Editing Ilmenite (section)

== Mineral chemistry ==
Pure ilmenite has the composition {{chem2|FeTiO3}}. However, ilmenite most often contains appreciable quantities of magnesium and manganese and up to 6 [[wt%]] of hematite, {{chem2|Fe2O3}}, substituting for {{chem2|FeTiO3}} in the crystal structure. Thus the full chemical formula can be expressed as {{chem2|(Fe,Mg,Mn,Ti)O3}}.<ref name=KleinHurlbut1993/> Ilmenite forms a solid solution with [[geikielite]] ({{Chem|Mg||Ti||O|3}}) and [[pyrophanite]] ({{Chem|Mn||Ti||O|3}}) which are magnesian and manganiferous end-members of the solid solution series.<ref name=HBM/>

Although ilmenite is typically close to the ideal {{Chem|Fe||Ti||O|3}} composition, with minor mole percentages of Mn and Mg,<ref name=HBM/> the ilmenites of [[kimberlite]]s usually contain substantial amounts of geikielite molecules,<ref>{{cite journal |last1=Wyatt |first1=Bruce A. |last2=Baumgartner |first2=Mike |last3=Anckar |first3=Eva |last4=Grutter |first4=Herman |title=Compositional classification of "kimberlitic" and "non-kimberlitic" ilmenite |journal=Lithos |date=September 2004 |volume=77 |issue=1–4 |pages=819–840 |doi=10.1016/j.lithos.2004.04.025|bibcode=2004Litho..77..819W |s2cid=140539776 }}</ref> and in some highly differentiated [[felsic]] rocks ilmenites may contain significant amounts of pyrophanite molecules.<ref name=SasakiEtal2003>{{cite journal |last1=Sasaki |first1=Kazuhiro |last2=Nakashima |first2=Kazuo |last3=Kanisawa |first3=Satoshi |title=Pyrophanite and high Mn ilmenite discovered in the Cretaceous Tono pluton, NE Japan |journal=Neues Jahrbuch für Mineralogie - Monatshefte |date=15 July 2003 |volume=2003 |issue=7 |pages=302–320 |doi=10.1127/0028-3649/2003/2003-0302}}</ref>

At temperatures above {{convert|950|C||sp=us}}, there is a complete solid solution between ilmenite and hematite. There is a [[miscibility gap]] at lower temperatures, resulting in a coexistence of these two minerals in rocks but no solid solution.<ref name=KleinHurlbut1993/> This coexistence may result in exsolution lamellae in cooled ilmenites with more iron in the system than can be homogeneously accommodated in the crystal lattice.<ref>{{cite journal |last1=Weibel |first1=Rikke |last2=Friis |first2=Henrik |title=Chapter 10 Alteration of Opaque Heavy Minerals as a Reflection of the Geochemical Conditions in Depositional and Diagenetic Environments |journal=Developments in Sedimentology |date=2007 |volume=58 |pages=277–303 |doi=10.1016/S0070-4571(07)58010-6|bibcode=2007DevS...58..277W |isbn=9780444517531 }}</ref> Ilmenite containing 6 to 13 percent {{chem2|Fe2O3}} is sometimes described as ''ferrian ilmenite''.<ref name=BuddingtonLindsley1964>{{cite journal |last1=Buddington |first1=A. F. |last2=Lindsley |first2=D. H. |title=Iron-Titanium Oxide Minerals and Synthetic Equivalents |journal=Journal of Petrology |date=1 January 1964 |volume=5 |issue=2 |pages=310–357 |doi=10.1093/petrology/5.2.310}}</ref><ref name=MurphyFrick2006>{{cite book |last1=Murphy |first1=P. |last2=Frick |first2=L. |year=2006 |chapter=Titanium |title=Industrial minerals & rocks: commodities, markets, and uses |editor-last1=Kogel |editor-first1=J. |publisher=SME |pages=987–1003 |isbn=9780873352338 |url=https://books.google.com/books?id=zNicdkuulE4C&q=Titanium |access-date=21 February 2022}}</ref>

Ilmenite [[Metasomatism|alters]] or [[Weathering|weathers]] to form the pseudo-mineral [[leucoxene]], a fine-grained yellowish to grayish or brownish material<ref name=KleinHurlbut1993/><ref>{{cite journal |last1=Mücke |first1=A. |last2=Bhadra Chaudhuri |first2=J.N. |title=The continuous alteration of ilmenite through pseudorutile to leucoxene |journal=Ore Geology Reviews |date=February 1991 |volume=6 |issue=1 |pages=25–44 |doi=10.1016/0169-1368(91)90030-B|bibcode=1991OGRv....6...25M }}</ref> enriched to 70% or more of {{chem2|TiO2}}.<ref name=MurphyFrick2006/> Leucoxene is an important source of titanium in [[heavy mineral sands ore deposits]].<ref name="VanGosenEtal2014">{{cite journal |last1=Van Gosen |first1=Bradley S. |last2=Fey |first2=David L. |last3=Shah |first3=Anjana K. |last4=Verplanck |first4=Philip L. |last5=Hoefen |first5=Todd M. |title=Deposit model for heavy-mineral sands in coastal environments |journal=U.S. Geological Survey Scientific Investigations Report |series=Scientific Investigations Report |date=2014 |volume=201--5070-L |page=13 |doi=10.3133/sir20105070L|doi-access=free |bibcode=2014usgs.rept...13V }}</ref>