Marcasite


Marcasite
Marcasite

Marcasite with tarnish (8x6 cm)
General
Category Sulfide mineral
Chemical formula FeS2
Strunz classification 02.EB.10a
Crystal symmetry Orthorhombic 2/m 2/m 2/m
Unit cell a = 4.436 Å, b = 5.414 Å, c = 3.381 Å; Z = 2
Identification
Molar mass 119.98
Color Tin-white on fresh surface, pale bronze-yellow, darkening on exposure, iridescent tarnish
Crystal habit

Crystals typically tabular on {010}, curved faces common; stalactitic,

reniform, massive; cockscomb and spearhead shapes due to twinning on {101}.
Crystal system Orthorhombic, Pnnm
Twinning Common and repeated on {101}; less common on {011}.
Cleavage Cleavage: {101}, rather distinct; {110} in traces
Fracture Irregular/Uneven
Tenacity Brittle
Mohs scale hardness 6-6.5
Luster Metallic
Streak Dark-grey to black.
Diaphaneity Opaque
Specific gravity 4.875 calculated, 4.887 measured
Pleochroism [100] creamy white; [010] light yellowish white; [001] white with rose-brown tint. Anisotropism: Very strong, yellow through pale green to dark green
References [1][2][3]

The mineral marcasite, sometimes called white iron pyrite, is iron sulfide (FeS2) with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide with cubic crystal structure. Both structures do have in common that they contain the disulfide S22- ion with a short, bonding distance between the sulfur atoms. The structures differ in how these dianions are arranged around the Fe2+ cations. Marcasite is lighter and more brittle than pyrite. Specimens of marcasite often crumble and break up due to the unstable crystal structure.

On fresh surfaces it is pale yellow to almost white and has a bright metallic luster. It tarnishes to a yellowish or brownish color and gives a black streak. It is a brittle material that cannot be scratched with a knife. The thin, flat, tabular crystals, when joined in groups, are called "cockscombs."

In marcasite jewellery, pyrite used as a gemstone is termed "marcasite". That is, marcasite jewellery is made from pyrite not from marcasite. In the late medieval and early modern eras the word "marcasite" meant both pyrite and marcasite (and iron sulfides in general).[4] The narrower, modern scientific definition for marcasite as orthorhombic iron sulfide dates from 1845.[2] The jewellery sense for the word pre-dates this 1845 scientific redefinition. Marcasite in the scientific sense is not used as a gem due to its brittleness.

Contents

Occurrence

Iridescent cluster of marcasite crystals (3.3 x 2.1 x 1.4 cm)
Two halves of a ball of radiating marcasite from France.

Marcasite can be formed as both a primary or a secondary mineral. It typically forms under low-temperature highly acidic conditions. It occurs in sedimentary rocks (shales, limestones and low grade coals) as well as in low temperature hydrothermal veins. Commonly associated minerals include pyrite, pyrrhotite, galena, sphalerite, fluorite, dolomite and calcite.[1]

As a primary mineral it forms nodules, concretions and crystals in a variety of sedimentary rock, such as at Dover, Kent, England, where it forms as sharp individual crystals and crystal groups, and nodules (similar to those shown here) in chalk.

As a secondary mineral it forms by chemical alteration of a primary mineral such as pyrrhotite or chalcopyrite.

Alteration

Marcasite reacts more readily than pyrite under conditions of high humidity. The product of this disintegration is iron(II) sulfate and sulfuric acid. The hydrous iron sulfate forms a white powder consisting of the mineral melanterite, FeSO4·7H2O.[5]

This disintegration of marcasite in mineral collections is known as "pyrite decay". When a specimen goes through pyrite decay, the marcasite reacts with moisture and oxygen in the air, the sulfur oxidizing and combining with water to produce sulfuric acid that attacks other sulfide minerals and mineral labels. Low humidity (less than 60%) storage conditions prevents or slows the reaction.[6]

References

  1. ^ a b Handbook of Mineralogy
  2. ^ a b Mindat.org
  3. ^ Webmineral data
  4. ^ CNRTL (in French)
  5. ^ Klein, Cornelis and Cornelius S. Hurlbut, Manual of Mineralogy, Wiley, 20th ed. 1985, p.286 ISBN 0-471-80580-7
  6. ^ http://www.nps.gov/history/museum/publications/conserveogram/11-02.pdf NPS Storage Concerns For Geological Collections, Conserv-O-Gram, April 1998

External links


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Look at other dictionaries:

  • Marcasite — Mar ca*site, n. [F. marcassite; cf. It. marcassita, Sp. marquesita, Pg. marquezita; all fr. Ar. marqash[=i]tha.] (Min.) A sulphide of iron resembling pyrite or common iron pyrites in composition, but differing in form; white iron pyrites. [1913… …   The Collaborative International Dictionary of English

  • marcasite — crystalized pyrite, early 15c., from M.L. marchasita, of obscure origin, perhaps via Spanish, probably from Arabic, though OED doubts this. Perhaps ultimately from Pers. marquashisha [Klein]. This name has been used for a number of substances but …   Etymology dictionary

  • marcasite — ► NOUN 1) a semi precious stone consisting of iron pyrites. 2) a piece of polished metal cut as a gem. ORIGIN Latin marcasita, from Arabic …   English terms dictionary

  • marcasite — [mär′kə sīt΄] n. [Fr marcassite < ML marcasita < Ar marqashītā < Pers marqashīshā] 1. an orthorhombic mineral, FeS2, that is dimorphic with pyrite; iron sulfide 2. this mineral cut and mounted on silver or other white metal to look like… …   English World dictionary

  • marcasite — marcasitical /mahr keuh sit i keuhl/, adj. /mahr keuh suyt /, n. 1. a common mineral, iron disulfide, FeS2, chemically similar to pyrite but crystallizing in the orthorhombic system. 2. any of the crystallized forms of iron pyrites, much used in… …   Universalium

  • Marcasite — Marcassite Marcassite Catégorie II : sulfures et sulfosels Marcassite à macles en fer de lance (sperkise), cap Blanc Nez, Pas de Calais, France …   Wikipédia en Français

  • marcasite — ● marcassite ou marcasite nom féminin (latin médiéval marchasita, de l arabe marqachītā) Sulfure naturel de fer, orthorhombique, se présentant le plus souvent en boules à structure radiée. ● marcassite ou marcasite (synonymes) nom féminin (latin… …   Encyclopédie Universelle

  • marcasite — noun Etymology: Middle English marchasite, from Medieval Latin marcasita Date: 15th century 1. a. crystallized pyrite b. a pale yellow to white mineral of the same composition and appearance as pyrite but of different crystalline organization and …   New Collegiate Dictionary

  • marcasite — markazitas statusas T sritis chemija apibrėžtis Rombinis FeS₂, mineralas. atitikmenys: angl. marcasite rus. марказит …   Chemijos terminų aiškinamasis žodynas

  • marcasite — noun a pale mineral, FeS Syn: white iron pyrite …   Wiktionary


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