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). 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). 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.