Editing Garnet (section)

== Geological importance ==
[[File:Grenat.jpg|thumb|upright=1.5|Main garnet producing countries]]
[[File:Garnet var. Spessartine, Putian City, Putian Prefecture, Fujian Province, China.jpg|thumb|Garnet var. Spessartine, Putian City, Putian Prefecture, Fujian Province, China]]
The mineral garnet is commonly found in metamorphic and to a lesser extent, igneous rocks. Most natural garnets are compositionally zoned and contain inclusions.<ref>{{Cite book|last=Nesse|first=William D.|title=Introduction to Optical Mineralogy|publisher=Oxford University Press|year=2013|isbn=978-0-19-984628-3|edition=International Fourth|location=New York|pages=252–255}}</ref> Its crystal lattice structure is stable at high pressures and temperatures and is thus found in green-schist facies metamorphic rocks including [[gneiss]], hornblende [[schist]], and mica schist.<ref name="Klein-1985">{{Cite book|last1=Klein|first1=C|title=Manual of Mineralogy|last2=Hurlbut|first2=C. D.|publisher=John Wiley and Sons|year=1985|isbn=0-471-80580-7|location=New York|pages=375–378}}</ref> The composition that is stable at the pressure and temperature conditions of Earth's mantle is pyrope, which is often found in [[peridotites]] and [[kimberlite]]s, as well as the [[Serpentine subgroup|serpentines]] that form from them.<ref name="Klein-1985" /> Garnets are unique in that they can record the pressures and temperatures of peak metamorphism and are used as geobarometers and geothermometers in the study of [[geothermobarometry]] which determines "P-T Paths", Pressure-Temperature Paths. Garnets are used as an index mineral in the delineation of [[isograd]]s in metamorphic rocks.<ref name="Klein-1985" /> Compositional zoning and inclusions can mark the change from growth of the crystals at low temperatures to higher temperatures.<ref name="Teaching Phase Equilibria">{{Cite web|title=P-T-t Paths|url=https://serc.carleton.edu/research_education/equilibria/PTtPaths.html|website=Teaching Phase Equilibria|language=en|access-date=2020-03-19}}</ref> Garnets that are not compositionally zoned more than likely experienced ultra high temperatures (above 700&nbsp;°C) that led to diffusion of major elements within the crystal lattice, effectively homogenizing the crystal<ref name="Teaching Phase Equilibria" /> or they were never zoned. Garnets can also form metamorphic textures that can help interpret structural histories.<ref name="Teaching Phase Equilibria" />

In addition to being used to devolve conditions of metamorphism, garnets can be used to date certain geologic events. Garnet has been developed as a U-Pb [[Geochronometry|geochronometer]], to date the age of crystallization<ref>{{Cite journal|last1=Seman|first1=S.|last2=Stockli|first2=D. F.|last3=McLean|first3=N. M.|date=2017-06-05|title=U-Pb geochronology of grossular-andradite garnet|url=http://www.sciencedirect.com/science/article/pii/S0009254117302541|journal=Chemical Geology|language=en|volume=460|pages=106–116|doi=10.1016/j.chemgeo.2017.04.020|bibcode=2017ChGeo.460..106S|issn=0009-2541}}</ref> as well as a [[Thermochronology|thermochronometer]] in the (U-Th)/He system<ref>{{Cite journal|last1=Blackburn|first1=Terrence J.|last2=Stockli|first2=Daniel F.|last3=Carlson|first3=Richard W.|last4=Berendsen|first4=Pieter|date=2008-10-30|title=(U–Th)/He dating of kimberlites—A case study from north-eastern Kansas|url=http://www.sciencedirect.com/science/article/pii/S0012821X08005323|journal=Earth and Planetary Science Letters|language=en|volume=275|issue=1|pages=111–120|doi=10.1016/j.epsl.2008.08.006|bibcode=2008E&PSL.275..111B|issn=0012-821X}}</ref> to date timing of cooling below a [[closure temperature]].

Garnets can be chemically altered and most often alter to serpentine, [[talc]], and [[Chlorite group|chlorite]].<ref name="Klein-1985" />

===Largest garnet crystal===
The open-pit Barton Garnet Mine, located at [[Gore Mountain (New York)|Gore Mountain]] in the [[Adirondack Mountains]], yields the world's largest single crystals of garnet; diameters range from 5 to 35&nbsp;cm and commonly average 10–18&nbsp;cm.<ref name="Geophere">{{cite journal | title= Megacrystic Gore Mountain–type garnets in the Adirondack Highlands: Age, origin, and tectonic implications | year= 2011 | publisher=Geological Society of America| doi= 10.1130/GES00683.1 | last1= McLelland | first1= James M. | last2= Selleck | first2= Bruce W. | journal= Geosphere | volume= 7 | issue= 5 | pages= 1194–1208 | bibcode= 2011Geosp...7.1194M | doi-access= free }}</ref>

[[Gore Mountain Garnet|Gore Mountain garnet]]s are unique in many respects, and considerable effort has been made to determine the timing of garnet growth. The first dating was that of Basu et al. (1989), who used plagioclase-hornblende-garnet to produce a Sm/Nd isochron that yielded an age of 1059 ± 19 Ma. Mezger et al. (1992) conducted their own Sm/Nd investigation using hornblende and the drilled core of a 50&nbsp;cm garnet to produce an isochron age of 1051 ± 4 Ma. Connelly (2006) utilized seven different fractions of a Gore Mountain garnet to obtain a Lu-Hf isochron age of 1046.6 ± 6 Ma. It is therefore concluded with confidence that the garnets formed at 1049 ± 5 Ma, the average of the three determinations. This is also the local age of peak metamorphism in the 1090–1040 Ma Ottawan phase of the [[Grenville orogeny|Grenvillian orogeny]] and serves as a critical data point in ascertaining the evolution of the megacrystic garnet deposits.<ref name="Geophere"/>