Ferrogedrite

{{short description|Amphibole, double chain inosilicate mineral}} {{infobox mineral | name = Ferrogedrite | boxwidth = 300px | image = Ferro-gedrite &amp; Sekaninaite.jpg | imagesize = | alt = | caption = Needles of ferro-gedrite on cleavage plane (001) of sekaninaite. | category = Inosilicates<br />Amphibole | formula = ☐Fe²⁺₂(Fe²⁺₃Al₂)(Si₆Al₂)O₂₂(OH)₂ | strunz = 9.DD.05 | dana = | system = Orthorhombic | class = Dipyramidal (mmm)<br />H-M symbol: (2/m 2/m 2/m) | symmetry = ''Pnma'' | unit cell = a = 18.52 Å, b = 17.94 Å, c = 5.31 Å; <br />Z = 4 | color = Pale greenish-gray to brown | habit = Crystals prismatic to bladed; fibrous | twinning = | cleavage = Perfect on {210}, with 54° and 126° intersections | fracture = | tenacity = Brittle | mohs = 5.5–6 | luster = Vitreous | streak = Gray white | diaphaneity = Transparent to translucent | gravity = 3.566 | density = | polish = | opticalprop = Biaxial (-) | refractive = n_α = 1.642 - 1.694<br />n_β = 1.649 - 1.710<br />n_γ = 1.661 - 1.722 | birefringence = δ = 0.019 - 0.028 | pleochroism = X = pale green; Y = brownish green; <br />Z = greenish blue | 2V = Measured: 82° | dispersion = r < v | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | prop1 = | prop1text = | references = <ref name=Mindat>[http://www.mindat.org/min-1515.html Ferrogedrite on Mindat.org]</ref><ref name=HBM>[http://rruff.info/doclib/hom/ferrogedrite.pdf Ferrogedrite in the Handbook of Mineralogy]</ref><ref name=Webmin>[http://www.webmineral.com/data/Ferrogedrite.shtml#.UsG7MtLjVVY Ferrogedrite data on Webmineral]</ref><ref>[http://pubsites.uws.edu.au/ima-cnmnc/ IMA Master List]</ref> }} '''Ferrogedrite''' is an amphibole mineral with the complex chemical formula of ☐Fe²⁺₂(Fe²⁺₃Al₂)(Si₆Al₂)O₂₂(OH)₂. It is sodium and calcium poor, making it part of the magnesium-iron-manganese-lithium amphibole subgroup. Defined as less than 1.00 apfu (atoms per formula unit) of Na + Ca<ref name=Leake>Leake, B. (2004) Nomenclature of amphiboles: Additions and revisions to the International Mineralogical Association's amphibole nomenclature. American Mineralogist. 89. 883-887 [http://ammin.geoscienceworld.org/content/89/5-6/883.full.pdf+html] </ref> and consisting of greater than 1.00 apfu of (Mg, Fe²⁺, Mn²⁺, Li) separating it from the calcic-sodic amphiboles.<ref name=Deer1997>Deer, William Alexander, Robert Andrew Howie, and Jack Zussman. Rock-forming minerals. 2B. Double-chain silicates. Vol. 2. Geological Society, 1997.</ref>{{rp|12–78}} It is related to anthophyllite amphibole and gedrite through coupled substitution of (Al, Fe³⁺) for (Mg, Fe²⁺, Mn) and Al for Si.<ref name=Deer1997 /> and determined by the content of silicon in the standard cell.<ref name=Leake/>

==Occurrence== Specimens of ferrogedrite have been collected in the greenstone belt of Africa, in the mountains of Norway, Greenland, Japan and in amphibole specimens from northwest America as well as the southern coast of California.<ref name=Deer1997 /> Ferrogedrite exists in low temperature, high pressure contact metamorphic geologic settings<ref name=Boniface/> and remain stable up to 600&nbsp;°C-800&nbsp;°C due to its iron content.

==Structure== As an end member of its subgroup<ref name=Deer1997 /> due to its aluminium content in the octahedral site, is sensitive to high pressures. The M4 site is the most important for classification<ref>Hawthorne, F.C. (1983): ''The crystal chemistry of the amphiboles,'' Can. Mineral. 21, 173-480</ref> housing the largest cation and causing behavior similar to monoclinic amphiboles. The linear relationship between the radius of M4 cations and the grand mean radius is varied and dependent on the M1, M2, and M3 sites inferring reliance on aluminium.<ref name="Hawthorneetal2008">{{Cite journal |author=F. C. Hawthorne |author2=M. Schindler |author3=Y. Abdu |author4=E. Sokolova |author5=B. W. Evans |author6=K. Ishida |title=The crystal chemistry of the gedrite-group amphiboles. II. Stereochemistry and chemical relations |journal=Mineralogical Magazine |year=2008 |doi=10.1180/minmag.2008.072.3.731 |volume=72 |issue=3 |pages=731–745|bibcode=2008MinM...72..731H |s2cid=98626831 }}</ref> The maximum content of aluminium in Ferrogedrite is 1.47 in the octahedral site.<ref name=Boniface/> A low-temperature solvus in the mineral causes exsolution changing the chemistry of a specimen<ref name="Hawthorneetal2008" /> resulting in confusing variations and close peaks in the diffraction pattern due to overlapping of phases.<ref>{{Cite journal |last=Stout |first=J. H. |title=Phase petrology and mineral chemistry of coexisting amphiboles from Telemark, Norway |journal=Journal of Petrology |year=1972|volume=13 |pages=99–145 | doi=10.1093/petrology/13.1.99 }}</ref>

Ferrogedrite is an orthorhombic dipyramidal amphibole with an H-M symbol of 2/m 2/m 2/m, and its space group is Pnma.<ref name=Deer1997 /> Cleavage is {110} perfect, {010} indistinct, and {110} indistinct with angles not at 90 degrees.<ref name=Nesse>Nesse, W.D. (2011) ''Introduction to Mineralogy,'' Oxford University Press, {{ISBN|978-0-19-982738-1}}</ref> Created by distinct events, exsolution during cooling suggests its structure can be in asbestos form.<ref>{{cite journal |author=E. F. Stoddard |author2=C. F. Miller |journal=Mineralogical Magazine |year=1990 |title=Chemistry and phase petrology of amphiboles and orthoamphibole-cordierite rocks, Old Woman Mountains, SE California, USA |volume=54 |issue=376 |pages=393–406 |url=http://rruff.info/doclib/MinMag/Volume_54/54-376-393.pdf | doi = 10.1180/minmag.1990.054.376.04 |bibcode=1990MinM...54..393S |s2cid=55088057 }}</ref>

This is a hydrous mineral that will exsolve and form course, elongated laths (230–1070&nbsp;mm,<ref name=Boniface/> or fine fibrous (10–70&nbsp;mm). This double chain inosilicate with two unequal double-chains of tetrahedral<ref name="NestolaPasqual2012">{{cite journal|last1=Nestola|first1=F.|last2=Pasqual|first2=D.|last3=Welch|first3=M. D.|last4=Oberti|first4=R.|title=The effects of composition upon the high-pressure behaviour of amphiboles: compression of gedrite to 7 GPa and a comparison with anthophyllite and proto-amphibole|journal=Mineralogical Magazine|volume=76|issue=4|year=2012|pages=987–995|issn=0026-461X|doi=10.1180/minmag.2012.076.4.14|bibcode=2012MinM...76..987N|s2cid=131713507}}</ref> the A and B chains. The Fe²⁺ are smaller cations in the M4 site than monoclinic amphiboles rich in Na and Ca and results in weakness under pressure.<ref name="NestolaPasqual2012" /> The higher Al content of the mineral strengthens the mineral as it increases the size of the tetrahedral and its placement in the M2 site.<ref name="NestolaPasqual2012" /> The rigidity of Al in the mineral counteracts the compressional weakness of the Fe²⁺ in high temperature and pressure environments.<ref name="NestolaPasqual2012" /> It is believed the edge-sharing ribbon of octahedra provides the strength and resistance to the structure.<ref name="NestolaPasqual2012" />

==Properties== Ferrogedrite is identified optically by its distinct cleavage and twinning with angles wider (650 and 1330)<ref>Law, A. (1982) Studies of the orthoamphiboles III. Hydroxyl Spectra of gedrites. Mineral Magazine 45. 63-71 http://www.minersoc.org/pages/Archive-MM/Volume_45/45-337-63.pdf {{Webarchive|url=https://web.archive.org/web/20131230233454/http://www.minersoc.org/pages/Archive-MM/Volume_45/45-337-63.pdf |date=2013-12-30 }}</ref> than its counterparts, its dark-green to brown rims.<ref name=Boniface/> and minor matrix. The hardness of ferrogedrite registers between 5.5-5.6 on the Mohs hardness scale and will scratch a knife blade and leaves a gray white streak on a ceramic plate. Ferrogedrite in its fibrous state, along with other amphiboles is considered due to medium-grade conditions. Found initially by Seki and Yamasaki in 1957 in Japan and approved by the IMA in 1978. It is often found as an inclusion in garnet crystals.<ref name=Boniface>Boniface, N. (2011) [http://www.ajol.info/index.php/tjs/article/view/73622/63783 ''Contact Metamorphism in the Supracrustal rocks of the Sukamaland Greenstone belt in the Northwest Tanzania''], Tanzania Journal of Science. 37. 1. 1-12</ref>

==References== {{reflist}}

Category:Amphibole group Category:Orthorhombic minerals Category:Minerals in space group 62