nontronite

{{Short description|Phyllosilicate mineral}}

{{Infobox mineral

| name = Nontronite

| category = Phyllosilicates
Smectite group

| boxwidth =

| image = Mineraly.sk - nontronit.jpg

| imagesize = 260px

| caption = Nontronite from Slovakia

| formula = (CaO_0.5,Na)_0.3Fe³⁺₂(Si,Al)₄O₁₀(OH)₂·nH₂O

| IMAsymbol = Non{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3 |pages=291–320|doi=10.1180/mgm.2021.43 |bibcode=2021MinM...85..291W |s2cid=235729616 |doi-access=free}}

| molweight =

| strunz = 9.EC.40

| system = Monoclinic

|symmetry = C2/m (no. 12)

| color = Yellow, olive-green, green, orange, brown

| habit = Earthy masses

| twinning =

| cleavage = Perfect basal

| fracture = uneven

| mohs = 1.5 to 2

| luster = Earthy to dull

| refractive = nα = 1.530–1.580; nβ = 1.555–1.612; nγ = 1.560–1.615

| opticalprop = Biaxial (−)

| birefringence = δ = 0.030–0.035

| pleochroism =

| streak = Colorless

| gravity = 2.3

| density =

| melt =

| fusibility =

| diagnostic =

| solubility =

| diaphaneity =

| other =

| references ={{cite web |last1=Anthony |first1=John W. |last2=Bideaux |first2=Richard A. |last3=Bladh |first3=Kenneth W. |last4=Nichols |first4=Monte C. |title=Nontronite |url=http://www.handbookofmineralogy.org/pdfs/nontronite.pdf |website=Handbook of Mineralogy |publisher=Mineral Data Publishing |access-date=28 July 2022 |date=2005}}{{Cite web|last1=Barthelmy|first1=David|year=2014|url=http://www.webmineral.com/data/Nontronite.shtml|access-date=28 July 2022|title = Nontronite Mineral Data|website=Webmineral.com}}{{mindat|id=2924|title=Nontronite|access-date=28 July 2022}}{{cite journal|last1=Dainyak|first1=Lidia G.|last2=Zviagina|first2=Bella B.|last3=Rusakov|first3=Viacheslav S.|last4=Drits|first4=Victor A.|title=Interpretation of the nontronite-dehydroxylate Mossbauer spectrum using EFG calculations|journal=European Journal of Mineralogy|date=2006|volume=18|issue=6|pages=753–764|doi=10.1127/0935-1221/2006/0018-0753|bibcode=2006EJMin..18..753D}}

}}

Nontronite is the iron(III) rich member of the smectite group of clay minerals. Nontronites typically have a chemical composition consisting of more than ~30% Fe₂O₃ and less than ~12% Al₂O₃ (ignited basis). Nontronite has very few economic deposits like montmorillonite.{{citation|last=Eggleton|first=R. A.|year=1977|title=Clay minerals|volume=12|pages=181–194}}{{cite journal|last=Keeling|title=Geology and Characterization of Two Hydrothermal Nontronites from Weathered Metamorphic Rocks at the Uley Graphite Mine, South Australia|display-authors=etal|year=2000|journal=Clays and Clay Minerals|volume=48|issue=5|pages=537–548|doi=10.1346/CCMN.2000.0480506|bibcode=2000CCM....48..537K|s2cid=129598259}} Like montmorillonite, nontronite can have variable amounts of adsorbed water associated with the interlayer surfaces and the exchange cations.

A typical structural formula for nontronite is Ca_.5(Si₇Al_.8Fe_.2)(Fe_3.5Al_.4Mg_.1)O₂₀(OH)₄.{{cite journal|title=Mountainville nontronite|last=Gates|display-authors=etal|year=2002|journal=Clays and Clay Minerals|volume=50|pages=223–239|doi=10.1346/000986002760832829|s2cid=95954306}} The dioctahedral sheet of nontronite is composed mainly of trivalent iron (Fe³⁺) cations, although some substitution by trivalent aluminium (Al³⁺) and divalent magnesium (Mg²⁺) does occur. The tetrahedral sheet is composed mainly of silicon (Si⁴⁺), but can have substantial (about 1 in 8) substitution of either Fe³⁺ or Al³⁺, or combinations of these two cations. Thus, nontronite typically is characterised by having most (usually greater than 60%) of the layer charge located in the tetrahedral sheet. The layer charge is typically balanced by divalent calcium (Ca²⁺) or magnesium (Mg²⁺).

Nontronite forms from the weathering of biotite and basalts, precipitation of iron and silicon rich hydrothermal fluids and in deep sea hydrothermal vents.{{citation|last=Bischoff|year=1972|title=Clays and Clay Minerals|volume=20|pages=217–223}}{{cite journal|last=Eggleton|first=R. A.|year=1975|title=Nontronite topotaxial after hedenbergite|journal=American Mineralogist|volume=60|pages=1063–1068}} Some evidence suggests that microorganisms may play an important role in their formation.{{citation|last=Kohler|display-authors=etal|year=1994|title=Clays and Clay Minerals|volume=42|pages=680–701}} Microorganisms are also involved in reduction of structural iron in nontronite when soils undergo anoxia, and the reduced form of the clay appears to be highly reactive towards certain pollutants, perhaps contributing to the destruction of these compounds in the environment.{{cite journal|author=Tor, J., C. Xu, J. M. Stucki, M. Wander, G. K. Sims|year=2000|title=Trifluralin degradation under micro-biologically induced nitrate and Fe(III) reducing conditions|journal=Env. Sci. Tech.|volume=34|issue=15 |pages=3148–3152|doi=10.1021/es9912473 |bibcode=2000EnST...34.3148T }}{{cite journal|author=Xu, J., J. W. Stucki, J. Wu, J. Kostka, and G. K. Sims|year=2001|title=Fate of atrazine and alachlor in redox-treated ferruginous smectite|journal=Env. Tox. & Chem.|volume=20|issue=12 |pages=2717–2724|doi=10.1002/etc.5620201210 |bibcode=2001EnvTC..20.2717X }}

The only known commercially viable and operational nontronite mine is located in Canterbury, New Zealand. The mine is operated by Palmer Resources and the finished products are used internationally in industrial applications (pulp & paper, surface coating) and in cosmetics marketed as New Zealand Glacial Clay.{{cite web|work=Palmer Resources Limited|url=https://static1.squarespace.com/static/58c2277c2e69cf49f9b715cb/t/5901120586e6c0aad56c839e/1493242397999/New+Zealand+Glacial+Clay+Fact+Sheet|title=New Zealand Glacial Clay Fact Sheet}}