a less commonly used but more volumetrically correct synonym.
Coconut-meat calcite Fabric in length-slow calcite defined by Folk and Assereto (1976) in their study of a flowstone from Carlsbad Caverns. It consists of "felted, crudely fibrous carbonate" with "the crude fibers oriented physically roughly perpendicular to" layering. The calcite "forms stubby undulose fiber-bundles, somewhat flame-like in shape, 0.1-0.2 mm long and 0.01-0.02 mm wide"; "c axes tend to lie subhorizontally, sweeping through an angle between 60 and 90 degrees to the fibers". The example described by Folk and Assereto had "an abundance of liquid and gas inclusions, together with some tiny carbonate inclusions, [giving] this layer a very turbid appearance in thin section and [making] the the fibers opaque and white in hand specimen; hence the name". See Figure 4 of Folk, R.L., and Assereto, R., 1976, Comparative fabrics of length-slow and length-fast calcite and calcitized aragonite in the Holocene speleothem, Carlsbad Caverns, New Mexico: Journal of Sedimentary Petrolology, v. 46, p. 486-496.
Note that the inclusions causing the name are not necessarily linked to the crystal orientation, crystal size, or length-slow nature of the defining material. In a discussion of the paper by Folk and Assereto, Kendall and Boughton (1977) pointed out that inclusion-rich length-fast palisade calcite and inclusion-rich aragonite both can have a white appearance in hand specimen. See Kendall, A.C., & Broughton, P.L., 1977, Discusion: Calcite and aragonite fabrics, Carlsbad Caverns: by R.L. Folk and Riccardo Assereto, Jour. Sed. Petrology. v. 46, p. 486-496: Journal of Sedimentary Petrology, v. 47, p. 1397-1400.
Column The union of a stalagmite and overlying stalactite that have grown so as to join between the floor and ceiling of a cave. A "pillar", by contrast, is a remnant of bedrock supporting the roof of a cave.
Columnar calcite An assemblage of elongate, roughly parallel, calcite crystals. Kendall and Broughton (1978) defined "columnar" as "crystals that are elongate but which are wider than 10 microns". They nonetheless used it interchangeably with palisade, which has an implication of parallelism. See Kendall, A.C., & Broughton, P.L., 1978, Origin of fabrics in speleothems composed of columnar calcite crystals: Journal of Sedimentary Petrology, v. 48, p. 519-538. Cf. palisade.
Couplet A pair of layers, usually in an alternating series. Couplets presumably represent alternating conditions of precipitation, as may occur where climate is highly seasonal (i.e., alternating from a very wet to a very dry season or very warm to very warm season).
Cross-polarized light Light passing through a petrographic microscope when both the lower and upper polarizing filters are in the light path. See Petrographic Microscope.
Diagenesis Change in a sediment after its deposition. Diagenesis in speleothems can include dissolution of primary minerals, alteration of primary minerals to more stable secondary minerals (e.g., aragonite to calcite, fine-grained calcite to coarser calcite), and/or precipitation of secondary minerals. Diagenesis in one layer of a speleothem can occur as the speleothem continues to grow, or after growth ceases.
Electron microscope A device that bombards a sample from above with a very thin electron beam and records the results of the beam's interaction with the sample. In a scanning electron microscope (SEM), the beam is rastered (moved systematically in successive parallel lines) across the sample. A secondary electron image generated by an SEM is a record of the abundance of electrons that are emitted from any one point on the sample and collected by a detector at one side of the sample, so that a three-dimensional image is produced. A back-scattered electron (BSE) image generated by an SEM is a record of the abundance of electrons reflected by the sample to a detector above the sample, so that an image is produced with little three-dimensional information but with qualititative information about the average atomic numbers of the various crystals or other components of the sample.
Equant Having nearly the same length in all directions. Carbonate petrologists use the term "blocky" for essentially the same morphology. A common morphology of calcite; almost never observed in aragonite in speleothems. Cf. fibrous and bladed.
Euhedral Having straight crystal boundaries dictated by the crystallography of the crystal in question. A crystal that has filled the space between the straight sides of two or more other pre-existing euhedral crystals is not euhedral.
Fabric The general appearance of a set of crystals that grow together to produce a distinctive shape or texture (González et al., 1992). "Fabric" is a term best applied to groups of crystals, or to invidividual crystals within groups, that have grown together so that their growing surfaces have encountered each other. Cf. habit.
Fibrous Needlelike; straight and much longer in one direction than in the other two. Defined by some (e.g. González et al. (1993)) as 6 to 10 times as long as wide; by others simply as >6 times as long as wide. A common morphology of aragonite but a possible morphology of calcite. Cf. acicular and bladed.
Flowstone A chemically precipitated mineral body that covers a wall and/or floor of a cave. Cf. stalagmite.
Habit The general appearance of a crystal as a result of the nature and prominence of crystal forms (i.e., faces or sets of faces) (González et al., 1992). "Habit" is a term best applied to invidividual crystals that have grown without their growing surfaces encountering any pre-exisiting solid. Cf. fabric.
Layer A curviplanar zone in a speleothem, parallel to the upper external surface of the speleothem, with uniform petrographic characteristics. Two layers in an alternating series are a couplet If the zone between two layers (i.e., the zone defining the distinction between two layers) has any measureable thickness, it is generally best to consider that intervening zone as a layer, and to view the layer and its overlying or underlying boundary zone as a couplet.
Micrite In its broadest sense, an aggregate of CaCO3 crystals less than 4 microns in size. However, the meaning of this term has varied remarkably since it first appeared in a publication in 1959, and its use has been the subject of vigorous debate.
Robert L. Folk defined "micrite" as a rock "with microcrystalline ooze matrix" (Folk, 1959, p. 17) and stated that "Type III limestones, almost entirely ooze, are designated simply as 'micrite' . . ." (Folk, 1959, p. 17). He emphasized that "'micrite' should be reserved strictly for those rocks that, under the petrographic microscope, are seen to consist almost entirely of microcrystalline calcite" (Folk, 1959, p. 26). Throughout his 1959 paper, Folk consistently used "micrite" as a rock name and used "ooze" or other terms to refer to sediments, particles of sediments, and particles of limestones.
Despite Folk's clear definition of "micrite" as a rock name in 1959, his use of the term had changed slightly by 1962. The third section of Folk's 1962 paper had the heading "Microcrystalline calcite ooze (Micrite)" and said that "The term 'micrite' was introduced as a contraction of 'microcrystalline calcite,' to serve (1) in referring to the matrix of microcrystalline calcite as a rock constituent (for example, brachiopods in a micrite matrix), (2) as a combining term in the calssification of carbonates (for example, 'biomicrite'), and (3) to serve alone as the designation for a rock made up almost entirely of microcrystalline calcite." He also stated that "'Micrite' refers only to clay-sized carbonate". Thus Folk's 1962 paper used the term both explicitly for rocks and at least implicitly for sediments, although Folk continued to use "ooze" when he referred to sediments, and he used "micrite" in his widely reproduced charts of rock names.
Subsequent authors have followed Folk's (1962) incipient and indefinite use of "micrite" as a sediment name for the fine-grained matrix of limey sediments and of limestones. This usage was so engrained that Bathurst (1975) defined "micrite" as "an abbreviation of 'microcrystalline ooze' . . . . ; forms crystals 1-4 microns in diameter (Folk, 1959, p. 7-8)", when in fact the word "micrite" did not appear at all on pages 7 or 8 of Folk's 1959 paper and didn't appear until page 17, as noted above. In a dispute about the meaning of "micrite", Milliman et al. (1985) likewise stated that "As originally defined, micrite referred to "microcrystalline carbonate ooze" (Folk, 1959, p. 8)", but the word "micrite" did not appear at all on page 8 of Folk's 1959 paper. These authors correctly stated that "micrite" was a contraction, but their definitions and/or derivations left out the "calcite" that Folk (1962) explcitly said was the source of the "ite" syllable, perhaps because carbonate sedimentologists realized by the 1970s that aragonite, rather than calcite, is the dominant mineral in shallow-water muddy carbonate sediments.
Folk's 1959 definition was sufficiently forgotten by the 1990s that Carozzi (1993) wrote that "carbonate mud was originally defined as formed by grains of microcrystalline calcite with a grain size of less than 4 microns and designated by the term micrite (Folk, 1962)". Rezak and Lavoie (1993) likewise stated that micrite "consists of grains 1 to 4 microns in diameter" and noted that "carbonate mud and micrite have been used interchangeably" since 1962. Seemingly the only persons today who remember that "micrite" is a rock name are undergraduate students taught to use Folk's classification of carbonate rocks.
By the 1980's, terms like "micrite cement" and "micrite envelope" were common (for example, see Milliman et al., 1985), so that "micrite" became a textural term used to describe microcrystalline components of sediments and/or rocks. In keeping with this expansion of meaning, Jones and Kahle (1995) defined micrite as "calcite particles less than 4 microns long . . . As such, it includes particles that formed as a sediment by precipitation, as a cement, and by the breakdown of pre-existing components". The definition by Jones and Kahle was unusual only because its last few words included the products of terrestrial weathering, whereas micrite had previously almost always been associated with marine sediments. Jones and Motyka (1987) likewise reported on micrite in stalactites.
To summarize, "micrite" has evolved from a name for fine-grained limestone (1959) to a name for fine-grained generally marine calcium carbonate sediment (1962-1970s) to a name for finely crystalline components of carbonate sediments and limestones (1970s-1980s) to a name for finely crystalline calcium carbonate of almost any sort (1980s-1990s). In light of the many different meanings of "micrite" and the exclusion of spelean material from most of those meanings, the use of "micrite" for material in speleothems seems likely to cause more confusion than communication, and microcrystalline calcite is suggested as a more descriptive alternative.
Microcrystalline Consisting of crystals so small that they are unresolvable, or nearly so, using a petrographic microscope (less than about 4 microns in size). Carbonate petrologists commonly use the word micrite for sediments consisting of microcrystalline CaCO3 and "micritic" for materials resembling micrite, but the use of "micrite" or "micritic" for material in speleothems is not advised.
Microfabric Texture or appearance of a geological material as viewed by a petrographic microscope or by an electron microscope. Variation in shape, size, clarity, orientation, and layering of crystals causes variation in microfabrics.
Micron One millionth of a meter, or one thousandth of a millimeter. A micron is 10,000 angstroms.
Overgrowth A crystal that has been precipitated on a pre-existing crystal. As commonly used, almost all overgrowths are syntaxial.
Palisade calcite An assemblage of elongate calcite crystals with straight parallel sides and with their long direction parallel to the direction of growth.
In their study of a flowstone from Carlsbad Caverns with length-fast (normal) calcite, Folk and Assereto (1976) identified palisade calcite as "columnar crystals 2-10 mm long" in which "these gross 'fibers' [were] nearly parallel, sometimes with a vary slight radiating or converging tendency, and [had] a semicomposite to weakly undulose extinction . . . so that it [was] almost impossible to define precisely the width of the crystals, but they [were] roughly 0.2-0.5 mm" wide. A sketch (Figure 2 of Folk and Assereto, 1976) showed crystals with straight and almost perfectly parallel edges (deviating no more than 1 degree from parallel). Photographs (Figures 3 and 5 of Folk and Assereto, 1976) showed more irregular crystal edges and edges deviating as much as 12 degrees from parallel. In light of the existence of "columnar" for crystals like those shown in Folk and Assereto's photographs, it seems reasonable to reserve "palisade" for the kind of fabric shown in their sketch (Figure 2 of Folk and Assereto, 1976).
Petrographic microscope A transmitted light microscope equipped from bottom to top with (at the very least) a light source, a lower polarizing filter, an iris diaphragm, a rotating sample stage, objective lenses, an upper polarizing filter, and an ocular lens or lenses. The sample is invariably a thin section. The two polarizing filters have perpendicular vibration directions. When only the lower one is in the light path, the sample is viewed in plane-polarized light; when both are in the light path, the sample is viewed in cross-polarized light. In the latter condition, the only light passing to the observer results from interference by the sample, yielding interference colors or birefringence useful in identification of minerals.
Plane-polarized light Light passing through a petrographic microscope when only the lower polarizing filter is in the light path. See Petrographic Microscope.
Soda straw A thin-walled hollow cylindrical stalactite through which dripwater passes and then precipitates mineral material at the bottom of the cylinder. Many stalactites begin as soda straws and later overgrow their original form.
Spar In its most general sense, any relatively large and clear crystal or crystals. In its specific sense, calcite crystals more the 4 microns in size (Bathurst, 1975) or more than 20 microns in size (Rezak and Lavoie, 1993). Cf. micrite and microcrystalline.
Speleothem A body of mineral material formed in a cave as the result of chemical precipitation from groundwater that has entered the cave. Speleothems are thus one kind of cave deposit. Examples of speleothems include stalagmites, stalactites, and flowstones. "Cave Formation" is a vague synonym designated as "unsatisfactory" by Lowe and Waltham. Cf. stal.
Spherulite A mass of elongate or acicular crystals radiating from one point into a roughly spherical space. Because crystals of a speleothem generally grow from a planar surface, they are more likely to make a botryoid than a spherulite. Cf. botryoid.
Stal Collective term for chemically precipitated mineral material formed in a cave or caves. As a collective term, "stal" has no plural. Lowe and Waltham consider "stal" to be a colloquial term. Cf. speleothem.
Stalactite A chemically precipitated elongate mineral body that has grown from, and hangs from, the roof of a cave. Stalactites are typically narrower and more pointed than stalagmites, which grow from the floor of a cave. Cf. stalagmite.
Stalagmite A chemically precipitated mineral body that has grown from, and sits on, the floor of a cave. Stalactites are typically broader and more blunt than stalactites, which grow from the roof of a cave. Cf. stalactite.
Syntaxial Pertaining to a crystal that has been precipitated on a pre-existing crystal lattice and has the same lattice orientation as that pre-existing lattice. Cf. Overgrowth.
Thin section A planar section of mineral material 30 microns thick, attached to a glass slide and commonly covered with a cover slip. Non-opaque minerals will allow passage of light at a thickness of 30 microns, allowing transmitted-light microscopy of the sample.
Travertine Relatively solid and dense deposits of CaCO3 chemically and/or biologically precipitated by groundwaters. This term is used in many senses, many of which conflict with each other. For example, Lowe and Waltham (1995) stated that "travertine" is "normally used only for deposits formed outside caves", whereas the American Geological Institute's Dictionary of Geological Terms (1976) stated that "travertine forms the stalagmites and stalactites of limestone caves". Given the existence of terms like "speleothem", "stalagmite", and "stalactite", it would seem reasonable to reserve "travertine" for things other than cave deposits. Cf. tufa.
Tufa Relatively porous or spongy deposits of CaCO3 chemically and/or biologically precipitated by groundwaters or surface waters. Like travertine, this term is used in many senses, and in many usages it overlaps with travertine.
Vaterite A mineral consisting of CaCO3, and thus a polymorph of calcite and aragonite, with very limited occurrence in nature. Cf. aragonite and calcite.
For a much more extensive treatment of cave minerals and of unusual speleothems, see Cave Minerals of the World by Carol Hill and Paolo Forti (National Speleological Society, 1997 (Second Edition), 463 pp.), which is sold by the National Speleological Society (2813 Cave Avenue, Huntsville, Alabama 35810 U.S.A.).
Back to the Index to the Atlas of Speleothem Microfabrics