Editing Goethite (section)

== Composition ==
Goethite is an [[Iron(III) oxide-hydroxide|iron oxyhydroxide]] containing [[ferric]] iron. It is the main component of [[rust]] and [[bog iron]] ore. Goethite's hardness ranges from 5.0 to 5.5 on the [[Mohs scale of mineral hardness|Mohs Scale]], and its [[specific gravity]] varies from 3.3 to 4.3. The mineral forms prismatic needle-like [[crystal]]s ("needle ironstone"<ref name=Barthelmy2012/>) but is more typically massive.<ref name=HurlbutKlein1985/>

[[Feroxyhyte]] and [[lepidocrocite]] are both [[Polymorphism (materials science)|polymorph]]s of the iron oxyhydroxide FeO(OH) which are stable at the pressure and temperature conditions of the Earth's surface. Although they have the same chemical formula as goethite, their different crystalline structures make them distinct minerals.<ref name=HOM/>

Additionally, goethite has several high-pressure and high-temperature [[Polymorphism (materials science)|polymorph]]s, which may be relevant to the conditions of the Earth's interior. These include ε-FeOOH, which has an orthorhombic crystal structure,<ref>{{cite journal |doi=10.1007/s00269-009-0319-x |title=High-pressure X-ray diffraction study of ε-FeOOH |journal=Physics and Chemistry of Minerals |volume=37 |issue=3 |pages=153–157 |year=2010 |last1=Suzuki |first1=Akio |bibcode=2010PCM....37..153S |s2cid=92941002 }}</ref> a cubic [[pyrite]]-type polymorph with<ref>{{cite journal |doi=10.1038/nature18018 |title=FeO<sub>2</sub> and FeOOH under deep lower-mantle conditions and Earth's oxygen–hydrogen cycles |journal=Nature |volume=534 |issue=7606 |pages=241–244 |year=2016 |last1=Hu |first1=Qingyang |last2=Kim |first2=Duckyoung |last3=Yang |first3=Wenge |last4=Liuxiang |first4=Yang |last5=Yue |first5=Meng |last6=Zhang |first6=Li |last7=Mao |first7=Ho-kwang|pmid=27279220 |bibcode=2016Natur.534..241H |s2cid=10702618 }}</ref> or without losing hydrogen<ref>{{cite journal |doi=10.1038/nature22823 |title=The pyrite-type high-pressure form of ε-FeOOH |journal=Nature |volume=547 |issue=7662 |pages=205–208 |year=2017 |last1=Nishi |first1=Masayuki |last2=Kuwayama |first2=Yasuhiro |last3=Tsuchiya |first3=Jun |last4=Tsuchiya |first4=Taku |pmid=28678774 |bibcode=2017Natur.547..205N |s2cid=205257075 }}</ref> and an ultradense hexagonal structure.<ref>{{cite journal |doi=10.1073/pnas.1720510115 |pmid=29507221 |title=Discovery of a hexagonal ultradense hydrous phase in (Fe,Al)OOH |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=547 |issue=12 |pages=205–208 |year=2017 |last1=Zhang |first1=Li |last2=Yuan |first2=Hongsheng |last3=Meng |first3=Yue |last4=Mao |first4=Ho-kwang |pmc=5866593 |doi-access=free }}</ref>

Goethite has the same crystal structure as [[diaspore]], the analogous aluminium oxide-hydroxide mineral. Oxygen and hydroxide ions form a hexagonal close-packed structure, with iron ions filling octahedral sites between the anions. The sites filled by iron ions form paired chains running the length of the crystal, with the two chains in each pair joined by hydroxide ions.{{sfn|Hurlbut|Klein|1985|p=392}}

<gallery>
File:Goethite c.png|The goethite crystal structure viewed along [001]. Red ions are iron, white are oxygen, and yellow are hydroxide.
File:Goethite b.png|The goethite crystal structure viewed along [010]
</gallery>