Iron disulfide (FeS2) with orthorhombic crystal structure
Marcasite
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Marcasite with tarnish (8×6 cm)
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Category
| Sulfide mineral
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Formula
(repeating unit)
| FeS
2
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IMA symbol
| Mrc
[1]
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Strunz classification
| 2.EB.10a
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Crystal system
| Orthorhombic
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Crystal class
| Dipyramidal (mmm)
H-M symbol
: (2/m 2/m 2/m)
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Space group
| Pnnm
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Unit cell
| a = 4.436 A,
b = 5.414 A,
c = 3.381 A; Z = 2
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|
Formula mass
| 119.98 g/mol
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Color
| Tin-white on fresh surface, pale bronze-yellow, darkening on exposure, iridescent tarnish
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Crystal habit
| Crystals typically tabular on {010}, curved faces common; stalactitic,
reniform, massive; cockscomb and spearhead shapes due to twinning on {101}.
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Twinning
| Common and repeated on {101}; less common on {011}.
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Cleavage
| Cleavage: {101}, rather distinct; {110} in traces
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Fracture
| Irregular/Uneven
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Tenacity
| Brittle
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Mohs scale
hardness
| 6?6.5
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Luster
| Metallic
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Streak
| Dark-grey to black.
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Diaphaneity
| Opaque
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Specific gravity
| 4.875 calculated, 4.887 measured
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Pleochroism
| [100] creamy white; [010] light yellowish white; [001] white with rose-brown tint. Anisotropism: Very strong, yellow through pale green to dark green
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References
| [2]
[3]
[4]
[5]
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The
mineral
marcasite
, sometimes called "
white iron pyrite
", is
iron sulfide
(FeS
2
) with
orthorhombic crystal structure
. It is physically and crystallographically distinct from
pyrite
, which is iron sulfide with
cubic crystal structure
. Both structures contain the disulfide S
2
2?
ion
, having a short bonding distance between the
sulfur
atoms. The structures differ in how these
di-anions
are arranged around the
Fe
2+
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 called "marcasite" ? that is, marcasite jewellery is made from pyrite, not from the mineral marcasite. Marcasite in the scientific sense is not used as a gem due to its brittleness. In the late medieval and early modern eras, the word "marcasite" meant all iron sulfides in general, including both pyrite and the mineral marcasite.
[6]
The narrower, modern scientific definition for marcasite as orthorhombic iron sulfide dates from 1845.
[4]
The jewelers' sense for the word "marcasite" pre-dates this 1845 scientific redefinition.
Occurrence
[
edit
]
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
.
[3]
As a primary mineral marcasite forms nodules, concretions, and crystals in a variety of
sedimentary rock
, such as in the
chalk
layers found on both sides of the
English Channel
at
Dover
,
Kent
,
England
, and at
Cap Blanc-Nez
,
Pas de Calais
,
France
, where it forms as sharp individual crystals and crystal groups, and nodules (similar to those shown here). Marcasite is also found in complex sulphide deposits. In the Reocin mine, Cantabria, Spain, appears as crystals grouped in the form of
cockscombs
.
[7]
As a secondary mineral, it forms by chemical alteration of a primary mineral, such as pyrrhotite or
chalcopyrite
.
Sedimentary marcasite and low pH
[
edit
]
In laboratory experiments, marcasite forms preferentially to pyrite at a
pH
of less than about 5.
[8]
Ab initio
calculations suggest that this is due to pyrite having a higher surface energy (thus being less thermodynamically stable) than marcasite at low pH.
[9]
Due to the association of marcasite with low pH, the occurrence of marcasite in sedimentary rocks in the
geologic record
implies the presence of highly acidic conditions during the formation and early
diagenesis
of those rocks. However, sedimentary pore waters below the modern ocean are typically buffered at near-neutral to slightly alkaline pH by dissolved
carbonate
species.
[10]
This raises the question of how sedimentary pore waters became sufficiently acidic to promote marcasite formation in the past.
Several theories have been proposed for the formation of early diagenetic marcasite, including: partial
oxidation
of primary pyrite by
molecular oxygen
infiltrating from the overlying water column,
[11]
and rapid anoxic organic matter decomposition and organic acid generation by
fermentation
and
methanogenesis
.
[12]
Varieties and blends
[
edit
]
Blueite
(S.H.Emmons):
Nickel
variety of marcasite, found in Denison Drury and Townships,
Sudbury District
,
Ontario
, Canada.
Lonchidite
(
August Breithaupt
):
Arsenic
variety of marcasite, found at Churprinz Friedrich August Erbstolln Mine (Kurprinz Mine), Großschirma Freiberg,
Ore Mountains
,
Saxony
, Germany; ideal formula Fe(S, As)
2
.
Synonyms for this variety:
- kausimkies,
- kyrosite,
- lonchandite,
- metalonchidite (Sandberger) described at Bernhard Mine near
Hausach
(Baden), Germany.
Sperkise
: designates a marcasite having twin spearhead crystal on {101}. Sperkise derives from the German Speerkies (
Speer
meaning spear and
Kies
gravel or stone). This twin is very common in the marcasite of a chalky origin, particularly those from the
Cap Blanc-Nez
.
Decay
[
edit
]
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
, FeSO
4
·7H
2
O.
[13]
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.
[14]
[15]
References
[
edit
]
- ^
Warr, L.N. (2021).
"IMA?CNMNC approved mineral symbols"
.
Mineralogical Magazine
.
85
(3): 291?320.
Bibcode
:
2021MinM...85..291W
.
doi
:
10.1180/mgm.2021.43
.
S2CID
235729616
.
- ^
"Marcasite"
.
Mineralien Atlas
(in German).
- ^
a
b
"Marcasite"
(PDF)
.
Handbook of Mineralogy
. U. Arizona.
- ^
a
b
"Marcasite"
.
Mindat.org
. 2571.
- ^
"Marcasite"
.
Webmineral
. data.
- ^
"marcassite"
.
CNRTL
(in French).
- ^
Calvo Rebollar, Miguel (2003).
Minerales y Minas de Espana. Vol. II. Sulfuros y sulfosales
[
Minerals and Mines of Spain. Sulphides and sulphosalts.
] (in Spanish). Vitoria, Spain: Museo de Ciencias Naturales de Alava. pp. 489?490.
ISBN
9788478215430
.
- ^
Murowchick, James B.; Barnes, H. L. (December 1986).
"Marcasite precipitation from hydrothermal solutions"
.
Geochimica et Cosmochimica Acta
.
20
(12): 2615?2629.
Bibcode
:
1986GeCoA..50.2615M
.
doi
:
10.1016/0016-7037(86)90214-0
– via Elsevier Science Direct.
- ^
Kitchaev, Daniil A.; Ceder, Gerbrand (14 December 2016).
"Evaluating structure selection in the hydrothermal growth of FeS2 pyrite and marcasite"
.
Nature Communications
.
7
(1): 13799.
Bibcode
:
2016NatCo...713799K
.
doi
:
10.1038/ncomms13799
.
PMC
5171653
.
PMID
27966547
.
- ^
Ben-Yaakov, Sam (January 1973).
"pH BUFFERING OF PORE WATER OF RECENT ANOXIC MARINE SEDIMENTS"
.
Limnology and Oceanography
.
18
(1).
John Wiley & Sons, Ltd
: 86?94.
Bibcode
:
1973LimOc..18...86B
.
doi
:
10.4319/lo.1973.18.1.0086
.
- ^
Schieber, Juergen (1 July 2011).
"Marcasite in Black Shales?a Mineral Proxy for Oxygenated Bottom Waters and Intermittent Oxidation of Carbonaceous Muds"
.
Journal of Sedimentary Research
.
81
(7): 447?458.
Bibcode
:
2011JSedR..81..447S
.
doi
:
10.2110/jsr.2011.41
– via GeoScienceWorld.
- ^
Bryant, R. N.; Jones, C.; Raven, M. R.; Owens, J. D.; Fike, D. A. (20 October 2020).
"Shifting modes of iron sulfidization at the onset of OAE-2 drive regional shifts in pyrite δ34S records"
.
Chemical Geology
.
553
: 119808.
Bibcode
:
2020ChGeo.55319808B
.
doi
:
10.1016/j.chemgeo.2020.119808
.
S2CID
224938768
– via
Elsevier Science Direct
.
- ^
Klein, Cornelis; Hurlbut, Cornelius S. (1985).
Manual of Mineralogy
(20th ed.). Wiley. p.
286
.
ISBN
0-471-80580-7
.
- ^
"Storage Concerns for Geological Collections"
(PDF)
. Conserv-O-Gram. U.S. National Park Service. April 1998.
- ^
Parafiniuk, J.; Stepisiewicz, M. (2000).
"Pyrite oxidation under room conditions"
. Geology.
www.geo.uw.edu.pl
. How Minerals Form and Change. Warsaw, Poland: University of Warsaw. Archived from
the original
on 24 November 2006.
External links
[
edit
]
Wikimedia Commons has media related to
Marcasite
.