Calcite
Calcite is a very common but extremely diverse mineral - Goldschmidt (1913) illustrates
2544 habits of Calcite crystals, and many collectors have formed exquisite collections of
Calcite. It is one of these minerals, that keeps surprising you even when you have 'seen
everything.'
Calcite often contains traces of foreign metals, and sometimes as much as several
percent. Particularly divalent metals like magnesium, iron, and zinc are common
impurities, but Calcite may also contain grains of other minerals, whether clay minerals
or more exotic species like Duftite or Aurorite, that give colour to specimens.
Cement
Calcite is calcium carbonate, and is the main component of most limestones. Limestone
that is used in agriculture to adjust the acidity of soils and to produce cement for
construction. It is the construction material of choice today, but has been known for
over 2000 years. The Romans knew cement as 'pulvis puteolanus', and made and used it
as we do. One of the few surviving intact Roman buildings, the Pantheon in Rome,
dating from approximately 25 BC (heavily rebuilt by Hadrian 115-126) has a dome
made from cement, but seemingly the technique was lost or forgotten, and not
rediscovered until the 17th Century.
Originally, cement was produced by burning limestone with a high content of silica,
typically clay minerals, but only few deposits have limestone of the right composition.
Most modern cement is so-called 'Portland cement', named for the deposits in Portland
(Devon, England), where it was produced the first time. It is made by mixing crushed,
fired clay and crushed limestone, chiefly composed of Calcite, in a furnace and then
heating it. The result, cement, is mixed with water and forms a solid mass of intergrown
crystals, when solidifying. Note that it solidifies, hardens, due to a chemical reaction, not
just because the water dries - cement can actually harden under water.
Ether and birefringence
Transparent Icelandic Calcite helped resolve one of the main scientific inquiries of the
late 17th Century. Many physicists at that time believed light traveled through an 'ether',
an aggregate of minute particles, filling all transparent objects. The main proponent of
the ether theory at that time was the physicist Christian Huygens (1629-1695), who
actually developed a testable model of the ether. The Danish astronomer Ole
Rømer (1644-1710) had discovered light travels at a finite speed - he had
observed, Jupiter's moons had longer periods of rotation and fewer eclipses, when Earth
moved away from Jupiter, than when it moved towards, and he interpreted that as light
traveling at a finite speed. Huygens also knew, light has wave properties, and combined
the observations into a model of the ether. He supposed the ether was continuous
throughout space, and was composed of minute, hard, elastic particles which transmitted
light without being displaced themselves. Each particle would vibrate around a fixed
position and transmit motion to neighbouring particles, but the motion had to move
around the particles thereby being slowed down. This would explain refraction, the
displacement of a light ray passing through a transparent object.
In 1669 the Danish doctor and physicist Rasmus Bartholin published a study of
'birefringent Icelandic spar.' He observed light passing through Calcite is indeed
displaced, refracted, but also split in two. If you look at a straight line on a piece of
paper through a piece of Calcite, the line appears double. One image appears stronger,
the other weaker. Huygens explained this by the ether particles in Calcite being
ellipsoidal, so light traveling one direction would need to go further than light in the
other direction. However, he also found that two crystals placed on top of each other
would produce four lines, and by rotating one crystal relative to the other, he could
interchange the weak and strong lines. This could not be explained by the model or any
however elaborate modification, and belief in the ether theory soon withdrew.
Science
By itself, this is a beautiful example of scientific progress. We summarise existing
knowledge into a testable model, a postulated relationship of mechanisms, cause and
effect. That model has certain properties, allowing us to predict the outcome of some
experiments. We perform the experiments, and evaluate the results. Some support the
model, others do not. We can then fine-tune the model and make additional experiments,
but may ultimately find the model can not explain observations in a coherent fashion,
and will have to reject it.
Note, Huygens was not an idiot, on the contrary. He was wrong, but not an idiot. He
made a valuable contribution but summarizing existing information into a testable
hypothesis. It was tested, and rejected, but that is really science at its finest. Bartholin
did work with transparent Calcite from Helgustadir in Eskifjord, Iceland, but similar
material has subsequently been found many other places. Most of the material sold to
collectors comes from Chihuahua in Mexico. Note, that many minerals are birefringent,
but Calcite just happens to have a very large birefringence.
Biology
Calcite is biologically interesting, forming skeletal structures of many organisms. The
skeletons of humans and other vertebrates are composed of phosphates (Carbonate-
hydroxyl-apatite if you must know), but many shells and other exoskeletons of
invertebrates are composed of Calcite. Most mollusk shells are composed of Aragonite
(also calcium carbonate, but with another crystal structure), but for example oyster
shells are predominantly Calcite. Modern corals form Aragonite skeletons, but many
Palaeozoic corals had Calcite skeletons. By volume, far the largest amount of calcium
carbonate deposited by organisms, is formed by coccoliths, microscopic calcareous
algae. Their shells form the majority of sedimentary limestone.
Echinoids, sea urchins, are quite extraordinary. Their shells are composed of Calcite,
but in such a fashion that each plate is a single crystal. In some sea urchins with very
thick spines, the so-called cidarids, the spine itself is a single crystal of Calcite even if it
has an intricate morphology. The spine is sort of rounded in cross section with bloated
and constricted sections, small ribs and knobs, and sometimes an asymmetric tip. The
morphology can be quite complex, but all the atoms are aligned to the same crystal
lattice, and the spine is thus a 'single crystal' even if it does not have any of the faces, we
normally associated with Calcite crystals. To top it off, the crystal lattices of the spine
and the underlying plate - that is a single crystal as well - are aligned, so even if they are
physically disjunct, their crystal lattices are coherent.
Calcite associate minerals
Calcite crystals from Sardegna, Italy
My records indicate that Calcite from a range of deposits occurs on specimens that also
carry one or more of the following minerals: Achantite, Actinolite, Adamite, Adamite
var. Cu-Adamite, Adelite, Ägirine, Aikinite, Aktashite, Alabandite, Albite, Algodonite,
Allactite, Allanite, Allargentum, Allemontite, Altaite, Analcime, Anatase, Andorite,
Andradite, Anhydrite, Ankerite, Annabergite, Anorthite, Antigorite, Antimony,
Apophyllite, Aragonite, Argentopyrite, Arsenic, Arseniosiderite, Arsenolamprite,
Arsenolite, Arsenopyrite, Arsenpolybasite, Arsentsumebite, Asbolane, Aschamalmite,
Atelesite, Athabascaite, Aurichalcite, Aurorite, Austinite var. Cu-Austinite, Azoproite,
Azurite, Babingtonite, Banalsite, Barysilite, Barite, Barytocalcite, Bayldonite,
Becquerelite, Bellidoite, Bementite, Bergslagite, Berthierite, Berzelianite, Berzeliite,
Betafite, Beudantite, Beyerite, Bianchite, Biotite, Birnessite, Bismuth, Bismuthinite,
Bismutite, Blatterite, Blixite, Bobbierite, Boltwoodite, Bornite, Boulangerite,
Bournonite, Braunite, Breithauptite, Brewsterite, Brochantite, Brucite, Brushite,
Burbankite, Bustamite, Calaverite, Calciovolborthite, Carbonate-hydroxylapatite,
Carminite, Carnotite, Carrollite, Caryinite, Caryinite, Cassiterite, Cavansite, Cechite,
Celadonite, Celestite, Ceriopyrochlore-(Ce) var. Koppite, Cerussite, Cesarolite,
Chabazite, Chabazite-Na (Herschelite), Chalcocite, Chalcophyllite, Chalcopyrite,
Chervetite, Chlorargyrite, Chondrodite, Chromite, Chrysocolla, Cinnabarite,
Clausthalite, Cliffordite, Clinobisvanite, Clinochlore, , Clinohedrite, Clinohumite,
Conichalcite, Copper, Corundum var. Ruby, Cowlesite, Crocoite, Crookesite, Cubanite,
Cuprite, Cymrite, Dadsonite, Danburite, Dannemorite var. Asbeferrite, Datolite,
Dawsonite, Descloizite, Diopside, Dioptase, Dolomite, Duftite, Duhamelite, Duranusite,
Dypingite, Dyscrasite, Edenite, Edingtonite, Ekmanite, Elbaite, Ellestadite, Embreyite,
Epidote, Epistilbite, Erionite, Erythrite, Eucairite, Eudialyte, Euxenite, Feitknechtite,
Ferro-axinite, Ferrohornblende, Ferrotschermakite, Fizelyite, Fluoborite, Fluor-
richterite, Fluorapatite, Fluorapophyllite, Fluorite, Fowlerite, Franklinite,
Fredrikssonite, Freibergite, Friedelite, Gahnite, Galena, Gamagarite, Ganomalite,
Ganophyllite, Garronite, Gersdorffite, Glauconite, Glaucophane, Gmelinite, Gobbinsite,
Goethite, Gold, Gonnardite, Gonyerite, Graphite, Greenalite, Greenockite, Grossular,
Gypsum, Gyrolite, Hammarite, Hardystonite, Harmotome, Hausmannite, Hawleyite,
Hedenbergite, Hedyphane, Hematite, Hematolite, Hemihedrite, Hemimorphite,
Hendricksite, Henritermierite, Heterogenite, Heteromorphite, Heulandite, Hillebrandite,
Hisingerite, Hodgkinsonite, Hsinghualite, Humite, Hyalophane, Hydrocerussite,
Hydromagnesite, Hydroxylapatite, Hydroxylherderite, Ilmenite, Ilvaite, Imiterite,
Inesite, Iodargyrite, Iowaite, Jacobsite, Jamborite, Jamesonite, Johannsenite, Joseite,
Jouravskite, Julgoldite, Kasolite, Katoptrite, Kentrolite, Kermesite, Kimzeyite,
Klockmannite, Kolicite, Kolwezite, Koutekite, Kraisslite, Lanarkite, Larnite, Latrappite,
Laumontite, Lautite, Lavendulane, Lawsonbauerite, Lead, Lennilenapeite,
Lepidocrocite, Leucophoenicite, Levyne, Liebigite, Linarite, Litharge, Livingstonite,
Lizardite, Lšllingite, Ludwigite, Luzonite, Långbanite, Macfallite, Magnesio-
aluminokatophorite, Magnesioferrite, Magnetite, Magnetoplumbite, Malachite,
Manganberzeliite, Manganhumite, Manganite, Manganosite, Manjiroite, Marcasite,
Margarosanite, Marokite, Marsturite, Maucherite, Mcgovernite, McKinstryite, Meionite,
Melanotekite, Melilite, Melonite, Mendipite, Merwinite, Mesolite, Microcline, Millerite,
Mimetite, Minrecordite, Mixite, Mizzonite, Molybdenite, Monazite, Montebrasite,
Monticellite, Mosesite, Mottramite, Murdochite, Muscovite, Nadorite, Namibite,
Natrolite, Naumannite, Nekoite, Nelenite, Neotocite, Neyite, Nickel-skutterudite,
Nickeline, Nitrocalcite, Norbergite, Novakite, Okenite, Olivenite, Opal, Orientite,
Orlymanite, Orpiment, Orthoclase, Orthopinakiolite, Owyheeite, Pabstite, Palygorskite,
Paralaurionite, Pararealgar, Parasepiolite, Paratellurite, Pargasite, Parisite,
Parkinsonite, Pearceite, Periclase, Perite, Perovskite var. Ce-Perovskite, Pharmacolite,
Phillipsite, Phlogopite, Phoenicochroite, Picropharmacolite, Pinakiolite, Plagionite,
Plattnerite, Plumboferrite, Polybasite, Polydymite, Polylithionite, Prehnite, Proustite,
Pyrargyrite, Pyrite, Pyroaurite, Pyrobelonite, Pyrochlore, Pyrochroite, Pyrolusite,
Pyromorphite, Pyrophanite, Pyrophyllite, Pyrosmalite, Pyrostilpnite, Pyroxmangite,
Pyrrhotite, Quartz, Quintinite-2H, Rammelsbergite, Rancieite, Rauenthalite, Realgar,
Rhodochrosite, Rhodonite, Richterite, Rickardite, Rimkorolgite, Roggianite, Romeite,
Rosasite, Roselite, Roselite-beta, Rutile, Safflorite, Sahlinite, Samsonite, Sarabauite,
Sarkinite, Scapolite, Schefferite, Scolecite, Scorodite, Semseyite, Sepiolite var.
Parasepiolite, Serendibite, Serpierite, Siderite, Siegenite, Silver, Skutterudite,
Smithsonite, Smythite, Spadaite, Spessartine, Sphalerite, Spinel, Spurrite, Stephanite,
Stetefeldite, Stibarsen, Stibnite, Stromeyerite, Strontianite, Sulfur, Sulvanite, Sussexite,
Svabite, Swedenborgite, Symplesite, Synadelphite, Synchysite, Szaibelyite, Taeniolite,
Takeuchiite, Tegengrenite, Tellurium, Tellurobismuthite, Tennantite, Tephroite,
Tetradymite, Tetrahedrite, Thaumasite, Thomsonite, Thorianite var. Uranothorianite,
Thorite, Tilasite, Tirodite, Tochilinite, Todorokite, Torbernite, Tremolite, Tungstenite,
Tyuyamunite, Umangite, Uraninite, Uranocircite, Uranopilite, Uranpyrochlore,
Uvarovite, Uvite, Vaesite, Valleriite, Vanadinite, Vandendriesscheite, Vauquelinite,
Vesignieite, Vesuvianite, Violarite, Wakabayashilite, Warwickite, Wavellite, Weeksite,
Weloganite, Whewellite, Whitlockite var. Strontiowhitlockite, Wightmanite, Willemite,
Witherite, Wolframite, Wollastonite, Wulfenite, Wurtzite, Xanthoconite, Xonotlite,
Zincite, Zinkenite, Zircon, and Znucalite.
Specimen Handling
Calcite is for all practical purposes stable in a normal household environment. It is not
harmed by light, changes in temperature in the normal comfort range, or known to
decompose. Calcite specimens can be brittle and should be handled with care like any
other mineral specimen - be particularly careful with thin, pointed or bladed crystals.
Calcite is slightly soluble in water, but a gentle rinse will normally not harm a specimen.
Calcite is readily soluble in acids and should never be treated with acid, vinegar, lemon,
etc.
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This page is written and maintaned by Claus
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