samples FKM-1 to FKM-25

 

Check out the thin section scans introduction page for more information on the variety of samples featured here, how the scans were taken & processed for web display, and what additional optical and analytical data I hope to include in the figure captions as I continue to update the site and add to the collection of thin sections.

There’s also a fully searchable index covering the complete thin section set, listing for each sample its locality, the anticipated major minerals, a brief generalized geologic environment description, and where appropriate, the nature of any unusual element enrichments.

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sample: FKM-1
locality: Merelani Hills, Lelatema Mtns., Simanjiro district, Manyara region, Tanzania.
rock type: grossular-zoisite “skarn”. Granulite facies calc-silicate with retrograde amphibolite facies overprinting; possibly a meta-marl, with accompanying metasomatism. Specimens such as this with coarse grossular (and those with coarse tanzanite… see sample FKM-13, also from Merelani) are reported to occur in locally “low pressure” pockets and veins.
major mineralogy: Vanadium-bearing grossular garnet (“tsavorite”; with higher V in patches and rims) associated with zoisite+quartz+calcite symplectite, with minor additional calcite, diopside, graphite, sulfides, and titanite (up to 0.4 wt% V, so considerably less V-enriched than titanite in sample FKM-26, also from Merelani). Sparse scattered apatite is also present.
photomicrographs of FKM-1
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-1
pyrrhotite (Fe0.90Ni0.01)S1.00
pyrite Fe1.00S2.00
fluorapatite (Ca5.03Y0.01)[P2.94Si0.04O12](F0.87[OH]0.12)
grossular (most Mn-rich) (Ca2.65Mn2+0.31Mg0.01)(Al2.00V0.01Fe3+0.01Mn3+0.01)[Si2.95Al0.040.01O11.96F0.04]
grossular (most V-rich) (Ca2.91Mg0.05Mn2+0.04)(Al1.85V0.10Ti0.02Cr0.01Mn3+0.01)[Si2.97Al0.020.01O11.96F0.04]
titanite Ca1.00(Ti0.75Al0.24V0.01)(O0.75F0.15[OH]0.10)[Si0.99Al0.01O4]
zoisite Ca1.00Ca1.00Al1.00Al1.00Al1.00O[Si2.00O7][Si1.00O4](OH)
diopside Ca1.00Mg1.00[Si2.00O6]

 



sample: FKM-2
locality: Prabornaz mine, Saint-Marcel, Val d’Acosta, Italy.
rock type: titanite-piemontite schist. Greenschist to amphibolite facies meta-Mn-rich sediment (distal volcanogenic exhalite?), with superimposed metasomatism.
major mineralogy: Strikingly pleochroic Sr-rich piemontite with Sb-rich titanite, with minor quartz and orthoclase. Minor braunite and sparse rutile are also present. Sample FKM-171 is also from the Prabornaz mine, but represents a different assemblage (mica-bearing and more rich in Mn-oxides).
photomicrographs of FKM-2
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-2
rutile (Ti0.98Mn4+?0.01Fe3+0.01)O2
braunite (Mn2+0.80Ca0.20)(Mn3+5.45Fe3+0.49Al0.02)O8[Si1.02O4]
titanite (most Sb-rich) (Ca1.00Na0.01)(Ti0.78Sb5+0.12Fe3+0.04Mn3+0.03Al0.02)(O0.99[OH]0.01)[Si0.98Al0.02O4]
piemontite (most Sr-rich) (Ca0.97Mn2+0.03)(Ca0.61Sr0.36)(Al0.62Fe3+0.38)Al(Mn3+0.77Fe3+0.18Mn2+0.05)O[Si2.02O7][Si1.01O4](OH)
orthoclase (K0.92Na0.06Ba0.01)[Si2.98Al1.02O8]

 



sample: FKM-3
locality: Palos Hill, Gria Spilia, Syros Island, Cyclades chain, Greece.
rock type: Na-(Ca) clinopyroxene-glaucophane schist. Blueschist facies transitional to (or perhaps retrogressed from) eclogite facies; metavolcanic or meta-volcaniclastic(?)
major mineralogy: Nybøite was reported for this sample, but all of the amphibole examined in this thin section was verified as glaucophane by EPMA. This sample also contains abundant jadeite/omphacite clinopyroxene solid solution and paragonite/phengite dioctahedral mica solid solution. Subordinate epidote (with Sr- and REE-rich growth zones), along with scattered titanite, rutile (also with ~716 ppm Cr) and monazite, are also present.
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-3
rutile (Ti0.98Fe3+0.01)O2
monazite-(Ce) (Ce0.44La0.21Nd0.14Y0.05Pr0.04Ca0.03Th0.03Sm0.02Gd0.02[HREE]0.01)[P0.99Si0.01O4]
titanite Ca0.99(Ti0.88Al0.10Fe3+0.02)(O0.89[OH]0.06F0.05)[Si1.00O4]
epidote (most Sr+REE-rich) (Ca0.93Mn2+0.06Na0.01)(Ca0.54Ce0.23La0.09Sr0.07Nd0.02Pr0.01[M+HREE]0.01)Al1.00Al1.00
(Fe2+0.43Fe3+0.34Al0.20Sc0.01Mg0.01Ti0.01)(O0.97F0.03)[Si2.02O7][Si1.01O4](OH)
epidote (bulk) (Ca0.89Mn2+0.08Fe2+0.03)(Ca0.95)Al1.00Al1.00
(Fe3+0.79Fe2+0.09Al0.08Sc0.02Cr0.01)O[Si2.03O7][Si1.01O4](OH)
jadeite-rich cpx ss (Na0.84Ca0.12Mg0.02)(Al0.52Fe3+0.29Fe2+0.12Mg0.07)[Si2.01O6]
omphacite-rich cpx ss (Na0.59Ca0.37Mg0.03)(Al0.38Mg0.28Fe3+0.18Fe2+0.15)[Si2.01O6]
aegirine-augite-rich cpx ss (Na0.71Ca0.24Mg0.02)(Fe3+0.36Al0.30Mg0.19Fe2+0.15)[Si2.02O6]
glaucophane (Na0.04K0.010.95)(Na1.81Ca0.19)(Mg1.89Al1.43Fe2+0.98Fe3+0.62Ti0.01Zn0.01Cr0.01)
[Si7.78Al0.22O22]([OH]1.91F0.07O0.02)
muscovite-rich dioct mica ss (K0.82Na0.050.12)(Al1.55Mg0.29FeT0.18Ti0.01Cr0.010.95)[Si3.42Al0.58O10]([OH]1.96F0.04)
paragonite-rich dioct mica ss (Na0.82K0.03Ca0.010.15)(Al1.99FeT0.05Mg0.010.94)[Si3.03Al0.97O10]([OH]1.98F0.02)

 



sample: FKM-4
locality: near Kionia, Tinos, Greece.
rock type: garnet-epidote-glaucophane schist. Bröcker & Enders, 2001 describe glaucophane- and epidote-bearing HP metamorphic rocks from near the village of Kionia as peak eclogite facies meta-ophiolite rocks subsequently variably retrogressed to either greenschist or blueschist (then greenschist) facies. A potential eclogite to blueschist to greenschist P-T path would appear to be consistent with the mineralogy of this sample.
major mineralogy: Porphyroblasts of epidote (greenschist overprint), minor garnet (relict eclogite?), and rare zones of winchite (originally reported to be ferrowinchite), with some actinolite (greenschist overprint) and muscovite, and abundant glaucophane (blueschist overprint). Scattered rutile (also with ~602 ppm Cr and ~183 ppm Mn) is also present.
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-4
rutile (Ti0.98Fe3+0.01)O2
magnetite Fe2+1.00(Fe3+1.97V0.01Si0.01)O4
fluorapatite Ca5.02[P2.99O12](F0.87[OH]0.13)
almandine (core) (Fe2+1.65Ca0.80Mn2+0.33Mg0.19)(Al1.91Fe3+0.09Ti0.01)[Si2.97Al0.03O12]
almandine (middle) (Fe2+1.79Ca0.78Mg0.22Mn2+0.19)(Al1.96Fe3+0.04Ti0.01)[Si2.98Al0.02O12]
almandine (rim) (Fe2+1.76Ca0.84Mg0.30Mn2+0.09)(Al1.97Fe3+0.03)[Si2.98Al0.01O12]
titanite Ca1.00(Ti0.95Al0.04Fe3+0.01)(O0.94[OH]0.06F0.01)[Si0.99Al0.01O4]
epidote (Ca0.94Fe2+0.05Mn2+0.01)Ca0.97Al1.00Al1.00(Fe3+0.50Al0.41Fe2+0.06Mg0.01V0.01Ti0.01)O[Si2.02O7][Si1.01O4](OH)
omphacite (Ca0.50Na0.47Mg0.03)(Mg0.40Al0.38Fe2+0.14Fe3+0.07)[Si2.00O6]
glaucophane 1.00(Na1.85Ca0.09Fe2+0.05)(Mg2.13Al1.76Fe2+1.05Fe3+0.05Cr0.01)[Si8.01O22](OH)2.00
winchite (Na0.02K0.020.95)(Ca1.29Na0.71)(Mg3.03Fe2+0.78Fe3+0.60Al0.52Mn2+0.04Cr0.02V0.01Ti0.01)
[Si7.51Al0.49O22]([OH]1.95F0.03O0.01)
actinolite (K0.010.99)(Ca1.66Na0.28Mn2+0.03Fe2+0.03)(Mg3.66Fe2+1.06Al0.25Fe3+0.03)[Si7.99Al0.01O22]([OH]1.98F0.02)
muscovite (K0.90Na0.020.07)(Al1.50Mg0.35FeT0.14Ti0.01Cr0.010.99)[Si3.50Al0.50O10](OH)2.00
clinochlore (Mg2.67FeT1.96Al1.21MnT0.040.11)[Si2.81Al1.19O10](OH)8.00

 



sample: FKM-5
locality: Cape Marmari, Grammata Bay, Syros Island, Cyclades chain, Greece.
rock type: heavily altered or retrogressed blueschist.
major mineralogy: The sample is now nearly entirely muscovite pseudomorphs after lawsonite, along with subordinate calcite and scattered actinolite, clinozoisite, pumpellyite-(Al), titanite and clinochlore. Originally reported to contain chromian lawsonite, but no relict lawsonite remains in this thin section.
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-5
titanite Ca1.00Ti1.00O[Si1.00O4]
pumpellyite-(Al) (Ca1.95Fe2+0.02Mn2+0.01Na0.01)(Al0.49Mg0.45FeT0.03Ti0.01V0.01Cr0.01)Al2.00
[Si2.01O7][Si1.00O4]([OH]~1.37O~0.55F0.08) . H2O
clinozoisite (Ca0.97Fe2+0.03)(Ca0.97Sr0.02)Al1.00Al1.00(Al0.73Fe3+0.25Cr0.01Ti0.01)O[Si1.99Al0.01O7][Si1.00O4](OH)
actinolite (Na0.02K0.010.97)(Ca1.64Na0.36)(Mg3.62Fe2+0.90Al0.31Fe3+0.13Ni0.02Cr0.01Mn2+0.01)[Si7.88Al0.12O22]([OH]1.95F0.04)
muscovite (K0.62Na0.32Ca0.02Sr0.010.04)(Al1.84Mg0.07FeT0.06Ti0.04Cr0.010.98)[Si3.03Al0.97O10](OH)2.00
clinochlore Ca0.01(Mg3.43FeT1.20Al1.15Ni0.02MnT0.010.18)[Si2.92Al1.08O10]([OH]7.98F0.02)

 



sample: FKM-6
locality: Ottré, Vielsalm, Stavelot massif, Luxembourg Province, Belgium.
rock type: chloritoid phyllite. Greenschist facies (chlorite zone) metapelite.
major mineralogy: Porphyroblasts of chloritoid (originally reported to be ottrélite, but Mn is only ~0.3 apfu, verified by EPMA), with abundant chlorite and muscovite, and tiny widepread hematite. Scattered apatite and monazite are also present.
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-6
fluorapatite (Ca4.97Na0.01Nd0.01Y0.01)[P2.97Si0.02O12](F0.75[OH]0.25)
chloritoid (core) (Fe2+1.53Mn2+0.29Mg0.19)Al3.96[Si1.01O4]2(OH)4.00
chloritoid (rim) (Fe2+1.47Mg0.28Mn2+0.27)(Al3.93Fe3+0.02)[Si1.01O4]2(OH)4.00
muscovite (K0.73Na0.05Ba0.010.21)(Al1.57Mg0.23FeT0.23Ti0.030.94)[Si3.42Al0.58O10]([OH]1.90F0.10)
clinochlore Na0.01(Mg2.31FeT1.70Al1.67MnT0.09Zn0.010.22)[Si2.57Al1.43O10]([OH]7.99F0.01)

 



sample: FKM-7
locality: Schellkopf, Brenk, Niederzissen, Eifel, Rhineland-Palatinate, Germany.
rock type: nosean phonolite.
major mineralogy: Large nosean and sanidine are abundantly scattered in a fine-grained to glassy felsic volcanic matrix. The aegirine-augite (with up to 4300 ppm V), nepheline, analcime, K-rich “natrolite”-like zeolite, and perovskite occur as relatively fine grains within the volcanic matrix, whereas the Na-rich leucite, diopside and apatite appear to be later-stage and limited to lining vugs. The rinkite was only identified as one very small grain. For the background justification in how the speciation of S in the sodalite group mineral (nosean) was estimated, see the more detailed discussion for lazurite in sample FKM-25. Note that for nosean, however, all S was presumed to S6+ with possibly some contribution from S4+. To aid in charge balance of excess SOx2-, nosean’s essential H2O was combined with the sulfur species to make neutral “acid” species (although the cages in nosean nominally contain Na4[SO4] and Na4[H2O]).
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-7
perovskite (Ca0.70Na0.10Ce0.10La0.04Nd0.03Pr0.01Sm0.01Sr~0.01)(Ti0.88Fe3+0.08Al0.02Nb0.01Si0.01)O3
fluorapatite (most F-rich) (Ca4.84Sr0.06Na0.04Ce0.03La0.02Nd0.01)[P2.91Si0.08S0.01O12](F0.85[OH]0.15)
fluorapatite (most OH-rich) (Ca4.90Sr0.03Na0.03Ce0.02La0.01)[P2.87Si0.11S0.01O12](F0.56[OH]0.44)
rinkite (Ca3.70Sr~0.14Ce0.06La0.03Nd0.01Pr0.01Y0.01){Na1.00(Ca1.59Na0.31Fe2+0.05Mn2+0.05)(F1.00O1.00)}
{(Ti0.92Zr0.07Nb0.06)[Si1.99Al0.01O7]2F2.00}
diopside (Ca0.90Na0.09Mn2+0.01)(Mg0.47Fe3+0.22Fe2+0.21Ti0.05Al0.03Mn2+0.01)[Si1.72Al0.28O6]
aegirine-augite
(most Ca-rich)
(Ca0.57Na0.41Mn2+0.01)(Fe3+0.40Fe2+0.31Mg0.22Mn2+0.04Ti0.01V0.01)[Si1.97Al0.03O6]
aegirine-augite
(most Na-rich)
(Na0.72Ca0.25Mn2+0.03)(Fe3+0.60Fe2+0.23Mg0.07Ti0.04Al0.02Mn2+0.02V0.02)[Si2.00O6]
sanidine
(core; most Na-rich)
(K0.79Na0.18Ba0.01Sr0.01)[Si2.95Al1.04Fe3+0.01O8]
sanidine
(mid crystal)
(K0.88Na0.09Ba0.02Sr0.02)[Si2.91Al1.06Fe3+0.02O8]
sanidine
(rim; most Ba-rich)
(K0.85Na0.11Ba0.04Sr0.01)[Si2.93Al1.06Fe3+0.01O8]
sanidine
(overgrowth; most Fe-rich)
(K0.90Na0.09)[Si2.97Al0.99Fe3+0.04O8]
nepheline (Na2.86K0.73Mn2+0.010.40)[Si4.44Al3.38O16]
leucite? (K0.40Na0.37Ca0.080.15)[Si1.74Al1.25O6]
nosean (Na5.80K0.19Sr0.01)[Si5.97Al5.99Fe3+0.04O24]
. (Na1.86Ca0.06Sr0.01)([SO4]2-?0.86?[H2SOx; x = 3, 4]0?0.35?Cl0.26F0.03)
analcime (Na0.89K0.02Ca0.010.08)[Si1.96Al1.03Fe3+0.01O6] . H2O
natrolite? ([Na2]0.35[K2]0.34Ca0.13)[Si2.96Al2.04O10] . ~2H2O

 



sample: FKM-8
locality: Franklin mining district, Sussex Co., NJ, USA.
rock type: high grade norbergite marble. Granulite facies metamorphosed siliceous dolostone.
major mineralogy: Abundant norbergite, partially altered (seemingly to serpentine ± brucite in the optical image, but not easily identified in BSE imaging), in carbonate (calcite > dolomite; very minor strontianite). Also present are scattered chondrodite (thus two different humite group minerals are present) separated from intergrown fluoro-pargasite by a thin rim of chlorite. Ba-rich phlogopite inclusions are present in the amphibole, and scattered tremolite (as a secondary mineral?) is also present. The norbergite and chondrodite normalizations include estimates of possible B present (0.03 wt% B and 0.16 wt% B, respectively). Although not verified by independent ICP-MS or SIMS analyses, these added low B estimates improve the T-site occupancy, are permissible based on studies of B incorporation into humite group minerals (Gerasimova et al., 2013 [← subscription required]; Woodford, 1995; Hinthorne and Ribbe, 1974), and are consistent with the notable B-enrichments observed in the Franklin area marbles (for example, see sample FKM-37 [with fluoborite] and sample FKM-184 [with warwickite]). Additional analytical notes: OH replacing O is calculated as equal to B apfu, according to the exchange vector [B(OH)]1[SiO]-1; O replacing (OH+F) is calculated as equal to 2*Ti apfu according to the exchange vector [TiO2]1[Mg(OH)2]-1 (akin to the “oxo” correction recommended for normalizing titaniferous amphiboles [Hawthorne et al., 2012: Appendix III]). These adjustments were undertaken to hopefully minimize the error introduced into the Fe3+/∑Fe calculation. Study of the light element content of these humite group minerals (and also possibly in any accompanying olivine) would be desirable. Samples FKM-36, FKM-112 and FKM-184 are additional examples of the high grade Franklin marble from the adjoining Sussex Co., NJ/Orange Co., NY area that contain humite group minerals.
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-8
norbergite (Mg2.97Fe2+0.02Ti0.01)[Si0.99B0.01O3.99(OH)0.01](F1.57[OH]0.42O0.01)
chondrodite (Mg3.84Fe2+0.10Fe3+0.05)[Si1.94B0.05O7.95(OH)0.05](F1.56[OH]0.44O0.01)
fluoro-pargasite (Na0.72K0.07Sr0.03Ca0.020.16)Ca2.00(Mg4.02Al0.75Fe3+0.08Ti0.06V0.05Cr0.01)
[Si5.96Al2.04O22](F1.08[OH]0.78O0.12Cl0.02)
tremolite (Na0.070.93)(Ca1.98Na0.02)(Mg4.88Fe2+0.08Fe3+0.02Al0.01)[Si7.91Al0.09O22]([OH]1.27F0.72)
phlogopite (K0.52Ba0.24Na0.10Ca0.010.13)(Mg2.90FeT0.07Al0.03Ti0.01)[Si2.84Al1.16O10]([OH]1.20F0.79)
clinochlore (Mg4.61Al1.14FeT0.160.09)[Si3.00Al1.00O10]([OH]7.52F0.48)

 



sample: FKM-9
locality: Bellerberg volcano, Ettringen, Mayan, Eifel, Rhineland-Palatinate, Germany.
rock type: metasomatized(?) sanidinite facies meta-calcareous ejectum.
major mineralogy: Gehlenite was reported, but not observed in this thin section. Optically- and BSE-zoned grossular garnet (variably enriched in Ti and Fe3+; some zones show significant schorlomite and andradite components) and abundant vuggy calcite occur in a non-descript, fine-grained matrix probably largely composed of carbonate and various calc-silicates.
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-9
grossular (core) (Ca2.94Mg0.04Fe2+0.02Mn2+0.01)(Al0.90Fe3+0.78Ti0.30V0.01)[Si2.71Fe3+0.29O12]
grossular (middle) (Ca2.93Mg0.06Fe2+0.01Mn2+0.01)(Al0.78Ti0.62Fe3+0.54Fe2+0.05V0.01)[Si2.43Fe3+0.57O12]
grossular (rim) (Ca2.93Mg0.03Fe2+0.01Mn2+0.01)(Al1.45Fe3+0.54Ti0.01)[Si2.97Fe3+0.03O12]

 



sample: FKM-10
locality: Crestmore quarries, Crestmore, Riverside Co., CA, USA.
rock type: high-grade monticellite marble. Contact aureole (high-T low-P; sanidinite(?) facies) metamorphosed siliceous dolostone, with possible additional metasomatic contribution from the causative intrusion.
major mineralogy: Monticellite in carbonate (calcite > dolomite). For comparison, another monticellite-bearing sample featured here, also from Crestmore, is FKM-179.
(left: unpolarized light; right: under crossed polars)

 



sample: FKM-11
locality: Fiskenæsset, Nuuk, Sermersooq, Greenland.
rock type: sapphirine-gedrite-phlogopite gneiss. Perhaps a granulite equivalent to a K-metasomatized cordierite-anthophyllite rock, or perhaps a granulite facies sepiolitic metapelite or argillic (montmorillonite+kaolinite±chlorite) alteration assemblage?
major mineralogy: Low-Fe sapphirine, “sodic-gedrite” (now disallowed as a name and roughly approximated by a composition along the gedrite-“rootname 1” join, according to the petrologically-inconsistent 2012 IMA update of amphibole nomenclature), pargasite, clinochlore and abundant phlogopite. For comparison, other sapphirine-bearing samples featured here include FKM-23 and FKM-28.
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-11
sapphirine (Mg3.71Fe2+0.28)(Al8.71Mg3.14Fe3+0.15)O4[Al8.87Si3.13O36]
gedrite-“rootname 1” join
(Hawthorne et al., 2012)
or “sodic-gedrite”
(Leake et al., 1997)
(Na0.580.42)(Mg1.36Fe2+0.51Ca0.13Mn2+0.01)(Mg3.69Al1.30)[Si6.16Al1.84O22]([OH]1.98F0.02)
pargasite (Na0.52K0.060.42)(Ca1.80Fe2+0.19Mn2+0.01)(Mg3.83Al0.98Fe2+0.18Ti0.01)
[Si6.42Al1.58P0.01O22]([OH]1.94F0.04Cl0.01O0.01)
phlogopite (K0.70Na0.15Ba0.010.14)(Mg2.48Al0.35FeT0.13Ti0.010.03)[Si2.80Al1.20O10]([OH]1.97F0.03Cl0.01)
clinochlore (Mg4.36Al1.37FeT0.190.08)[Si2.74Al1.26O10](OH)8.00

 



sample: FKM-12
locality: Nine Mile nine, Broken Hill district, Yancowinna Co., NSW, Australia.
rock type: gahnite-metaquartzite. Granulite facies possible Zn-rich siliceous volcanogenic meta-exhalite(?)
major mineralogy: Abundant gahnite, slightly altered to an unidentified low-Zn sheet silicate, with sparse ilmenite, in quartz. For comparison, another gahnite-bearing sample featured here (admixed with sulfides rather than quartz) is FKM-178. Additionally, several of the Franklin, NJ samples show a somewhat atypical orange to yellow gahnite (e.g. FKM-45 and FKM-48).
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-12
ilmenite (Fe2+0.92Zn0.05Mn2+0.02)Ti1.01O3
gahnite (Zn0.67Fe2+0.25Mg0.07)Al2.00O4

 



sample: FKM-13
locality: Merelani Hills, Lelatema Mtns., Simanjiro district, Manyara region, Tanzania.
rock type: zoisite-quartz “skarn”. Retrograde amphibolite facies overprint of a granulite facies calc-silicate; possibly a meta-marl, with accompanying metasomatism. Specimens such as this with coarse tanzanite (and those with coarse tsavorite garnet… see sample FKM-1, also from Merelani) are reported to occur in locally “low pressure” pockets and veins.
major mineralogy: Abundant quartz with subordinate V-bearing zoisite (“tanzanite”). Scattered diopside, fluorite, V-bearing grossular (particularly as inclusions in zoisite) and graphite are present. The sample is vuggy and heavily fractured, with abundant fine-grained alteration/cementation material (calcite ± halloysite?) filling fractures, along with patches of unidentified Fe-oxides, a separate Mn-oxide, and minor amounts of an Fe-rich sheet silicate that appears to normalize to an unusual (and perhaps new) Ca-dominant stilpnomelane.
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-13
grossular (most V-rich) (Ca2.85Mn2+0.08Mg0.06)(Al1.31V0.56Cr0.08Ti0.03Mg0.01)[Si3.02O12]
grossular (most Al-rich) (Ca2.88Mn2+0.07Mg0.04)(Al1.61V0.32Cr0.04Ti0.02Mg0.01)[Si3.02F0.01O11.99]
zoisite Ca0.98Ca1.00Al1.00Al1.00(Al0.93V0.06Cr0.01)O[Si2.00O7][Si1.00O4](OH)
diopside (Ca0.99Na0.01)(Mg0.95Al0.09V0.01)[Si1.97Al0.03O6]
“Ca-stilpnomelane” (Ca0.55Na0.05K0.020.37)(FeT6.92Mg0.32Zn0.09Ni0.05MnT0.01V0.010.60)
[Si11.50Al0.43Fe3+0.08O~30](OH)~6 . ~4.5H2O

 



sample: FKM-14
locality: Langesundfjorden, Larvik, Vestfold, Norway.
rock type: alkali feldspar syenite. Neither quartz nor any feldspathoid was observed in the thin section, so the sample appears to be critically silica-saturated (on the quartz/foid boundary).
major mineralogy: Coarse astrophyllite with scattered variable composition patches and swirls, locally sufficiently Nb-rich to be niobophyllite. Within the astrophyllite are scattered tiny grains of an unidentified Nb-rich mineral. The other abundant minerals in the thin section are aegirine, microcline and albite. One finely- and complexly-zoned Na+REE-rich apatite is present (this grain was initially misidentified as analcime… see note below). In what appears to be a more-erratically zoned overgrowth of apatite, abundant inclusions of substituted(?) “calciobritholite-(Ce)” are scattered along the apatite outer edges and along cracks. Measured Si+P alone fail to fill the “T” site, so an additional light “T” site cation such as boron (so grading compositionally towards tritomite-(Ce), which is reported from numerous Langesundfjorden localities), or possibly carbon, seems likely to be present; in this case, adding in 0.16 apfu estimated B simultaneously fills the “T” site, provides overall charge balance, and brings the analytical total to 100.03 wt%. Sparse scattered small in-fillings of what appear to be a zeolite (possibly tetranatrolite? [material seems too Ca-rich to be natrolite]) are also present. The characterization and analysis of this sample offered several good learning experiences. Astrophyllite group minerals can be challenging to analyze and normalize, in part due to potential vacancies, the possibility of interlayer H2O, variable transition metal valences, variable O for F+[OH] substitution in the φ anion site, and the likely occurrence of rare elements (e.g. Rb, Cs, Ta, Hf) that may be overlooked in routine analytical protocols. For the normalizations of the astrophyllite group mineral analyses presented here, a somewhat modified approach to that suggested in Piilonen et al., 2003 was adopted. Rather than an anion-based normalization, the normalization routine used here was based on ∑(T) = 8, where Si, B, Be, P, S, Ge, As and Al were considered to be the “T” site cations. The “D” site was subsequently populated with Nb, Ta, Zr, Hf and Sn, and then filled with Ti up to 2.00 apfu; excess Ti (typically less than 0.05 apfu) was carried over to the “C” site. Rb, Cs, Hf and Ta were not measured (to date… these may be evaluated later by LA-ICP-MS), but were presently estimated from a crude average of the Langesundfjorden samples described in Piilonen et al., 2003. As Piilonen et al., 2003 also observed with their analyses, the analytical totals reported here are similarly a bit low; these low totals were slightly improved by assuming that cation deficiencies in the “A” site were filled with interlayer H2O. An additional adjustment was made in the φ anion site occupancy based on the measured Nb+Ta content: [OH] is replaced by O = 0.5*[Nb+Ta] apfu to account for the charge imbalance resulting from the partial replacement of [Ti+Zr+Hf+Sn]4+ with [Nb+Ta]5+. Full replacement of [4+] cations with [5+] cations in the “D” site would additionally require the replacement of one [Fe2++Mn2+] with Na in the “C” site (akin to what is observed in magnesium-astrophyllite); hence, a properly charge-balanced niobophyllite end-member formula (no commas!) should be given as K2Na[Na(Fe2+)6]Nb2[Si8O26](OH)4O. Another promising outcome of this normalization routine is that after the addition of estimated Rb & Cs and the partial replacement of [OH] by O, the resultant calculated Fe3+/∑Fe ratio is in the range of 0.01 to 0.05, consistent with the Mössbauer-derived values measured for the Norwegian samples in Piilonen et al., 2003.
 
*Note: prior to examining this thin section on the microprobe, the apatite in this sample was originally thought to be analcime, and is misidentified as such in Isotropic Minerals in Thin Section; I regret the error, and offer a correction here. But there’s also a valuable lesson to be learned from this mistake. Experienced petrographers tend to identify many common minerals by sight, and may forego certain optical tests when an unknown mineral appears familiar. However, especially in atypical “exotic” rocks, relying on familiarity alone can sometimes lead to surprising misidentifications, as was the case here. Several atypical visual features of the unknown mineral helped contribute to its misidentification; indeed, for it to have been apatite rather than analcime, the single crystal in this sample would have had to have been both texturally and optically anomalous (and indeed it was!) First, that it is the only apatite in the thin section is perhaps unusual. It is also not euhedral, and is instead oddly interstitial between larger astrophyllite and microcline crystals. Optically it is essentially isotropic (probably primarily due to crystal orientation, although chemical substitutions also affect the refractive indices), but shows faint very low birefringence sectors (this effect is presumably due to its marked compositional zoning). It also shows moderate relief. All of these properties, along with the noted occurrence of analcime with astrophyllite at several Langesundfjorden localities, seemed indicative of “familiar” analcime. Critically, however, the nature of the moderate relief was not determined (for example, by observing the movement of the Becke line against the adjacent feldspar): analcime has moderate negative relief whereas apatite has moderate positive relief. That simple but commonly neglected test would have readily eliminated analcime as a possibility (although apatite would almost certainly not have been an alternative identification given its other anomalous properties… so ultimately this grain was destined for the microprobe!) It is quite likely that the significant Na+REE substitution for Ca in this apatite (along with the marked zoning) had a modifying effect on its physical and optical properties. But that realization only emphasizes an important and humbling lesson: especially in unusual rocks, a “familiar” mineral may not always be what it seems, and the effort of extra care is never for naught!
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-14
fluorapatite (low z) (Ca4.61Sr0.09Na0.06Y0.050.19)[P3.00O12]F1.07
fluorapatite (mod z) (Ca4.54Na0.15Y0.12Sr0.04Gd0.01[HREE]0.010.13)[P3.00O12](F0.91[OH]0.09)
fluorapatite (mod-high z) (Ca4.36Na0.29Y0.13Ce0.06Nd0.03[HREE]0.03Sr0.02La0.02Gd0.02Pr0.01Sm0.010.02)
[P3.00O12](F0.93[OH]0.07)
“calciobritholite-(Ce)” (Ca1.64Ce1.46Nd0.85La0.46Pr0.21Sm0.12Na0.12Gd0.04Y0.04Sr0.04[HREE]0.03)
[Si2.46P0.38B~0.16O12]([OH]0.55F0.44Cl0.01)
aegirine
(in astrophyllite)
(Na0.94Ca0.05)(Fe3+0.86Fe2+0.06Al0.05Ti0.02)[Si1.99Al0.01O6]
aegirine
(in microcline; main)
(Na0.91Ca0.08Mn2+0.01)(Fe3+0.81Fe2+0.10Al0.06Ti0.02Mg0.01)[Si1.99Al0.01O6]
aegirine
(in microcline; high z core)
(Na0.89Ca0.12)(Fe3+0.83Fe2+0.10Al0.02Ti0.02Mg0.01Mn2+0.01Zr0.01)[Si1.98Al0.02O6]
astrophyllite-rich
astrophyllite group ss
(main)
(K1.51Rb0.10?Na0.03Ba0.01Cs0.01?[H2O]0.32?)(Na0.87Ca0.13)
(Na0.36Fe2+4.00Mn2+2.32Fe3+0.22Ti0.05Mg0.03V0.01)(Ti1.21Nb0.77Zr0.01Sn0.01)
[Si7.81Al0.19O26](OH)4(F0.46O0.39[OH]0.15)
astrophyllite-rich
astrophyllite group ss
(low z swirls in main)
(K1.45Na0.35Ba0.01[H2O]0.19?)(Na0.96Ca0.04)
(Na0.04Fe2+5.53Mn2+1.29Mg0.06Ti0.04Fe3+0.02V0.01)(Ti1.86Nb0.12Zr0.01Sn0.01)
[Si7.99Al0.01O26](OH)4(F0.70[OH]0.23O0.06)
niobophyllite-rich
astrophyllite group ss
(high z swirls in main)
(K1.46Rb0.11?Na0.02Ba0.01Cs0.01?[H2O]0.39?)(Na0.99Ca0.01)
(Na0.48Fe2+3.69Mn2+2.57Fe3+0.17Mg0.05Ti0.03V0.02)(Nb1.26Ti0.74)
[Si7.97Al0.03O26](OH)4(O0.63F0.22[OH]0.15)
microcline (K0.97Na0.04)[Si2.99Al1.01O8]
albite Na1.02[Si2.99Al1.01O8]
tetranatrolite? ([Na2]0.80Ca0.31)[Al2.30Si2.70O10] . ~2.2H2O

 



sample: FKM-15
locality: Laytonville quarry, Laytonville, Mendocino, Co., CA, USA.
rock type: spessartine-stilpnomelane-metaquartzite. Blueschist facies ferruginous metaquartzite.
major mineralogy: Scattered stilpnomelane and abundant tiny spessartine, in quartz. This sample was originally reported to contain zussmanite, but it does not (however, see FKM-147 from the same locality, which does contain abundant zussmanite). Stilpnomelane is a challenging mineral to normalize, owing to possible vacancies in the A and M sites, likely Fe3+ and/or Mn3+ in the M site, and variable hydroxyl and excess water. For this sample, the stilpnomelane is normalized to three simplifying assumptions: (a) 20 T+M cations (permits no vacancies in the M site), (b) 6[OH] per 36 total [O+OH] (this conforms to a sheet silicate T4O10 skeleton and also yields a reasonable M3+/∑M [M = Fe+Mn] ≈ 0.32), and finally (c) 5 moles of excess H2O (which brings the overall total to almost exactly 100 wt%). Although this normalization results in a permissible formula, this material would benefit from additional characterization.
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-15
spessartine (Mn2+1.56Fe2+1.10Ca0.27Y0.03Mg0.02Na0.02)(Al1.93Fe3+0.02Mg0.02V0.01Ti0.01)[Si2.95Al0.03P0.02O12]
stilpnomelane (K0.32Ca0.08Na0.06Ba0.050.49)(FeT5.60MnT1.17Mg0.75Al0.39V0.06Cr0.01Ni0.01)[Si10.76Al1.24O~30](OH)~6 . ~5H2O

 



sample: FKM-16
locality: Tsitondroina, Ikalamavany district, Fianarantsoa province, Madagascar.
rock type: chrysoberyl “amphibolite”. This rock occurs as ~10 cm veins and is likely of metasomatic origin.
major mineralogy: Scattered chrysoberyl in a matrix of partially altered anorthite and amphibole (widely reported in the mineral collector community to be taramite, but in this thin section verified as fluorian pargasite by EPMA). For comparison, another chrysoberyl-bearing sample featured here is FKM-156, although as an Al-rich oxide in an amphibole+anorthite rock, this particular sample also bears some resemblance to the corundum-bearing amphibolite sample FKM-24.
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-16
chrysoberyl analysis pending
pargasite (Na0.74K0.17Ca0.020.07)Ca2.00(Mg4.06Al0.67Fe3+0.15Fe2+0.11)[Si6.27Al1.73O22]([OH]1.09F0.86Cl0.04)
anorthite (Ca0.98Na0.02)[Si2.00Al2.00O8]

 



sample: FKM-17
locality: Sengischorr Mountain, Lovozero massif, Kola Peninsula, Murmanskaja Oblast’, Russia.
rock type: nepheline syenite.
major mineralogy: specimen acquired for lamprophyllite. Based on petrography, in addition to lamprophyllite, abundant eudialyte, aegirine, alkali feldspar and nepheline are present. The nepheline shows alteration rims, presumably of cancrinite.
(left: unpolarized light; right: under crossed polars)

 



sample: FKM-18
locality: Pargas, Finland.
rock type: high grade meionite-diopside marble. The host rock is reported to be a “limestone” interbedded between a diopside amphibolite and a garnet-cordierite gneiss (see Chapter 2 [p. 54] in The Mines and Quarries of Finland [1954]). The characterization of the rock as a limestone coupled with the abundance of K and F indicates metamorphism was probably not entirely isochemical, and suggests the possibility of accompanying metasomatism.
major mineralogy: Abundant diopside, minor fluorite and meionitic scapolite (partially altered), in calcite. The sample was originally reported to contain pargasite, but this thin section contains no amphibole. However, pargasite and K- and F-rich pargasite family amphiboles do occur at Pargas (the type locality).
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-18
diopside (Ca0.99Na0.02)(Mg0.66Fe2+0.24Fe3+0.04Al0.04Ti0.01Mn2+0.01)[Si1.92Al0.08O5.99F0.01]
meionite (Ca2.52Na0.46K0.02Mg0.01)[Si6.45Al5.53Fe3+0.01O24] . Ca1.01(CO3)1.01

 



sample: FKM-19 (separate specimen from sample FKM-19b)
locality: Gjerdingselva, Nordmarka, Lunner, Oppland, Norway.
rock type: “elpidite ekerite” (soda-granite).
major mineralogy: Quartz and K-feldspar (perthitic and heavily altered), with riebeckite, aegirine, kupletskite and elpidite.
(left: unpolarized light; right: under crossed polars)

 



sample: FKM-19b (separate specimen from sample FKM-19)
locality: Gjerdingselva, Nordmarka, Lunner, Oppland, Norway.
rock type: “elpidite ekerite” (soda-granite).
major mineralogy: Quartz and K-feldspar (perthitic and heavily altered), with riebeckite, aegirine, kupletskite and elpidite.
(left: unpolarized light; right: under crossed polars)

 



sample: FKM-20
locality: Franklin mining district, Sussex Co., NJ, USA.
rock type: zincite-franklinite-willemite “marble”. Granulite facies Zn-Mn metasomatite.
major mineralogy: Classic Franklin-type assemblage of zincite, franklinite, and willemite (partially altered to gageite), all hosted in calcite.
(left: unpolarized light; right: under crossed polars)

 



sample: FKM-21
locality: Greenwood mine, Tuxedo, Orange Co., NY, USA.
rock type: K-Fe metasomatite associated with hydrothermal Fe-oxide mineralization. The occurrence of metasomatic K-Cl-bearing hastingsite (sensu lato) with magnetite and apatite is characteristic of the ore zone in many IOCG (iron-oxide-Cu-Au) systems (Mazdab, 2003).
major mineralogy: Calcic amphibole is the dominant mineral in this sample. It is reported as potassic-fluoro-hastingsite (Lupulescu et al., 2009), but in this thin section the examined amphibole was verified by EPMA as Cl-F-bearing potassic-magnesio-hastingsite. This amphibole is broadly similar in composition, and of similar occurrence, to the hastingsite in sample FKM-33. Magnetite, apatite and a plagioclase-quartz symplectite are also abundant. Subordinate orthoclase is also present, both as small masses intergrown with the symplectite, and as larger crystals of “perthite” (possibly relict from an earlier igneous or hydrothermal event). Additional minor minerals present include laths of ilmenite in the magnetite, abundant very tiny monazite inclusions in the apatite, and some cpx (patchy zoned augite to diopside) in the amphibole (relict, possibly from an earlier higher temperature hydrothermal event).
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-21
ilmenite (Fe2+0.96Mn2+0.03)Ti1.00O3
magnetite Fe2+1.00(Fe3+1.96Al0.02Ti0.01Fe2+0.01)O4
monazite-(Ce) (Ce0.49La0.34Nd0.07Pr0.04Ca0.03)[P1.00Si0.02O4]
fluorapatite (Ca4.92Ce0.02La0.01Nd0.01Y0.01Na0.01)[P2.98Si0.03O12](F0.68[OH]0.30Cl0.02)
diopside-rich cpx ss (Ca0.87Mg0.07Na0.04Mn2+0.01)(Mg0.48Fe2+0.43Fe3+0.07Al0.02)[Si1.94Al0.06O6]
augite-rich cpx ss (most Mg-rich) (Ca0.84Mg0.11Na0.04Mn2+0.01)(Fe2+0.46Mg0.46Fe3+0.05Al0.03)[Si1.96Al0.04O6]
augite-rich cpx ss (most Fe-rich) (Ca0.70Mg0.25Na0.03Mn2+0.02)(Fe2+0.58Mg0.32Fe3+0.06Al0.03)[Si1.94Al0.06O6]
potassic-magnesio-hastingsite (K0.42Na0.310.27)(Ca1.79Na0.21)(Mg1.89Fe2+1.66Fe3+1.08Al0.26Ti0.07Mn2+0.03)
[Si6.12Al1.87O22]([OH]1.01F0.53Cl0.32O0.15)
orthoclase (with symplectite) (K0.87Na0.10Ba0.01)[Si2.96Al1.03O8]
“perthite” (orthoclase host) (K0.88Na0.08)[Si2.98Al1.02O8]
“perthite” (“oligoclase” lamellae) (Na0.72Ca0.29K0.01)[Si2.67Al1.32Fe3+0.01O8]
“andesine” (symplectite) (Na0.69Ca0.31K0.01)[Si2.66Al1.34O8]

 



sample: FKM-22
locality: Cascade Canyon, San Gabriel Mtns., Los Angeles Co., CA, USA.
rock type: test.
major mineralogy: specimen acquired for corundum.
(left: unpolarized light; right: under crossed polars)

 



sample: FKM-23 (billet from Univ. Arizona petrology collection, courtesy of J. Ganguly)
locality: Anantagiri, Eastern Ghats belt, Anantagiri district, Andhra Pradesh, India.
rock type: sapphirine-opx-garnet-cordierite gneiss. Granulite facies (sapphirine zone) metapelite.
major mineralogy: Sapphirine (mostly mantled by sillimanite coronas), with cordierite, almandine, orthopyroxene, as well as abundant quartz, feldspar (both plagioclase and orthoclase), scattered rutile (also with ~748 ppm Cr) and hercynite, and minor monazite. For added interest, a number of the included analyses represent mineral pairs (where the measurements were taken adjacent to other minerals). Additional sapphirine-bearing samples featured here include FKM-11 and FKM-28, for comparison.
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-23
spinel (enclosed in garnet) (Mg0.56Fe2+0.41Zn0.02)(Al1.92Fe3+0.06Cr0.01)O4
hercynite (rimmed by sillimanite) (Fe2+0.56Mg0.41Zn0.02)(Al1.89Fe3+0.08Cr0.02)O4
hercynite (adjacent to sapphirine) (Fe2+0.54Mg0.44Zn0.02)(Al1.92Fe3+0.06Cr0.02)O4
ilmenite (Fe2+0.89Mg0.09Fe3+0.01)(Ti0.99Fe3+0.01)O3
rutile Ti0.99O2
monazite-(Ce) (Ce0.48La0.23Nd0.14Pr0.05Th0.04Ca0.03Sm0.01)[P0.98Si0.01O4]
pyrope (adjacent to spinel) (Mg1.46Fe2+1.41Ca0.09Mn2+0.04)(Al1.92Fe3+0.08)[Si3.00O12]
almandine (adjacent to biotite) (Fe2+1.51Mg1.35Ca0.10Mn2+0.04)(Al1.94Fe3+0.06)[Si3.00O12]
sillimanite (Al1.98Fe3+0.01)O[Si0.98Al0.02O4]
cordierite (adjacent to sapphirine) (Mg1.67Fe2+0.27Fe3+0.06)(Al2.98Fe3+0.02)[Al1.08Si4.92O18] . Na0.01
“hypersthene” (Mg0.99Mn2+0.01)(Fe2+0.61Mg0.19Al0.14Fe3+0.06Ti0.01)[Si1.79Al0.21O6]
sapphirine (adjacent to hercynite) (Mg2.23Fe2+1.74Mn2+0.01Ni0.01Ca0.01Na0.01)(Al8.36Mg3.04Fe3+0.52Cr0.05V0.02Ti0.01)O4[Al8.96Si3.04O36]
sapphirine (adjacent to cordierite) (Mg2.21Fe2+1.77Mn2+0.01Ni0.01)(Al8.40Mg3.04Fe3+0.43Cr0.09V0.02Ti0.02)O4[Al8.97Si3.03O36]
sapphirine (adjacent to biotite) (Mg2.64Fe2+1.33Mn2+0.01Ni0.01K0.01)(Al8.42Mg3.11Fe3+0.41Cr0.03Ti0.02V0.01)O4[Al8.90Si3.10O36]
phlogopite (adjacent to garnet+sapphirine) (K0.93Na0.010.06)(Mg2.20FeT0.41Ti0.27Al0.11)[Si2.82Al1.18O10]([OH]1.02O0.55F0.42Cl0.01)
phlogopite (most Fe-rich) (K0.940.06)(Mg1.90FeT0.63Ti0.26Al0.18Cr0.01V0.010.01)
[Si2.82Al1.18O10]([OH]1.14O0.52F0.32Cl0.02)
orthoclase (K0.94Na0.02Ca0.01Sr0.01)[Si2.94Al1.05P0.01O8]
“andesine” (Na0.64Ca0.33K0.03)[Si2.62Al1.37O8]

 



sample: FKM-24
locality: Fotodrevo area, Ejeda commune, Ampanihy district, Tuléar province, Madagascar.
rock type: corundum amphibolite. Granulite facies amphibolite (estimated at below ~900° C and below ~10 kbars, according to experimental phase relations; estimated at 820°C and 7.1 kbars [hence consistent with the phase equilibria] using the edenite-richterite formulation of Holland and Blundy, 1994 [run at several of the highest permitted An content feldspar compositions and then further extrapolated to An0.94 and An0.95] coupled with Anderson et al., 2008… the alternative edenite-tremolite formulation gives 935°C and 2.1 kbars [a bit hot to be consistent with the phase equilibria]; lastly, estimated at 970° C and 3 kbars from the single-mineral amphibole thermobarometry equations of Ridolfi et al., 2010 [but this thermobarometer seems to commonly yield higher T & P estimates relative to that obtained from most amphibole-plagioclase pairs, and in this case is also too hot to be consistent with the phase equilibria]). This sample is essentially a lower pressure version of sample FKM-68. Another also similar corundum-bearing amphibolite is sample FKM-158, but that rock contains abundant garnet and its plagioclase is much more Na-rich (oligoclase). This sample also seems to have a higher bulk Mg/(Mg+Fe) ratio than FKM-158.
major mineralogy: Corundum (in the hand sample, but not in this thin section) in a matrix of tschermakite (near the tschermakite/ferri-tschermakite composition boundary) and anorthite. Minor alteration chlorite also occurs in the sample.
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-24
tschermakite (Na0.18K0.030.79)(Ca1.67Na0.33)(Mg2.97Al1.04Fe3+0.94Cr0.02Mn3+0.02Ti0.02)[Si6.07Al1.93O22]([OH]1.97O0.03)
anorthite (Ca0.95Na0.05Sr0.01)[Si2.03Al1.97O8]
clinochlore (Mg3.82Al1.41FeT0.65Ni0.010.11)[Si2.66Al1.34O10](OH)8.00

 



sample: FKM-25
locality: Sar-e-Sang, Koksha valley, Badakhshan province, Afghanistan.
rock type: phlogopite-forsterite-diopside-lazurite marble. Upper amphibolite to granulite facies calcareous meta-evaporite, almost certainly with superimposed autologous(?) metasomatism.
major mineralogy: The rock is largely calcite with abundant silicate-rich bands of lazurite, forsterite (bright orange CL), diopside (dark blue CL), phlogopite, nepheline and pyrite. The thin section was cut too thin, and the observed birefringence values of the anisotropic minerals are much lower than what one is accustomed to seeing. Carbonate-hydroxylapatite is also present; the significant carbonate component is inferred and estimated from the low analytical total and from the necessity to charge balance the substantial Na content. Interestingly, despite the notable Cl content of the lazurite and the occurrence of Cl-essential sodalite and marialite in associated parageneses from Sar-e-Sang (for example, see sample FKM-155), neither the apatite nor the mica in this particular sample show significant Cl contents. Compared to this sample, sample FKM-155 may represent an evaporite facies depositionally-related to the carbonate-dominated facies represented here.
 
*Note: the proper characterization of “lazurite” (sensu lato) is complicated by several factors. The material occurs in several polymorphs that are probably not chemically distinguishable (or at least not readily so). The material in this sample appears to be an intergrowth of isotropic and anisotopic phases, and thus two or more polymorphs may be present (and are not attempted to be differentiated here, at present). Another complicating factor is that the speciation of S in lazurite is reportedly complex and still controversial. The deep blue color in lazurite (and synthetic “ultramarine”) has been attributed to the [S3]2- polysulfide species and it is presumably present here. Tauson et al., 2012 [← subscription required] observe for a set of Russian lazurite samples that in many instances, treating S as simply any combination of sulfate and/or monosulfide markedly exceeds the net negative charge necessary to charge balance the formula, and that is the case here as well. Using X-ray photoelectron spectroscopy (XPS), they determined that a variety of other S-bearing species are typically present, including [S3]2- polysulfide (to concentrations over 0.2 apfu), thiosulfate (to over 0.6 apfu), sulfite (to over 1.2 apfu), and even S0 (to ~0.3 apfu); all of these species except for sulfite would help to reduce the charge balance deficit. Nonetheless, [SO4]2- is generally considered the dominant S-species in lazurite (suggesting it may simply be a sulfide-rich variety of haüyne), and indeed in a paper by Hettmann et al., 2012, they report that essentially all of the S in a measured Afghani lazurite is present as [SO4]2-. An alternative approach to reducing the charge deficit is to allow for possible unanalyzed positive charge such as the presence of hydrogen. Water is reported in some analyses of lazurite (here and here, for example), and the possibility of species such as [HSO4] or [H2SO4]0 in lieu of or in addition to [SO4]2- seems plausible. For the purpose of this normalization, a combination of both approaches is used. Without the addition of hydrogen, achieving charge balance would require either assigning essentially all S to [S3]2-, or alternatively treating a significant portion (>1.1 apfu) of the S as S0… neither scenario seems consistent with published data. Ultimately, even with care, analyzing Na-rich feldspathoids in general can be challenging due to potential alkali migration and the tendency for samples to be damaged by even a defocused and rastered beam; hence, this reported analysis should not be considered evidence for either the purported abundance of particular S-bearing species, or the presence of hydrogen. Rather, this composition is simply put forth as an attempt to present a plausible charge-balanced formula. Although the characteristics of Sar-e-Sang lazurite (sensu lato) are certainly well-documented in the literature, further study of both the polymorphs present and the details of their compositions can always be worthwhile.
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-25
pyrite analysis pending
calcite analysis pending
carbonate-hydroxylapatite
(most Na+OH-rich)
(Ca4.70Na0.28)([P2.78S0.01Si0.01O~11.20][CO3]~0.21)([OH]0.58F0.33Cl0.09)
carbonate-hydroxylapatite
(most Ca+F-rich)
(Ca4.77Na0.22)([P2.83S0.01O~11.36][CO3]~0.16)([OH]0.50F0.44Cl0.07)
forsterite Mg1.00Mg1.00[Si1.00O4]
diopside (Ca0.96Na0.03Mg0.01)(Mg0.96Al0.03)[Si1.99O6]
phlogopite (K0.94Na0.01Ca0.010.06)(Mg2.81Al0.09Ti0.080.02)[Si2.96Al1.04O10]([OH]1.54F0.29Cl0.17)
nepheline (Na0.75K0.24)[Si1.00Al1.00O4]
lazurite (see text) (Na5.71K0.260.03)[Si6.03Al5.97O24] . (Na0.58Ca0.62)([S2O3]2-?0.62?[H2SOx; x = 3, 4]0?0.47?[S3?]2-0.21?Cl0.17)

 



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