Dolomite

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Dolomite is an important rock-forming mineral that named is French mineralogist Déodat Gratet de Dolomieu. It is a colorless to white, pale brown, grayish, reddish, or pink mineral. Its crystals are commonly rhombohedral or tabular, often have curved faces, and sometimes cluster in saddle-shaped aggregates. Dolomite may be striated horizontally and twinned. Some crystals may be up to 2 in (5 cm) long. It can also be coarse to fine granular, massive, and, rarely, fibrous.

Polymorphism & Series: Forms two series, with ankerite and with kutnohorite.

Mineral Group: Dolomite group.

Name: Honors Dieudonne (D´eodat) Sylvain Guy Tancr`ede de Gratet de Dolomieu (1750–1801), French geologist and naturalist, who contributed to early descriptions of the species in dolostone.

Association: Fluorite, barite, calcite, siderite, quartz, metal sulfides (hydrothermal); calcite, celestine, gypsum, quartz (sedimentary); talc, serpentine, magnesite, calcite, magnetite, diopside, tremolite, forsterite, wollastonite (metamorphic); calcite, ankerite, siderite, apatite (carbonatites).

Formation

Dolomite is the main constituent in dolomite rocks and dolomitic marbles. It occurs as a replacement deposit in limestone affected by magnesium-bearing solutions, in talc schists, and in other magnesium-rich metamorphic rocks. Dolomite is found in hydrothermal veins associated with lead, zinc, and copper ores. It is also found in altered, silica-poor igneous rocks, in some carbonatites, and in serpentinites. Crystals of dolomite frequently form in cavities in limestone and marble.

Chemical Properties of Dolomite

Chemical Classification	Carbonate
Chemical Composition	CaMg(CO3)2
Common Impurities	Fe,Mn,Co,Pb,Zn

Physical Properties of Dolomite

Color	Colorless, white, pink, green, gray, brown, black
Streak	White
Luster	Vitreous, pearly
Cleavage	Perfect, rhombohedral, three directions
Diaphaneity	Transparent to translucent
Mohs Hardness	3.5 to 4
Crystal System	Hexagonal
Tenacity	Brittle
Density	2.84 – 2.86 g/cm3 (Measured)    2.876 g/cm3 (Calculated)
Parting	Noted in lamellar twins on {0221}. Twin gliding on {0221};

Optical Properties of Dolomite

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RI values:	nω = 1.679 – 1.681 nε = 1.500 – 1.503
Twinning	Common as simple contact twins
Optic Sign	Uniaxial (-)
Birefringence	0.179
Relief	Moderate
Optical Extinction	Parallel

Occurrence

Formed by diagenesis or hydrothermal metasomatism of limestone; a primary phase in hypersaline sedimentary environments; a major component of some contact metamorphic rocks and marbles; a gangue in hydrothermal veins; in carbonatites and ultramafic rocks.

Uses Area

• Construction aggregate
• Cement manufacture
• Dimension stone,
• Calcined to produce lime,
• Sometimes an oil and gas reservoir,
• A source of magnesia for the chemical industry, agricultural soil treatments,
• Metallurgical flux

Distribution

A major rock-forming mineral, abundant worldwide with numerous commercial uses. Some localities for fine examples include:

• In Italy, from Traversella and Brosso, Piedmont.
• Exceptional crystals from Eugui, Navarra Province, Spain.
• At Trieben and Hall, Tirol, Austria. From Freiberg and Schneeberg, Saxony, Germany.
• At Lengenbach, Binntal, Switzerland. From Trepca, Serbia, Yugoslavia. At Frizington, Cumbria, England.
• In the Vuoriyarvi carbonatite complex, Kola Peninsula, Russia.
• Fine crystals from Brumado, Bahia, and in the Morro Velho gold mine, Nova Lima, Minas Gerais, Brazil.
• At Naica, Chihuahua, Mexico.
• In the USA, in New York, from Lockport, Niagara Co., eastward to Walworth, Wayne Co.; at Stony Point, Alexander Co., North Carolina; in the Mississippi Valley region, in the Tri-State district, at Joplin, Jasper Co., Missouri; Galena, Cherokee Co., Kansas; and Picher, Ottawa Co., Oklahoma.

References

• Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
• Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
• Mindat.org. (2019). Orpiment: Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].
• Smith.edu. (2019). Geosciences | Smith College. [online] Available at: https://www.smith.edu/academics/geosciences [Accessed 15 Mar. 2019].