Editing Pyrite (section)

==Hazards==
[[File:GoldinPyriteDrainage acide.JPG|thumb|A pyrite cube (center) has dissolved away from a host rock, leaving behind trace gold]]
Iron pyrite is unstable when exposed to the [[redox|oxidizing]] conditions prevailing at the Earth's surface: iron pyrite in contact with atmospheric [[oxygen]] and water, or damp, ultimately decomposes into [[iron oxides|iron oxyhydroxides]] ([[ferrihydrite]], FeO(OH)) and [[sulfuric acid]] ({{chem|H|2|S|O|4}}). This process is accelerated by the action of ''[[Acidithiobacillus]]'' bacteria which oxidize pyrite to first produce [[ferrous ion]]s ({{chem|Fe|2+}}), [[sulfate]] ions ({{chem|SO|4|2-}}), and release protons ({{H+}}, or {{H3O+}}). In a second step, the ferrous ions ({{chem|Fe|2+}}) are oxidized by {{O2}} into [[ferric ion]]s ({{chem|Fe|3+}}) which [[hydrolysis|hydrolyze]] also releasing {{H+}} ions and producing FeO(OH). These oxidation reactions occur more rapidly when pyrite is finely dispersed (framboidal crystals initially formed by [[Sulfate-reducing microorganisms|sulfate reducing bacteria]] (SRB) in argillaceous sediments or dust from mining operations).

===Pyrite oxidation and acid mine drainage===
{{Main|Acid mine drainage}}
Pyrite oxidation by atmospheric {{O2}} in the presence of moisture ({{H2O}}) initially produces ferrous ions ({{chem|Fe|2+}}) and [[sulfuric acid]] which dissociates into [[sulfate]] ions and [[hydronium|protons]], leading to [[acid mine drainage]] (AMD). An example of acid rock drainage caused by pyrite is the [[2015 Gold King Mine waste water spill]].<ref>{{cite web| url = http://www.westech-inc.com/en-usa/industry-solutions/mineral-overview/acid-mine-drainage| title = Acid Mine Drainage}}</ref>

:{{chem2 | 2 FeS2 (s) + 7 O2 (g) + 2 H2O (l) -> 2 Fe(2+) (aq) + 4 SO4(2-) (aq) + 4 H+ (aq) }}

===Dust explosions===
Pyrite oxidation is sufficiently [[exothermic]] that underground [[coal mine]]s in high-sulfur coal seams have occasionally had serious problems with [[spontaneous combustion]].<ref>{{cite journal |last1=Deng |first1=Jun |last2=Ma |first2=Xiaofeng |last3=Zhang |first3=Yutao |last4=Li |first4=Yaqing |last5=Zhu |first5=Wenwen |title=Effects of pyrite on the spontaneous combustion of coal |journal=International Journal of Coal Science & Technology |date=December 2015 |volume=2 |issue=4 |pages=306–311 |doi=10.1007/s40789-015-0085-y|doi-access=free |bibcode=2015IJCST...2..306D }}</ref> The solution is the use of buffer blasting and the use of various sealing or cladding agents to [[Hermetic seal|hermetically seal]] the mined-out areas to exclude oxygen.<ref>{{cite journal |last1=Onifade |first1=Moshood |last2=Genc |first2=Bekir |title=A review of spontaneous combustion studies – South African context |journal=International Journal of Mining, Reclamation and Environment |date=17 November 2019 |volume=33 |issue=8 |pages=527–547 |doi=10.1080/17480930.2018.1466402|bibcode=2019IJMRE..33..527O |s2cid=116125498 }}</ref>

In modern coal mines, [[limestone]] dust is sprayed onto the exposed coal surfaces to reduce the hazard of [[dust explosion]]s. This has the secondary benefit of neutralizing the acid released by pyrite oxidation and therefore slowing the oxidation cycle described above, thus reducing the likelihood of spontaneous combustion. In the long term, however, oxidation continues, and the [[hydrated]] [[sulfate]]s formed may exert crystallization pressure that can expand cracks in the rock and lead eventually to [[cave-in|roof fall]].<ref>{{cite journal |doi=10.1016/j.coal.2005.03.013 |title=Colliery and surface hazards through coal-pyrite oxidation (Pennsylvanian Sydney Coalfield, Nova Scotia, Canada) |year=2005 |last1=Zodrow |first1=E |journal=International Journal of Coal Geology |volume=64 |issue=1–2 |pages=145–155|bibcode=2005IJCG...64..145Z }}</ref>

===Weakened building materials===
{{Main|Concrete degradation|Sulfate attack in concrete and mortar}}
Building stone containing pyrite tends to stain brown as pyrite oxidizes. This problem appears to be significantly worse if any [[marcasite]] is present.<ref>Bowles, Oliver (1918) [https://books.google.com/books?id=OksMAAAAYAAJ&lpg=PA25&pg=PA25 The structural and ornamental stones of Minnesota]. Bulletin 663, United States Geological Survey, Washington. p. 25.</ref> The presence of pyrite in the [[Construction aggregate|aggregate]] used to make [[concrete]] can lead to severe deterioration as pyrite oxidizes.<ref>{{cite journal|doi=10.1016/j.cemconres.2004.06.030|title=Internal deterioration of concrete by the oxidation of pyrrhotitic aggregates|year=2005 |last1=Tagnithamou|first1=A|last2=Sariccoric|first2=M |last3=Rivard|first3=P |journal=Cement and Concrete Research|volume=35|pages=99–107}}</ref> In early 2009, problems with [[Chinese drywall]] imported into the [[United States]] after [[Hurricane Katrina]] were attributed to pyrite oxidation, followed by microbial sulfate reduction which released [[hydrogen sulfide]] gas ({{H2S}}). These problems included a foul odor and [[corrosion]] of [[copper]] wiring.<ref>Angelo, William (28 January 2009) [http://www.enr.com/articles/2472-a-material-odor-mystery-over-foul-smelling-drywall A material odor mystery over foul-smelling drywall]. Engineering News-Record.</ref> In the United States, in Canada,<ref>"[https://acqc.ca/sites/default/files/pdf/pyrihouse.pdf Pyrite and your house, what home-owners should know] {{webarchive|url=https://web.archive.org/web/20120106014634/http://www.consommateur.qc.ca/acqc/PyriHouse.pdf |date=2012-01-06 }}" – {{ISBN|2-922677-01-X}} – Legal deposit – National Library of Canada, May 2000</ref> and more recently in Ireland,<ref>Shrimer, F. and Bromley, AV (2012) "Pyritic Heave in Ireland". ''Proceedings of the Euroseminar on Building Materials''. International Cement Microscopy Association (Halle Germany)</ref><ref>[http://www.irishtimes.com/newspaper/ireland/2011/0611/1224298735366.html Homeowners in protest over pyrite damage to houses]. The Irish Times (11 June 2011</ref><ref>Brennan, Michael (22 February 2010) [http://www.independent.ie/irish-news/devastating-pyrite-epidemic-hits-20000-newly-built-houses-26634603.html Devastating 'pyrite epidemic' hits 20,000 newly built houses]. ''Irish Independent''</ref> where it was used as underfloor infill, pyrite contamination has caused major structural damage. [[Concrete]] exposed to sulfate ions, or sulfuric acid, degrades by [[sulfate attack]]: the formation of expansive mineral phases, such as [[ettringite]] (small needle crystals exerting a huge crystallization pressure inside the concrete pores) and [[gypsum]] creates inner [[tensile force]]s in the concrete matrix which destroy the hardened [[cement]] paste, form cracks and fissures in concrete, and can lead to the ultimate ruin of the structure. Normalized tests for [[construction aggregate]]<ref>I.S. EN 13242:2002 [https://www.roadstone.ie/revision-s-r-21/ Aggregates for unbound and hydraulically bound materials for use in civil engineering work and road construction] {{Webarchive|url=https://web.archive.org/web/20180802162539/https://www.roadstone.ie/revision-s-r-21/ |date=2018-08-02 }}</ref> certify such materials as free of pyrite or marcasite.