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14.7.1: Calcite Group Minerals

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    18672
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    Calcite CaCO3

    Origin of Name
    From the Latin word calx, meaning “burnt lime.”

    14.361.jpg
    Figure 14.361: Calcite cleavage fragments
    14.362.png
    Figure 14.362: Calcite cleavage fragments

    Hand Specimen Identification
    Calcite is identified by its hardness of 3, rhombohedral cleavage, and effervescence in cold dilute HCl. It may be confused with dolomite or aragonite. Dolomite however, does not react as readily to HCl, and aragonite is orthorhombic. Calcite can also be distinguished from dolomite using a red alizarin stain that turns calcite pinkish.

    14.363.png
    Figure 14.363: Pseudohexagonal prisms of calcite from Northfield, Massachusetts, FOV is 11 cm across
    14.364.png
    Figure 14.364: Cluster of scalenohedral calcite crystals, the specimen is 16 cm across

    Some calcite specimens contain euhedral crystals, but many are rhombohedral cleavage fragments like those in Figures 14.361 and 14.362. The photo in Figure 14.361 shows calcite with its most common whitish color. The photo in Figure 14.362 shows (less common) blue calcite cleavage fragments. Other colors too, are possible.

    When euhedral, calcite may form crystal clusters such as those seen in Figures 14.363 and 14.364. Note that the crystals in these two photos have significantly different shapes. Calcite commonly twins; Figure 7.40 (Chapter 7) shows one example.

    Physical Properties

    hardness 3
    specific gravity 2.71
    cleavage/fracture perfect rhombohedral {101}/conchoidal
    luster/transparency vitreous/transparent to translucent
    color colorless to white; may also be tinted many other light colors when impure
    streak white

    Properties in Thin Section
    Calcite is colorless in thin section and has extremely high birefringence, resulting in pale, washed out, or “pearl” white interference colors. Polysynthetic twinning is nearly always visible. This mineral shows high variable relief upon stage rotation. In thin section, calcite may be confused with other hexagonal carbonates. Orthorhombic carbonates, however, have parallel extinction and are biaxial. Calcite is uniaxial (-), ω = 1.658, ε = 1.486, δ = 0.172.

    Crystallography
    Calcite forms trigonal crystals. a = 4.99, c = 17.04, Z = 6; space group \(R\overline{3}\dfrac{2}{c}\); point group \(\overline{3}\dfrac{2}{m}\).

    Habit
    Calcite has many habits. Coarse crystals are highly variable. The most common forms are hexagonal prisms with simple to complex terminations, scalenohedra (often with combinations of other forms), rhombohedra (either acute or flattened), and tabs with well-developed basal faces. Polysynthetic twinning is common but usually requires a microscope to detect. Besides forming coarse crystals, calcite may be a fine-grained massive rock-forming mineral, it may make up fine to coarse granular aggregates, and it may precipitate as nodules or crusts or in speleothems.

    Structure and Composition
    Calcite is isostructural with other members of the calcite group. In calcite, Ca2+ ions alternate with (CO3)2- groups in a three-dimensional array. The atomic arrangement is related to the structure of cubic salts, such as halite or periclase, but is not cubic because the structure has been squashed along the equivalent of a main diagonal of the cube. The shortened direction is the c-axis in calcite; planar (CO3)2- groups are perpendicular to c, giving the structure a 3-fold axis of symmetry in that direction only. Mg, Fe, Mn, Zn, and a number of others may substitute for some of the Ca; except for Mn, most solid solutions are quite limited.

    Occurrence and Associations
    Calcite is a common and widespread mineral. It is an essential and major mineral in limestones and marbles, occurs in cave deposits, and occurs as a vein mineral with other carbonates, sulfides, barite, fluorite, and quartz. Calcite also occurs in some rare carbonate-rich igneous rocks and is a common cement in some sandstones. Calcite is an accessory mineral in rocks of many sorts. It is also a common weathering product. Organic calcite is found in shells and skeletal material.

    14.365.png
    Figure 14.365: Iceland spar exhibiting double refraction

    Varieties
    Several different varieties of calcite, having special properties, have their own name. Iceland spar, for example, is the name given to clear calcite, usually in rhombohedral cleavage fragments. Iceland spar is one of a very few mineral varieties that cause double refraction that can be seen without the aid of a petrographic microscope. An image behind a crystal appears double when viewed through the crystal. Figure 14.365 is a photo of Iceland spar that displays this property.

    14.336.jpg
    Figure 14.366: Dogtooth spar, 19 cm tall
    14.337.jpg
    Figure 14.367: Nailhead spar. the largest crystals are about 4.5 long

    Related Minerals
    Calcite has two polymorphs, aragonite, and vaterite. It is isostructural with magnesite, MgCO3; siderite, FeCO3; sphaerocobaltite, CoCO3; smithsonite, ZnCO3; nitratite, NaNO3; dolomite, CaMg(CO3)2; and gaspeite, (Ni,Mg,Fe)(CO3). Calcite and rhodochrosite form extensive solid solutions at room temperature and a complete solid solution above about 550 ºC (1,020ºF). Calcite forms limited solid solutions with ankerite, CaFe(CO3)2; dolomite, CaMg(CO3)2; and kutnohorite, CaMn(CO3)2, at all temperatures.

    Magnesite MgCO3

    Origin of Name
    The name refers to its composition.

    14.368.png
    Figure 14.368: Nondescript white magnesite
    14.369.png
    Figure 14.369: A clear rhomb of magnesite from Minas Gerais, Brazil, 2.4 cm across

    Hand Specimen Identification
    Massive forms of magnesite may be chalky or porcelain-like, such as the specimens seen in Figure 14.368. When massive and fine-grained like the samples shown in this photo, mineral identification may be uncertain.

    Magnesite may be confused with other soft white minerals, especially other carbonates. It is occasionally confused with chert but has inferior hardness. Coarse crystals of magnesite are rhomb-shaped and display rhombohedral cleavage (Figure 14.369) – identifying it as a carbonate – but it may be difficult to distinguish from other rhombohedral carbonates. However, magnesite is denser than dolomite and does not react to cold HCl like calcite, which helps with identification.

    Physical Properties

    hardness 3.5 to 5
    specific gravity 3.00
    cleavage/fracture perfect rhombohedral {101}/conchoidal
    luster/transparency porcelainous/transparent to translucent
    color white, gray, brown, or yellow
    streak white

    Properties in Thin Section
    Magnesite is similar to calcite in thin section but has higher index of refraction (see calcite optics). Uniaxial (-), ω = 1.700, ε = 1.509, δ = 0.191.

    Crystallography
    Magnesite forms trigonal crystals. a = 4.59, c = 14.87, Z = 6; space group \(R\overline{3}\dfrac{2}{c}\); point group \(\overline{3}\dfrac{2}{m}\).

    Habit
    Coarse crystals are rare; magnesite is usually massive, granular, fibrous, or earthy.

    Structure and Composition
    Magnesite is isostructural with calcite and other members of the calcite group (see calcite structure, above). Large amounts of Fe commonly substitute for Mg. Mn, Ca, Ni, and Zn may also be present in small amounts.

    Occurrence and Associations
    Magnesite is most common in veins or masses as an alteration product of mafic minerals. It also occurs in some Mg-rich schists and as a primary mineral in some rare chemical sediments. It is sometimes found as a replacement for calcite or dolomite in limestone.

    Varieties
    Breunnerite is a Fe-rich variety of magnesite; hoshiite is an Ni-rich variety.

    Related Minerals
    Magnesite forms complete solid solutions with siderite, FeCO3, and with gaspeite, (Ni,Mg,Fe)CO3. Other related minerals include hydromagnesite, Mg5(CO3)4(OH)2•4H2O.

    Siderite FeCO3

    Origin of Name
    From the Greek word sideros, meaning “iron.”

    14.370.png
    Figure 14.370: Siderite from the Tazna Mine, Bolivia; 8.2 cm in longest dimension

    Hand Specimen Identification
    Coarse siderite crystals have the same rhombohedral shape and cleavage as other rhombohedral carbonates (Figure 14.370). Siderite can be distinguished from the other carbonates by its high specific gravity and brownish-green color. It effervesces slightly in warm HCl. It may be confused with sphalerite (which can have a similar brown color and the same hardness).

    Physical Properties

    hardness 3.5 to 4
    specific gravity 3.96
    cleavage/fracture perfect rhombohedral {101}/subconchoidal
    luster/transparency vitreous/translucent
    color various shades of brown or brown-green are typical
    streak white

    Properties in Thin Section
    Siderite is colorless to pale yellow-brown in thin section. It appears similar to calcite and other carbonates, but the other carbonates have a lower index of refraction and lack color. Uniaxial (-), ω = 1.875, ε = 1.633, δ = 0.242.

    Crystallography
    Siderite forms trigonal crystals. a = 4.72, c = 15.46, Z = 6; space group \(R\overline{3}\dfrac{2}{c}\); point group \(\overline{3}\dfrac{2}{m}\).

    14.371.png
    Figure 14.371: Reticulated aggregate of siderite crystals

    Habit
    Siderite crystals are typically rhombohedra, often with curved faces. Fine- to coarse-grained aggregates, such as the reticulated aggregate seen in Figure 14.371, are common. Aggregates may also be colloform, globular, botryoidal, fibrous, or earthy.

    Structure and Composition
    Siderite is isostructural with calcite and other members of the calcite group (see calcite structure). Mn and Mg often substitute for Fe. Small amounts of Ca, Zn, and Co may be present.

    Occurrence and Associations
    Siderite is a relatively common mineral found in veins with galena, pyrite, chalcopyrite, and tetrahedrite; as a rock-forming mineral associated with limestone, clay, shale, coal or ironstone; as a replacement mineral in limestone; and less commonly in metamorphic rocks.

    Related Minerals
    Siderite is isostructural with calcite and a number of other minerals (see calcite related minerals, above). It forms complete solid solutions with rhodochrosite, MnCO3, and magnesite, MgCO3.

    Rhodochrosite MnCO3

    Origin of Name
    From the Greek words meaning “rose” and “color,” referring to its rose-pink color.

    14.372.png
    Figure 14.372: Rhodochrosite from Silverton, Colorado; FOV is 9 cm across
    14.373.png
    Figure 14.373: Rhodochrosite rhombs from Silverton, Colorado; the specimen is 2.4 cm tall

    Hand Specimen Identification
    Pink color and rhombohedral carbonate morphology identify rhodochrosite. The two photos here show examples from Silverton, Colorado. Some rhodochrosite specimens have a light-pink to beige color, like the one in Figure 14.372. Others may be deeply colored, like the rhombs in Figure 14.373. Figure 7.30 is a photo of pink rhodochrosite rhombs with quartz.

    Its rhomb-shaped crystals help identify this mineral but, if crystal morphology is uncertain, rhodochrosite may be mistaken for rhodonite (also a pink Mn-mineral). Rhodonite, however, is significantly harder (H = 5.5 to 6, compared with rhodochrosite’s 3.5 to 4). Rhodochrosite also effervesces slightly with cold dilute hydrochloric acid; rhodonite does not effervesce at all.

    Physical Properties

    hardness 3.5 to 4
    specific gravity 3.70
    cleavage/fracture perfect rhombohedral {101}/uneven
    luster/transparency vitreous, pearly/transparent to translucent
    color rose-red, light pink to dark brown
    streak white

    Properties in Thin Section
    Rhodochrosite is colorless or pale pink in thin section, has extremely high birefringence, and three perfect cleavage directions. It may be confused with calcite and other carbonates. Uniaxial (-), ω = 1.816, ε = 1.597, δ = 0.219.

    Crystallography
    Rhodochrosite belongs to the trigonal system. a = 4.74, c = 15.51, Z = 6; space group \(R\overline{3}\dfrac{2}{c}\); point group \(\overline{3}\dfrac{2}{m}\).

    Habit
    Rhodochrosite forms rare rhombohedral crystals. It is usually fine-grained and massive, sometimes granular, botryoidal, columnar, or crusty.

    Structure and Composition
    Rhodochrosite is isostructural with calcite and other members of the calcite group (see calcite structure, above). Zn commonly replaces some Mn; Ca, Mg, Cd, and Co may be present in limited amounts.

    Occurrence and Associations
    Rhodochrosite is uncommon. It is found with other manganese minerals in Mn-rich metamorphic rocks, as a primary mineral in sulfide veins and some replacement bodies, and as a secondary mineral in residual deposits.

    Related Minerals
    Rhodochrosite forms solid solutions with calcite, CaCO3; siderite, FeCO3; and kutnohorite, CaMn(CO3)2.

    Smithsonite ZnCO3

    Origin of Name
    Named after James Smithson (1754–1829), founder of the Smithsonian Institute.

    14.374.png
    Figure 14.374: Smithsonite from Greece
    14.375.png
    Figure 14.375: Smithsonite from Sinaloa, Mexico; the specimen is 6.8 cm across
    14.376.png
    Figure 14.376: Smithsonite from Sonora, Mexico; the specimen is 8.9 cm across

    Hand Specimen Identification
    Rhombohedral carbonate habit, color (typically light green or purplish), high density, and association with other Zn-minerals identify smithsonite. If not distinctly colored, it may be difficult to tell from other dense carbonates. It is occasionally confused with hemimorphite, another commonly light-green soft mineral. The light-green and lilac colors seen in the first two photos above are typical; the stronger green-turquoise color in Figure 14.376 is less common.

    Physical Properties

    hardness 4 to 4.5
    specific gravity 4.43
    cleavage/fracture perfect rhombohedral {101}/subconchoidal
    luster/transparency pearly, vitreous/transparent to translucent
    color typically green; also purple, lilac, pink, and other colors
    streak white

    Properties in Thin Section
    Smithsonite is uniaxial (-), ω = 1.850, ε = 1.625, δ = 0.225.

    Crystallography
    Smithsonite belongs to the trigonal crystal system. a = 4.61, c = 14.88, Z = 6; space group \(R\overline{3}\dfrac{2}{c}\); point group \(\overline{3}\dfrac{2}{m}\).

    Habit
    Crystals, when they are euhedral or subhedral, show rhombohedral form and cleavage. More typical smithsonite is massive, colloform, earthy, stalactitic, or forms crusts.

    Structure and Composition
    Smithsonite is isostructural with calcite and other members of the calcite group (see calcite structure, above). It typically contains substantial amounts of Fe. It may also contain smaller amounts of Ca, Co, Cu, Cd, Mg, or Mn, and traces of Ge or Pb.

    Occurrence and Associations
    Smithsonite is a secondary mineral found in zinc deposits. Associated minerals include sphalerite, hemimorphite, cerussite, malachite, azurite, and anglesite.

    Related Minerals
    Smithsonite forms limited solid solutions with most other carbonates, including otavite, CdCO3.


    This page titled 14.7.1: Calcite Group Minerals is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Dexter Perkins via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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