Structure of the lecture ØSpinel group minerals - cubic oxides of general formula Ø- AB2O4 Øcrystalochemistry of the spinel group minerals Ømineral formula calculation and graphic presentation ØCr-spinels ØAl-spinels ØFe3+-spinels spinel group minerals Øcommon accessory minerals in the mantle and crustal rocks Øeconomically interesting ore deposits (magnetite, chromite) Øtransparent, translucent and attractively colored varieties are respected gemstones (from ancient times) Øinteresting for petrogenetic interpretations (PT conditions) -geothermometers Ø Skupina spinelu (spinelidy) Øgeneral formula AB2O4 Ø Øcrystallize in the cubic symetry, space group Fd-3m Øcommon crystal forms are octahedron (usually twinned); rarely rhombic dodecahedron, tetrahedron, (and their simple or complicated combinations). Øusually fine- to coarse grained aggregates ØIt has an imperfect octahedral cleavage and a conchoidal fracture. ØIts hardness is 8, Øits specific gravity is 3.5-4.1 Ø ØSpinel group minerals are subdivided into three series - based on dominant trivalent cation in B-site: •Spinel series (B = Al) •Magnetite series (B = Fe3+) •Chromite series (B = Cr) Spinel structure Øgeneral formula AB2O4 Ø ØA-site: Mg, Fe2+, Zn, Mn, Ni – divalent cations ØB-site: Al, Cr, Fe3+, V, Ti– trivalent cations Ø Ø Ø ØStructure of spinel was solved at 1915 (Bragg W.H., Nishikawa S.) ØSpinels have so-colled „normal“ and „inverse“ spinel structure ØStructure - oxygen atoms in spinel have a cubic close-packed structure –tetrahedral points are smaller than the octahedral points –„normal“spinel structure (a common example is MgAl2O4) •the cations A2+ occupy 1/4 of the tetrahedral sites •and cations B3+ occupy half of the octahedral sites –„inverse“ spinel structure (a common example is FeFe2O4) •If the A2+ ions have a strong preference for the octahedral site, they will force their way into it and displace half of the B3+ ions from the octahedral sites to the tetrahedral sites. spinel_structure A-site B-site Spinel group minerals ØSpinel series (B = Al) Ø Ø Ø Ø Ø Ø Ø Ø Ø Chromite seies (B = Cr) name T-site O-site structure Hercynite Fe2+ Al2 Normal Spinel Mg Al2 Normal Gahnite Zn Al2 Normal Galaxite Mn0,71Al0,29 Mn0,29Al1,71 29% Inverse name T-site O-site structure Chromite Fe2+ Cr2 Normal Magnesiochromite Mg2+ Cr2 Normal •Magnetite series (B = Fe3+) – Ø Ø Ø Ø Ø Ø ØTi-spinels Ø Někdy se přiřazují k Fe-spinelům, ale na rozdíl od nich neobsahují Fe3+ Ø name T-site O-site structure Ulvöspinel Fe2+ Fe2+Ti4+ 100% inverse Quandilite Mg2+ Mg2+Ti4+ 100% inverse name T-site O-site strukture Magnetite Fe3+ Fe2+Fe3+ Inverse Magnesioferrite Mg0,1Fe3+0,9 Mg0,9Fe3+1,1 from 90% inverse Franklinite Zn Fe3+2 Normal Jaccobsite Mn0,85Fe3+0,15 Mn0,15Fe3+1,85 from 15% Inverse Trevorite Fe3+ Fe3+Ni Inverse Spinel group minerals Mineral formula calculation and graphic presentation Ø AB2O4 Ø Ømineral formula recalculation on the basisi of 3 cations ØAll trivalent cations are assigned to the B site (Al, Cr, V). Elevated Si content indices that alteration takes a place. ØTake a care to the ulvöspinel component Ti4+Fe2+2O4. Assign the Ti in to the A site and the double amount of Fe2+ assign to the B-site. ØThe divalent cations (except Fe) should be assigned to the A-site. ØThe rest of Fe is splited between the A-site (Fe2+) and the B-site (Fe3+) Øthe FeO/Fe2O3 is calculated on the basis of the Fe2+/Fe3+ atomic ratio (The EMP measure only FeO or Fe2O3) prepocet2 Graphical presentation Øuseful combination of dominant cations in A-site and B-site –A-site on X-axis and B-site on Y-axis Øe.g. for Cr-spinels Ø in plots so-called Cr-number (Cr#) – equals to Cr/(Cr + Al) »Mg-number (Mg#) – equals to Mg/(Mg+Fe2+) »Fe3+ -number (Fe3+#) – equals to Fe3+/(Fe3+ + Cr + Al) »Fe -number (Fe#) – equals to Fe2+/(Mg+Fe2+) graf Cr spinely ukazka bez pop Øternary plots Ødisplay of 3 major components in the A-site or B-site Ø Ø suitable for variable B-site Cr spinely trojuhelnik CrAlFe Chemical composition of detrital Cr-rich spinels in the Moravo-Silesian Culm Basin, Drahany Upland (Čopjaková 2007) Graphical presentation Spinelidy ØVery good miscibility among end-members at high temperature, even among Fe, Cr, Al and Ti spinels. ØSpinels often forms solid state solution of many end-members, e.g. spinel-hercynite-chromite-magnetite; chromite-hercynite-magnetite-franklinite Ø Ø Ø ØSpinels are often strongly chemically heterogeneous-evolution through several different members of the spinel group, even within one rock. Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø => Difficult terminology – how to name such heterogeneous grains? graf Cr spinely ukazka tab pestrost Cr-spinels ØCr-spinels (Mg,Fe2+)(Cr,Al,Fe3+)2O4 ØAccording to the new classification: chromite (FeCr204); magnesiochromit (MgCr2O4); nichromite (NiCr2O4); zincochromite (ZnCr2O4) … Ø Ø klas Stevens OLD classification ternary plot Stevens (1944), based on the B-site trivalents ions Note, the old naming is commonly used in the recent literature, e.g. ferrichromite Cr-spinels Ø Cr-rich spinels – solid solution of chromite-magnesiochromite-spinel-hercynite Ø ØCr-rich spinels are ubiquitous accessory minerals in various mafic and ultramafic rocks such as mantle peridotites, gabbros, mafic volcanic rocks (basalts, tholeites) and their metamorphic equivalents. ØCr-spinels crystallized typically as an early stage magmatic mineral. ØContinuous spinel crystallization together with other silicate phases (olivine, pyroxenes and plagioclase) over a significant range of temperatures is common. In some cases the crystallization temperature interval for Cr-spinel may exceed 200–250 °C. ØChemical composition of Cr-spinels is controlled by several factors and as the most important are considered •PT conditions, •Melt composition, •degree of partial melting, •fractional crystallization, •subsolidus reequilibration with the co-existing silicates •oxygen fugacity •(see e.g., Irvine, 1965; Hill and Roeder 1974; Medaris, 1975; Pinsent and Hirst, 1977; Fisk and Bence, 1980; Dick and Bullen, 1984; Kepezhinskas et al., 1993). Ø Ø Factors govering chemical composition of Cr-spinels ØThe ratio of Fe2+/Fe3+ is sensitive to variations in fO2, it increases dramatically in magmas as they ascend (Sato, 1978). Ø ØThe effect of pressure on chromian spinel composition is controlled by the partition coefficient for Cr between crystals and melts –at high pressure conditions - partition coefficient for Cr is close to 1 –high partition coefficient are accepted for low pressures of melt segregation (Kurat et al. 1980; Dick and Bullen, 1984). Ø ØSpinel Cr # (Al2O3 abundances) depend on the melt composition, which is a function of pressure, temperature, and degree of partial melting Ø ØDuring fractional crystallization or partial melting - Cr and Mg are strongly partitioned into the solid, and Al and Fe into the melt Ø ØFractional crystallization (decreasing melt Mg # and temperature) should lead to related change in the Mg # of co-crystallizing olivine and spinel. ØPartitioning of Mg and Fe2+ between chromian spinel, silicate melts and minerals is strongly temperature dependent (thermometry) Ø ØSpinel Mg # should be interpreted with caution, as it is a complex function of a number of factors, the most important of which are –(1) Mg # of the parental melt; –(2) temperature dependent –(3) partitioning of Al and Cr in spinel, and hence Al2O3 in the melt; –(4) post-entrapment re-equilibration with silicate minerals Ø(3) The distribution coefficient Kd = (Mg/Fe)olivine/(Mg/Fe2+)spinel varies significantly from 2.5 (cr-number 20–30) to 10–12 (cr-number 80–90) => spinel coexisting with olivine becomes more Mg rich with decreasing cr-number. Ø(4) => the cooling rate and spinel grain size are important factors. Consideration of the kinetics of olivine–spinel Mg–Fe2+ interdiffusion precludes significant subsolidus re-equilibration for rapidly cooled, volcanics - insufficient time for re-equilibration at near-magmatic temperatures. Slowly cooled subaerial thick lava flows and intrusive rocks (cooling rates of 0.1–0.001°C/h) can show extensive re-equilibration. Factors govering chemical composition of Cr-spinels Cr-spinels from mantle rocks - peridotites ØCr-rich spinels crystallized typically as an early stage magmatic mineral. ØCr-rich spinel is common in peridotite in the uppermost Earth's mantle, between approximately 20 - 120 km, possibly to lower depths depending on the chromium content. •At significantly shallower depths, calcic plagioclase is the more stable aluminous mineral in peridotite, •garnet is the stable phase deeper in the mantle below the spinel stability region. • • • • Ø Cr-spinel from „peridotites “ – mantle origin Cr-spinel in BSE image Cr-spinels form: - fine- to coarse-grained aggregates - massive cummulates - disseminated anhedral grains - disseminated euhedral grains (octahedron) no melt inclusions ! Cr-spinels from mantle peridotites chromity peridotity Ø Alpinotype peridotites Ø Ocean floor peridotites Discriminant fields according to Cr-spinel database from Barnes and Roeder (2001) Cr-spinel from volcanic rocks vs. Cr-spinel from peridotites Ø Chemical composition Ø ØPlutonic and volcanic complexes representing various geotectonic settings show remarkable differences in composition of chromian spinel (e.g., Dick and Bullen, 1984; Arai, 1992; Kepenzhinskas et al., 1993). Ø ØIn order to distinguish between intrusive peridotitic and extrusive volcanic spinels TiO2 content and Fe2+/Fe3+ ratio are used. Ø ØThe contents of Ti and Fe3+ from mantle rocks (peridotites) are consistently low (TiO2 < 0.25 wt. % and Fe3+ < 5 wt. %) (Dick and Bullen, 1984; Lee, 1999). Ø ØVolcanic spinels with TiO2 bellow 0.2 wt. % are uncommon, only sporadically some arc boninites and tholeites or low-Ti MORB have lower TiO2 contents (Barnes and Roeder, 2001; Lenaz et al., 2000). Ø ØOn the other side, comparing with peridotitic spinels, volcanic spinels have higher Fe3+ contents, and Fe2+/Fe3+ ratio is usually < 4 (Kamenetsky et al., 2001). Cr-spinels from peridotites have Fe2+/Fe3+ ratio usually > 3 Ø Ø Cr-spinels from „volcanic “ rocks Cr-spinel in BSE image - never massive - usually euhedral to subhedral discrete grains - smaller grains - commonly form inclusions in other major rock forming minerals (olivine, pyroxene) - presence of melt inclusions in Cr-spinel - usually show zonality Subhedral Cr-spinel from volcanic “ rock with melt inclusion Zonal Cr-spinel from volcanic “ rock - range from Mg-chromite in the core to aluminous Ti-magnetite in the rim Cr-spinels from „volcanic “ rocks ØSpinel Al2O3 vs TiO2 - a guide to magma chemistry and tectonic provenance ØThe compositional pairs of coexisting spinel and glass were studied to examine the effects of melt composition on the abundances of Al2O3 and TiO2 in spinel (Kamenetsky et al. 2001). ØA positive correlation between Al2O3 and TiO2 contents in spinel and coexisting melt is demonstrated over significant intervals of averaged spinel and melt compositions (e.g. 3–39 and 4·6–18 wt % Al2O3, and 0·04–3·9 and 0·07–3·9 wt % TiO2), sampled from a variety of magmatic types and tectonic environments ØObserved relationships are primarily controlled by magmatic Al2O3 and TiO2 contents. MORB – middle ocean ridge basalts BABB – back arc basin basalts OIB – ocean island basalts Island Arc – basalts of volcanic island arcs Continuous and dashed lines in (b) are for the high-Al and low-Al compositions Cr-spinels from „volcanic“ rocks basalty a andesity vulkanických oblouků boninity vulkanických oblouků OFB BABB gabra oceánského dna vnitrodeskové basalty chromity vulkanity MgCr cislo a Al2O3 a TiO2 Discriminant fields according to Cr-spinel database from Barnes and Roeder (2001), Kepezhinskas et al. (1993) and Kamenetsky et al. (2001) Ø Al2O3 vs TiO2 diagram to discriminate spinels that crystallized from different magmas in different geodynamic settings Melt inclusions in Cr-spinels ØMelt inclusions found in mineral grains (olivine, pyroxene, Cr-spinel) as tiny droplets trapped during crystal growth offer a unique way of catching instantaneous melt composition as magma cools due to their effective isolation from the influence of later processes, and thus they can reveal the melt evolution that may not be recorded in bulk-rock data (Watson, 1976; Roeder and Poustovetov,2001). ØCr-spinel = early-formed mineral => melt inclusion reflect composition of primary melt ØMelt inclusions – offer a unique way for study of melt chemistry •glass •usually mixture of glass and crystals Ø Melt inclusion - clinopyroxene crystals with spinifex-like texture (Zhou et al.) Melt inclusion containing very small partially crystallized minerals. (Zhou et al.) Melt inclusion in Cr-spinel ØMelt inclusions – offer a unique way for study of melt chemistry •glass •mixture of glass and crystals Ømelt inclusions composed from mixture of glass and crystals – Cr-spinels must be heated ( for X0 hours at T ~ 1200-1400 °C) and quenched ØBoth unheated (glass) or heated (homogenised) melt inclusions are studied with electron microprobe (a beam diameter 5-10 mm). Unheated melt inclusion in Cr-spinel (diameter 0.04 mm) containing a mixture of glass (dark gray) and pyroxene crystals (light gray - occupy over 70% of the surface). Heated and quenched (from 1360 °C) melt inclusion in Cr-spinel (diameter 0.06 mm) – contain glass with no silicate crystals (Sigurdsson et al. 2000). ØSigurdsson et al. (2000) observed two groups of primitive melts as inclusions in Cr-spinels in a picrite. ØThese primitive melts were isolated inside the crystals before the two types of magma had an opportunity to mix. ØThe inclusions are probably close to being true samples of these two types of primary melt. Ø Ø Melt inclusions in Cr-spinels Zonality in Cr-spinel – mantle peridotites Ø Cr spinely sp myslejovice Decreasing Mg # - with decreasing T and/or due to subsolidus re-ekvilibrations with silicates ØZonal spinel groups mineral from basaltic rock (Mrázová 2007) Ø Compositional evolution from - Cr-rich spinel in the core (up to 30 wt. % Cr2O3) Ø to - Ti-magnetite in the rim (up to 18 wt. % TiO2) Ø small Ti-magnetites are common in matrix Ø usually observed in intraplate basalts and lamprophyres cpx cpx sp sp Zonality in Cr-spinel – volcanic rocks Zonality in Cr-spinel – volcanic rocks Ø Cr spinely protivanov Commonly observed zonality in OFB, BABB and volcanic arc basalts and andesites (low TiO2) This zoning pattern reflect decreasing PT conditions (a) and increasing fO2 (b) - BABB - volcanic arc Cr-spinel thermometry ØOlivine-spinel Fe2+-Mg thermometers –Roeder et al, 1979 –O’Neill and Wall (1987), modified by Ballhaus et al. (1991) –Sack and Ghiorso, 1991 –Jianping et al., 1995 ØTo evaluate the T dependence of olivine-spinel Fe-Mg partitioning in analyzed pairs Ø ØSpinel-olivine empirical and theoretical thermometers are calibrated for mantle rocks (low Fe3+ and very low Ti4+), although spinel-olivine pairs are also common in basaltic volcanic rocks and in shallow mafic intrusions Ø ØOlivine-spinel pairs for volcanic rocks (basanite; Fedele and Tracy 2003) appears to underestimate absolute crystallization T, due to significant concentrations of the usp end member in the magmatic spinels. –Ti is not taken into account in thermometers of Roeder et al. and Jianping et al.) and apparently is not fully accounted in the model of Sack and Ghiorso. –approximately linear relationships was observed between Ti content and T- underestimation for all 3 thermometers Cr-spinel thermometry ØOrthopyroxene-spinel Fe2+-Mg thermometers –Mukherjee and Viswanath 1987; Mukherjee et al. 1990 –Liermann and Ganguly 2003; 2007 ØTo evaluate the T dependence of orthopyroxene -spinel Fe2+-Mg partitioning in analyzed pairs ØEffect of Fe3+ content in spinel –Although a thermodynamic correction was proposed for the effect of Fe3+ content of spinel, practical application of this correction is problematic due to inaccurate determination of Fe3+ content in spinel. –The Fe3+/ΣFe in minerals is usually determined from the microprobe data by imposing the condition of electroneutrality. However, this procedure absorbs the error of the analytical data into the estimated Fe3+ content. a)lnKD(Fe–Mg) between orthopyroxene and spinel versus a temperature (K) b)YCr(Sp), i.e., Cr/(Cr+Al) in spinel at 1.2–1.3 GPa, 1,000 C. The lnKD values with correction for the effect of Al substitution in orthopyroxene are shown as closed symbols, whereas those without correction for this effect are shown as open symbols. The triangles and circles stand for the different estamation of Fe3+ Øolivine-spinel Fe2+-Mg thermometer Ø(Ballhaus et al. 1991) Øvs. Øorthopyroxene-spinel Fe2+-Mg thermometer –(Liermann and Ganguly 2003; 2007) Ø ØThere are no significant differences between the temperatures estimates according to olivine-spinel and orthopyroxene-spinel thermometers at T < 900 °C. Above 900°C, T °C (Opx-Sp) is 30–75 °C higher than T °C (Ol-Sp) that may be due to the problem of resetting of olivine-spinel thermometer. Ø Cr-spinel thermometry Comparison of temperatures of Cr-spinel bearing calculated from the orthopyroxene–spinel thermometer and the olivine–spinel thermometer of O’Neill and Wall (1987), as modified by Ballhaus et al. (1991) ØMetamorphic processes do not modify chemical composition of chromian spinels up to lower amphibolite facies conditions. Ø ØPinsent and Hirst (1977) noticed different behavior of chromian spinels with variable Cr-content. –chromian spinels with high Cr# are more stable in metamorphic conditions. –For Al-rich chromian spinels with Cr# <0.5 the reaction spinel + serpentine + brucite = “ferritchromite” + chlorite proceeded in peridotites. –Metamorphism of chromian spinels with Cr# >0.5 shifts their composition to “ferritchromite” and Cr-enriched magnetite, and chlorite does not originate. – ØSubstitution of Mg by Fe2+ (or Zn, Ni, Mn) as a result of exchange with associated silicates in subsolidus changes significantly Mg# in highly metamorphosed rocks; ØSubstitution of Cr and Al by Fe3+ proceeded during oxidation process; ØConsequently, metamorphosed chromian spinels are largely Fe2+ -enriched and locally Fe3+-enriched relative to their magmatic precursors (e.g., Oberhänsli et al., 1999; Barnes, 2000; Barnes and Roeder, 2001). Ø ØCr-spinels from metamorphosed rock (≥ amphibolite facies) can be enriched in Zn (up to 7 wt. % ZnO), Mn (up to 3 wt. % MnO), or Ti compared with their magmatic precursor. Higher Si and Ca contents are typical for these metamorphosed ferritchromites and Cr-rich magnetite. Cr-spinels from metamorphic rocks Cr-spinels in BSE image Al-rich chromian spinels with Cr# <0.5 spinel (Sp I) “ferrichromite” (Sp II) + chlorite Cr-rich magnetite Cr-spinels from metamorphic rocks Sp I Chl I Sp II Replacement reaction of magmatic spinel from the Ransko gabbro-peridotite massif Replacement reaction of magmatic spinel from the Moldanubian spinel peridotite Cr-spinels in BSE image Metamorphism of chromian spinels with Cr# >0.5 Cr-rich spinel ferrichromit or Cr-enriched magnetite Cr-spinels from metamorphic rocks Cr-spinels in BSE image Metamorphism of chromian spinels with Cr# >0.5 Cr-rich spinel ferrichromit or Cr-enriched magnetite Cr-spinels from metamorphic rocks Primary magmatic Cr-rich spinels can be fully replaced by secondary metamorphic Cr-rich magnetite Ø Secondary Cr-spinel (so called ferrichromite) in BSE image – porous, enriched in SiO2 and CaO replacing primary magmatic Cr-spinel in serpentinites observed Mellini et al. (2005). MediaObjects/s00410-005-0654-yfhb1.jpg Cr-spinels from metamorphic rocks Ø Transmission electron microscopic investigation shows that ferritchromit actually consists of a complex, nanometric association of Cr-rich magnetite Ø (Mg0.03Fe2+0.97 Al0.11Cr0.89Fe3+1.00O4), chlorite and lizardite, with (001)chl/liz always parallel to (111) Cr-mag. Ø Bellow the resolution of electron microprobe Ø Mg and Al, released from the altered Cr-spinel, giving rise to chlorite, lizardite. MediaObjects/s00410-005-0654-yfhb5.jpg Cr-spinels from metamorphic rocks TEM images of ferritchromit (Mellini et al. 2005) Cr-spinels from the Bohemian Massif Øa,b) Moldanubian Zone Ø red field – primary Cr-spinels from spinel and garnet peridotites; Ø full red circle – secondary Cr-spinels from spinel peridotites; Ø křížky – tremolite rock (metamorphosed ultrabasic cumulate); Ø yellow square – durbachites (Třebíč massif) Ø ØČopjaková et al. (2005b), Medaris et al. (2005), Sulovský (2001), Van der Veen and Maaskant (1995) and unpublished data of Štědrá and Čopjaková ZH Crspinely CM cast I Ø c,d) Letovice-Rehberg ofiolite and Nasavrky massif; Ø Letovice-Rehberg ofiolite - white square – primary Cr-spinels; Ø black rhombus – secondary Cr-spinels; Ø Nasavrky massif – red field – garnet and spinel mantle peridotites; Ø yellow dashed line field – gabbro and spinel peridotite – Alaskan type; Ø yellow dot-and-dashed line field – secondary Cr-spinel from the Alaskan type gabbro; Ø Ø Čopjaková et al. (2005b), Medaris et al. (2005), Sulovský (2001), Van der Veen a Maaskant (1995) and unpublished data of Štědrá and Čopjaková ZH Crspinely CM part II Cr-spinels from the Bohemian Massif ØCr-spinels are important HM in siliciclastic sediments for provenance studies. Ø ØThey are stable during diagenesis and quite resistant to late hydrothermal alterations particularly relative to other high temperature minerals of mafic and ultramafic rocks such as olivines and pyroxenes. Ø ØThey represent a unique provenance indicator for various mafic and ultramafic rocks and subsequently for geotectonic implications. Ø ØUsing chemical composition of chromian spinels for provenance and geotectonic implications - e.g., Utter, 1978, Press, 1986, Arai and Okado, 1991, Cookenboo et al., 1997, Oberhänsli et al., 1999 Ø ØChemistry of detrital spinels and melt inclusion compositions in the detrital volcanic spinels from the Claut/Clauzetto and Julian Basins (N Italy and NW Slovenia) is used to constrain petrological and geochemical affinities and tectonic provenance of the source rocks – Lenaz et al. (2000)This is the first study of melt inclusions in detrital spinels. Ø Ø Detrital Cr-spinels in siliciclastic sedimentary rocks Melt inclusions in detrital Cr-spinels from volcanic rocks (Drahany Upland) - reflect origin from arc-related volcanic rocks (basaltic andesites and andesites) Øa) TAS diagram according to Le Maitre et al. (1989) inkluze v Cr spinelech grafy b) diagram according to Le Maitre et al. (1989) – and Rickwooda (1989) Detrital Cr-spinels in siliciclastic sedimentary rocks Al-spinels Ø Spinel s.s. MgAl2O4 ØPure is colorless, often is colorized by chromophorm elements: bluish (Co), greenish chlorospinel (Fe3+), redish (Cr) ØTypical mineral of HT metamorphosed rocks. •Common mineral in metamorphosed dolomite marbles - together with diopside and forsterite (e.g. In the Varied Unit, Moldanubian Zone – mineral assemblage Cal+Dol+Fo+Sp+Phl+Ch+Cho - (Čopjaková et al. 2008) – spinel-gahnite; 660-730°C, 3-4 kbar (Novák 1989)) – – – •Mg-rich skarnes •granulites (spinel-hercynite) or HT metapelites •From mafic and ultramafic rocks (Cr-enriched) Ø ØMagmatic rocks rich in Al2O3 – pegmatites, gabros ØTypical alluvial mineral (high density, mechanical and chemical durability), e.g. in pyrope gravelites in České středohoří (Třebívlice, Měrunice), Jizerska Louka tab spinel Al-spinels Ø Hercynite Fe2+Al204 ØUsually from metamorphosed (granulite grade) Fe-rich sediments ØLess common from mafic to ultramafic magmatites, pyroxenites and felsic granulites Ø Tabule - spinels – composition hercynite-spinel from felsic granulites - (Čopjaková 2007) tab hercynit Garnet-hercynite-plagioclase domain in felsic granulite originate by decomposition of kyanite during isothermal decompression – hercynite is partially replaced by AlOOH – diaspore? (Čopjaková 2007) Al-spinels Ø Gahnite ZnAl2O4 ØUsually forms solid solution with spinel and hercynite ØCommon mineral in metamorphosed contaminated marbles (Al enriched) and skarns enriched in Zn Øgahnite-spinel solid solution in Bohemian Massif –marbles in the Polička Crystalline Unit; marbles in the Hraničná Group (Staré Město Crystalline Unit) – mineral assemblagy Cal+Dol+Tr+Phl+Di (Novák, Houzar, Šrein 1997) –marbles in Varied Unit, Moldanubian Zone (min. assemblagy Cal+Dol+Fo+Sp+Phl+Ch+Cho) (Čopjaková et al. 2008) Ø Ø Ø Ø –granitic pegmatites (Maršíkov, Otov, Přibyslavice) Ø Øgahnite-hercynite solid solution - metapelites - staurolite bearing garnet mica schist – inclusions in staurolite (Svratka Crystalline Unit) – (Buriánek, Čopjaková 2008) Ø Ø Ø ØMetamorphosed ore deposits (Franklin, USA; Broken Hill, Austrálie) Ø Ø Galaxite MnAl2O4 ØVery rare, described from Mn rich vein deposits. Ø tab gahnit tab gahnit II Fe-spinels Ø ØMagnetite – the most common spinel group mineral ØOccurs in the wide variety of magmatic and metamorphic rocks-basic to intermediate magmatites (volcanic and plutonic), scarn deposits of economic importance, volcano-sedimentary and sedimentary rocks. ØAmphibolites, metamorphic ultrabasic rocks, metapelites, Alpine veins and authigenic Ø ØMagnesioferrite - jako koncový člen vzácný (vysokoteplotní - fumaroly, hořící haldy, Mg-mramory a skarny). Obvykle v pevném roztoku s magnetitem (obvykle do několika molárních %) ØFranklinit – vzácnější; •vyskytuje na Zn ložiscích Franklin a Sterling Hill (USA) - metasomatický, vzniklý reakcí hydrotermálních fluid s okolními horninami •metamorfované bazické a ultrabazické horniny obsahující Cr-bohaté spinely minimálně ve spodní amfibolitové facii – obvyklá minoritní komponenta ve spinelidech ( ØJaccobsit – metasomatická ložiska Mn ØTrevorit – vzácný; známý z mastkových fylitů v Jižní Africe, Ni-bohatý serpentinit Ø Fe-spinels Ø Magnetite ØElevated Usp component signalize elevated temperature. Increase of fO2 cause the rise in Fe3+ content in the system and the magnetite component as well. ØGood miscibility between magnetite and ulvospinel molecule at elevated temperature (Ti-rich magnetite from volcanic rock) ØThe slow decrease of temperature usually generate exsolution of ilmenite lamellas (commonly observed in gabros, amphibolites) Ø ØThe rock containing both Mt-Usp (magnetite) and Ilm-Hmt(ilmenite) –Its chemical composition is unambiguously determined by T and fO2 at the time of the equilibrium –can be used as geothermometers Ø Ø Magnetite ØGabbro – Nasavrky massif (Mrázová 2007) Ø exsolved ilmenite from magnetite; bright in BSE image – magnetite; dark in BSE image - ilmenite cpx cpx plg plg mag ilm ilm mag Magnetite as provenance indicator ØDetrital magnetite grains carry textural and chemical features that can be used in provenance research. Petrographic analysis of cca 3000 detrital magnetite grains from Holocene sands was performed by Grigsby (1990) Ø Chemical compostition ØTiO2, MgO, V2O3, and Al2O3 contents in magnetite best discriminate between detrital magnetite grains from felsic plutonic and volcanic, intermediate volcanic, and mafic plutonic parent rocks. ØHowever, grains from mafic volcanic and metamorphosed mafic/ultramafic sources were not chemically distinct, emphasizing the importance of integrating petrographic and chemical analyses in provenance research. Ø Texture ØSands from mafic volcanic sources are characterized by a 1:1 relationship between polymineralic (grains with trellis- or composite-type magnetite-ilmenite intergrowths) and monomineralic grains. ØMonomineralic (homogeneous) grains are characteristic of sands derived from felsic plutonic and intermediate volcanic sources. Ø V and Ni in spinels ØV – no V dominant end member Ø ØV - enters into the Cr-spinel (mafic and ultramafic rocks) and Fe-spinel (magnetite – mafic magmatic rocks) Øin Al-spinel – usually close to the detection limit of EMP • •In Cr-rich spinel peridotites – usually bellow 0.3 wt. % V2O3) •In Cr-rich spinel from volcanic arcs – usually bellow 0.8 wt. % V2O3 •In magnetite and Ti-magnetite from basalts and andesites - usually bellow 0.6 wt. % V2O3 •V-deposit Abitibi - layered gabbro intrusion, ilmenite and magnetite; V-rich magnetite (up to 1.5 wt.% V2O3) • ØNi (nichromite – NiCr2O4, trevorite NiFe3+2O4) –usually enters Cr-rich spinel from mantle peridotites – up to 0.3 wt.% NiO; –and secondary ferrichromite – during metamorphism of primary Cr-spinel in peridotites (amphibolite facie) – > 1 wt. % NiO • • Ø