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sample: FKM-351
locality: Farbište, Poniky, Banská Bystrica region, Slovakia.
rock type: oxidation zone of a small rhyolite-/rhyolitic-tuff-hosted As-rich copper occurrence.
major mineralogy: The specimen was acquired for tyrolite and azurite. A variety of other rare arsenates may also be present.

accompanying videos: Short videos featuring the mineral associations and optical properties of the azurite in this thin section offer a more detailed look at this sample.

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sample: FKM-352
locality: Taseq slope, Ilímaussaq complex, Narsaq, Kujalleq, Greenland.
rock type: test.
major mineralogy: The specimen was acquired for tugtupite.

accompanying videos: Short videos featuring the mineral associations and optical properties of the tugtupite and ussingite in this thin section offer a more detailed look at this sample.

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sample: FKM-353
locality: Långban, Filipstad, Värmland, Sweden.
rock type: test.
major mineralogy: The specimen was acquired for manganberzeliite.

accompanying videos: Short videos featuring the mineral associations and optical properties of the caryinite in this thin section offer a more detailed look at this sample.

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sample: FKM-354
locality: according to Hatipoğlu, 2016, the primary source of gemmy purple jadeitite (“turkiyenite”) occurs a few km SE of Akpınar, in the region of Harmancık, Tavşanlı Zone, Bursa Province, Turkey.
rock type: test.
major mineralogy: The specimen was acquired for manganian jadeite, orthoclase, quartz, epidote, chloritoid and phlogopite.

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sample: FKM-355
locality: Fuka mine, Fuka, Bicchi-cho, Takahashi City, Okayama prefecture, Japan.
rock type: test.
major mineralogy: The specimen was acquired for morimotoite.

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sample: FKM-356
locality: So, the labeling of this sample might be a little ambiguous, or perhaps more likely, there are some inconsistencies with the locality hierarchy on mindat. The label states “Twin Lakes limestone deposit, Lakeshore, Fresno Co., CA”, which would appear to refer to this locality: Twin Lakes limestone deposit, Lower Twin Lake, Twin Lakes, Fresno Co., California, USA. But on mindat there’s a second geologically similar locality just 1 km to the east, with this notably different hierarchy: unnamed skarn occurrence [1], Lakeshore, Fresno Co., California, USA. So the first locality includes “Twin Lakes” in multiple places in the name and hierarchy but no “Lakeshore”, while the second locality includes “Lakeshore” but no reference to “Twin Lakes”. My suspicion is that by name alone, the first one is the correct locality; however, the second one has way more photos on mindat, suggesting it might be the more popular collecting locality. Nonetheless, the mineralogy seems a bit more consistent with the former locality (for example, the color of the spinel). To further complicate matters, one collector’s label of spinel + phlogopite from the area states: “Locality #2, Twin Lakes area, Fresno Co., CA”. So there may really be a bunch of little occurrences of high-grade magnesian marble in the area, and which particular one my sample is from may never be unequivocally known.
rock type: test.
major mineralogy: The specimen was acquired for hydroxylclinohumite and geikielite.

accompanying videos: Short videos featuring the mineral associations and optical properties of the forsterite and hydroxylclinohumite in this thin section offer a more detailed look at this sample.

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sample: FKM-357
locality: Big Creek, Rush Creek deposit, Big Creek-Rush Creek Mining District, Fresno Co., California, USA.
rock type: test.
major mineralogy: The specimen was acquired for fencooperite and titantaramellite.

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sample: FKM-358
locality: Gozaisho mine, Iwaki City, Fukushima Prefecture, Japan.
rock type: test.
major mineralogy: The specimen was acquired for nambulite, iwakiite and rhodonite.

accompanying videos: Short videos featuring the mineral associations and optical properties of the nambulite and albite in this thin section offer a more detailed look at this sample.

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sample: FKM-359
locality: Allt a’Mhuillin, Loch Borralan, Assynt, NW Highlands, Scotland, UK.
rock type: “borolanite”, a local name for a nepheline syenite containing nepheline (and/or its alteration products), melanite garnet, orthoclase and biotite.
major mineralogy: The specimen was acquired for vishnevite, orthoclase, nepheline, melanite garnet and biotite.

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sample: FKM-360
locality: unnamed mountain “rosalinda” occurrence, near Tambo Colorado archaeological site, Pisco province, Ica, Peru.
rock type: “Rosalinda”, a gem trade name for a presumably metasomatic rock consisting of scapolite and pink epidote.
major mineralogy: The specimen was acquired for marialite, calcite and a pink Mn-enriched member of the epidote group (either piemontite or manganian epidote).

accompanying videos: Short videos featuring the mineral associations and optical properties of the clinozoisite and marialite in this thin section offer a more detailed look at this sample.

sample: FKM-361
locality: Liebenbergite slag dump, Agios Konstantinos, Lavrion slag localities, Lavreotiki, East Attica, Attica, Greece.
rock type: metallurgical slag of anthropogenic origin.
major mineralogy: The specimen was acquired for liebenbergite, schäferite with likely additional vanadate garnet molecules present, trevorite, bannermanite (or a similar “vanadium bronze”), and a whitlockite group phosphate.

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sample: FKM-362
locality: Although reported by the seller as being from the “Olekminskiy district, Yakutia, Russia”, the actual locality is probably better represented as the non-hierarchical Kedrovyi alkaline massif, Murunskii Massif, confluence of the Chara and Tokko rivers, Aldan Shield, Russia, a portion of the greater Murun Alkaline Complex near the boundary between the Olekminskiy district in the Sakha Republic (Yakutia) and the Bodaybinsky District in Irkutsk Oblast.
rock type: the rock has characteristics of a K-fenite; it is marketed under the trade name “dianite”.
major mineralogy: The specimen was acquired for blue strontium-bearing potassic-richterite. Another sample from the same locality (or presumably at least from a nearby locality) is FKM-363 (below).

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sample: FKM-363 (dealer sample number 6913)
locality: Unlike the previous ostensibly similar sample offered by a lapidary dealer, the label from this mineral specimen dealer explicitly locates this sample from the Kedrovyi alkaline massif.
rock type: test.
major mineralogy: The specimen was acquired for blue strontium-bearing potassic-richterite. Another sample from the same locality (or presumably at least from a nearby locality) is FKM-362 (previous).

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sample: FKM-364 (dealer sample number 7278)
locality: Kovdor Zheleznyi Mine (Iron Mine), Kovdor Massif, Murmansk Oblast, Russia.
rock type: carbonatite-associated phlogopite-dominated metasomatite?
major mineralogy: The specimen was acquired for clinohumite, forsterite and magnetite.

accompanying videos: Short videos featuring the mineral associations and optical properties of the clinohumite in this thin section offer a more detailed look at this sample.

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sample: FKM-365 (dealer sample number 7930)
locality: From an unspecified outcrop of the Dora Maira coesite-bearing unit, Cuneo Province, Piedmont, Italy.
rock type: eclogite facies pyrope whiteschist.
major mineralogy: The specimen was acquired for pyrope, with inclusions of relict coesite. For other samples also from Dora Maira massif, see also FKM-42 and FKM-258.

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sample: FKM-366 (dealer sample number 2278)
locality: From an unspecified locality in the northern Ladoga Region, Republic of Karelia, Russia.
rock type: high grade marble.
major mineralogy: The specimen was acquired for fluoro-edenite.

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sample: FKM-367 (dealer sample number 7787)
locality: Tazheranskii Massif, Lake Baikal area, Irkutskaya Oblast’, Prebaikalia, Eastern-Siberian Region, Russia.
rock type: high grade calcareous skarn xenoliths in alkali syenite.
major mineralogy: The specimen was acquired for kirschsteinite. For another sample also from the Tazheran massif, see also FKM-283.

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sample: FKM-368 (dealer sample number 7003)
locality: Östanmossa mine, Röberg ore field, Norberg, Västmanland County, Sweden.
rock type: test.
major mineralogy: The specimen was acquired for dollaseite-(Ce). Somewhat similar material, from the Malmkärra mine, is represented by sample FKM-370, as well as by sample FKM-188 from an unspecified mine in the area.

accompanying videos: Short videos featuring the mineral associations and optical properties of the dollaseite-(Ce) and tremolite in this thin section offer a more detailed look at this sample.

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sample: FKM-369 (dealer sample number 7769)
locality: Dealer label only goes to the level of detail of “Slyudyanka, Lake Baikal area, Irkutskaya Oblast’, Prebaikalia (Pribaikal’e), Eastern-Siberian Region, Russia”, but the sample likely more specifically comes from one of the local marble quarries where Cr-enriched minerals have been explicitly documented, such as the Pereval marble quarry or the Buruntui quarry (no mindat link).
rock type: marble.
major mineralogy: The specimen was acquired for tremolite (possibly Cr-bearing and/or V-bearing).

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sample: FKM-370 (dealer sample number 7948)
locality: Malmkärra Mine, Norberg, Västmanland County, Sweden.
rock type: test.
major mineralogy: The specimen was acquired for fluorbritholite-(Ce), västmanlandite-(Ce) and allanite-(Ce). Somewhat similar material, from the Östanmossa mine, is represented by sample FKM-368, as well as by sample FKM-188 from an unspecified mine in the area.

accompanying videos: Short videos featuring the mineral associations and optical properties of the gadolinite-(Ce), fluorbritholite-(Ce), “ferridissakisite-(Ce)”-dollaseite-(Ce) ss and västmanlandite-(Ce) in this thin section offer a more detailed look at this sample.

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sample: FKM-371 (dealer sample number 7688)
locality: Tayezhnoe Fe-B skarn (Tayozhnoye), Aldan, Sakha Republic (Yakutia), Russia.
rock type: test.
major mineralogy: The specimen was acquired for serendibite.

accompanying videos: Short videos featuring the mineral associations and optical properties of the tourmaline and serendibite in this thin section offer a more detailed look at this sample.

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sample: FKM-372 (dealer sample number 7723)
locality: Dealer label only goes to the level of detail of “Vielsalm, Stavelot Massif, Belgium”, but it’s likely the sample more specifically comes from Salmchâteau.
rock type: test.
major mineralogy: The specimen was acquired for ardennite-(As) and quartz. For another sample also from the Salmchâteau region, see also FKM-336.

accompanying videos: Short videos featuring the mineral associations and optical properties of the ardennite-(As) in this thin section offer a more detailed look at this sample. Notably for this mineral, these videos may help clear up some inconsistencies in the optical properties, as described below.

Older optical data for ardennite-(As), for example that compiled and reported in the Handbook of Mineralogy, are incomplete and are contradictory to some more recent observations. In the older work(s), nα and nγ have not been determined, nβ is reported as a range from 1.74 to 1.78, birefringence is reported as a range from 0.15 to 0.20 (note that this range corresponds to a very high order creamy white interference color for a 30 μm sample thickness), and the 2Vz° is reported as a range from 0° to 70° (hence B(+)). These incomplete data and large ranges of measured and derived optical properties for this mineral, first described in the early 1870s, indicate that the optical properties of ardennite-(As) are poorly understood.

Some newer published data are available, but these are not without problems. Bermanec et al., 2021 characterized ardennite-(As) from Nežilovo, North Macedonia, but report unrealistically low R.I. values of nα = 1.537(2) and nβ = 1.579(1). These values are almost certainly erroneous, as they would require ardennite-(As) to have low relief in thin section (indeed, low negative relief in one orientation!) when the X-Y optic plane is parallel to the microscope stage; this is clearly not the case as the mineral shows high positive relief in all orientations (see PPL videos below). Bermanec et al., 2021 also report δ = 0.0181(2), corresponding to a 1st order reddish-purple interference color for a 30 μm sample thickness, but note that their reported δ value would not derive from their measured nγ – nα data: (1.741 – 1.537 = 0.204). Rounding out their quantitative optical data, Bermanec et al., 2021 also report 2Vx° = 49(1)° for their Nežilovo ardennite-(As); hence, their sample would appear to be B(-).

Given the preceding issues with some of both the older and newer published data, the accompanying videos are used to assess the veracity of these published values, and to estimate values for optical parameters that are missing or contradictory. From the accompanying XP video below (and also assessing the other crystals in the thin section), the maximum interference color observed for this Belgian ardennite-(As) is 1st order orange-red; this corresponds to δ = ~0.015(1), which is comparable to data of Bermanec et al., 2021 above for their North Macedonian material. From the accompanying optic axis figure video below, the Belgian ardennite-(As) from sample FKM-371 is B(+) with 2Vz° = ~30(5)°. This is in contrast to the results of North Macedonian material from Bermanec et al., 2021, but compares more favorably with the data reported for Belgian material in the older literature. Of course, there is considerable compositional variability in ardennite-(As), so some variability in optical properties should be expected. From the new δ and 2V° data, estimated values of nα and nγ can be calculated based on the older literature value of nβ = 1.76 (for simplicity, the midpoint of the reported range). These calculated estimates are: nα = ~1.759 and nγ = ~1.774. The estimated nγ value is broadly comparable to the nγ = 1.741(1) of Bermanec et al., 2021, and also a good match to the nγ = 1.775(5) measured for a related mineral, ardennite-(V), by Barresi et al., 2007 from a sample from Italy. Barresi et al., 2007 also report nα > 1.765(5) for the same Italian ardennite-(V), a value also not too dissimilar from that estimated here for the Belgian ardennite-(As).

Both the older and newer literature make additional semi-quantitative observations about other optical properties such as pleochroism and dispersion. From the accompanying PPL videos below (especially the polarizer rotation video, where the eye is not distracted by the rotation of the stage), pleochroism is observed to be weak at best, in tones of medium brownish-yellow. This observation is consistent with those of both Bermanec et al., 2021 and Barresi et al., 2007 for their materials, but notably differs from what is reported in Handbook of Mineralogy. From the accompanying optic axis figure video, dispersion is very weak at best, and if recognizable, appears to be r > v. This is consistent with the observations of Barresi et al., 2007 for their Italian ardennite-(V), in which they did not observe dispersion. However, Bermanec et al., 2021 reported strong v > r dispersion in their North Macedonian ardennite-(As). As with other optical properties that are composition dependent, variations in the degree and nature of dispersion for different compositions should not be unexpected.

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sample: FKM-373 (dealer sample number 3072)
locality: Saranovskii mine, Saranovskaya village, Permskaya Oblast’ (middle Ural Mtns. region), Russia.
rock type: metasomatized chromatite.
major mineralogy: The specimen was acquired for clinochlore and Cr-bearing titanite. For other samples also from Saranovskii mine, see also FKM-149, FKM-215, and FKM-316.

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sample: FKM-374 (dealer sample number 7355)
locality: Dyadina Gora, Dyadino lake, Tedino, Loukhsky District, Republic of Karelia, Russia.
rock type: high grade aluminous amphibolite.
major mineralogy: The specimen was acquired for gedrite; magnesiohögbomite, cordierite, sapphirine and other minerals may be present.

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sample: FKM-375
locality: sub-locality #1, at Mautia Hill, Kongwa, Kongwa District, Dodoma Region, Tanzania.
rock type: whiteschist-associated UHP metamorphic rock.
major mineralogy: The specimen was acquired for magnesio-hornblende (pink in hand sample), gedrite (yellow in hand-sample), yoderite, talc and quartz.

accompanying videos: Short videos featuring the mineral associations and optical properties of the yoderite in this thin section offer a more detailed look at this sample.

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