samples FKM-251 to FKM-275


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.

Note: Depending on the speed of your internet connection, it could take a minute or two for the images to load.


sample: FKM-251 (billet from the Univ. Arizona petrology collection, sample Wards 32)
locality: Mont Saint-Gregoire (Mont Johnson; Monoir), Le Haut-Richelieu RCM, Montérégie, Québec, Canada.
rock type: essexite.
major mineralogy: specimen acquired for nepheline.
(left: unpolarized light; right: under crossed polars)


sample: FKM-252
locality: reportedly from “Kiev, Ukraine”. However, this locality is not entirely consistent with the sample’s purported combination of minerals; here an overview of the geology of Ukraine is instructive. Devonian and Permian evaporites in Ukraine form a belt in the east of the country, and Jurassic and Miocene evaporites form a sub-parallel belt in the west of the country, as well as a smaller exposure in the south near Kerch, Crimea (Hryniv et al., 2007) [← subscription required]. In contrast, running up through the center of the country (including in the vicinity of Kiev) is the 3.0 Ga Golovanevsk suture zone, which hosts a number of mafic-ultramafic bodies that also include chromatites (Gornostayev et al., 2004) [← subscription required]. So this being a chromite-bearing sample from the Kiev vicinity is certainly plausible. However, the possibility of boracite in this samples seems extremely unlikely.
rock type: apparently a chromatite clot in dunite.
major mineralogy: This specimen was labeled as “boracite with chromite”, and although the association of a chloro-borate typical of a sedimentary evaporite with a Cr-bearing spinel typical of an ultramafic igneous rock seems suspect, I was intrigued enough to purchase the sample (thinking, for example, that an ultramafic body could have intruded an evaporite). Although the billet is currently still out for thin section preparation, I prepared a small epoxy mount of the material to examine in advance. EPMA of this epoxy mount verifies abundant magnesiochromite and forsterite, with small amounts of Cr-enriched diopside. There is a widespread very low z Mg-silicate phase veining the olivine that seems inconsistent with either expected talc or serpentine (with an analytical total only 72 wt% and molar Mg:Si ≈ 11:4), so there is still some intrigue associated with this sample that will have to wait for resolution until the thin section comes back.
(left: unpolarized light; right: under crossed polars)

mineral representative mineral compositions in FKM-252
magnesiochromite (Mg0.66Fe2+0.33Mn2+0.01)(Cr1.51Al0.42Fe3+0.05Fe2+0.01)O4
forsterite Mg1.00(Mg0.90Fe2+0.08Ni0.01)[Si1.00O4]
diopside (Ca0.98Na0.01Mg0.01)(Mg0.94Fe2+0.02Cr0.02Al0.01Fe3+0.01)[Si1.97Al0.03O6]
low z Mg silicate? analysis pending


sample: FKM-253
locality: Darai-Pioz glacier, Alai range, Tien Shan Mtns., Tajikistan.
rock type: highly-evolved B-rich alkali granite.
major mineralogy: specimen acquired for stillwellite-(Ce).
(left: unpolarized light; right: under crossed polars)


sample: K-65 (covered and yellowing thin section from Univ. Arizona petrology collection [unfortunately, the original billet & hand sample appear to be lost], Krantz collection, sample K-65)
locality: Kaltes Tal, Bad Harzburg, Harz Mtn. region, Niedersachsen, Germany.
rock type: mica peridotite (as small lenses in norite). Also known locally as “glimmerite”.
major mineralogy: Based on petrography, predominately forsterite and red Ti-rich(?) biotite, with gray spinel and minor oxide (believed to be ilmenite).
(left: unpolarized light; right: under crossed polars)


previous page (samples FKM-226 to FKM-250)

This “add photo(s)” image is just a placeholder in advance of adding new samples, so check back periodically to see my progress.