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LocalitiesLapanouse-de-Sévérac slag locality, Sévérac-d'Aveyron, Rodez, Aveyron, Occitanie, France

6th Jan 2017 12:43 UTCŁukasz Kruszewski Expert

My current research (with P. Gatel & V. Thierry, just to be published = under way) shows, that most of the rocks from this locality are not slags but parabasalts. A typical slag formed due to natural fossil fires, like those at L. d S., is exclusively a rock formed due to the calcination of carbonate rocks (usually carbonate concretions).

6th Jan 2017 13:47 UTCUwe Kolitsch Manager

The working name for such rocks is, according to the IUGS-SSMR (SSMR = Subcommission on the Systematics of Metamorphic Rocks), "Burnt rocks".

The products of combustion metamorphism have received different names in the past, for example, thermantide, thermantide porcellanite, fused shale, porcellanite, porcelain jasper. However, most of these names have also been used for the products of contact metamorphism in volcanic or near-surface settings. Thus the SG decided to find a general name for all rocks generated by combustion metamorphism and an agreement was found for the term burned (or burnt) rock, which has been defined as follows: Burned rock: General term for a compact, vesicular or clinkery, glassy to holocrystalline metamorphic rock of various colours, produced by the combustion metamorphism of pre-existing sedimentary rocks. In the fused varieties the glass coexists with refractory grains and/or newly formed minerals (e.g.melilite, wollastonite, mullite, cristobalite, spinel), whose nature reflects both the very high temperature metamorphic conditions and the variable chemical composition of the original rock. The term burned rock supersedes such old names as thermantide, thermantide porcellanite, fused shale, porcellanite, porcelain jasper. The glassy or glass-bearing varieties of burned rocks are called buchite (coal-fire buchite) or fritted rock respectively. The term burned rocks also includes some typical ash deposits of refractory material remaining after he combustion of coal seams, giving rise to soft, clay-like rocks resembling volcanic cinerites, and for which the SG proposes the name of coal-fire ashto distinguish it from ash deposits of volcanic origin.

http://www.bgs.ac.uk/SCMR/docs/papers/paper_10.pdf

See also

Grapes, R. (2006): Pyrometamorphism. Springer, Berlin, 276 pp.

7th Jan 2017 11:47 UTCŁukasz Kruszewski Expert

Thanks Uwe, this is v. useful. I usually refer to the above "Pyrometamorphism" book of Grapes (2006); he gives numerous examples of old and new names, lots of very detailed data on the mineralogy of various pm rocks found mostly in the natural environments (incl. the very first buchites from Eifel). Considering this book, the main pm rock types are parabasalts (also called "paralavas"), clinkers (usually formed from organic-rich shales; those seem to include porcellanites), buchites (incl. "obsidianic" buchites), and slags. Another good position is the "Combusion Metamorphism" by Sokol et al. (2005).

I often find a convertive use of various rock-type names used for the same lithologies; buchites are usually vesicular (but may resemble obsidiane, too), formed from sandstones and similar rocks, may be rounded, pumice-like, and their colour is quite characteristic, as it is often slighly violet-black to violet gray (although the burning heap samples are often orange, sometimes with green internal parts, resembling the "fake" bricks), but I've also seen yellow, light gray and white examples; slags are exclusively formed from the carbonate rocks (and thus don't fit to most "industrial slags"); parabasalts in the heaps are actually quite scarce.

7th Jan 2017 15:43 UTCUwe Kolitsch Manager

Hi Łukasz,

Thanks for your observations.

22nd Jan 2017 16:31 UTCPhilip Bluemner Expert

Hi,

which locality is this about?

Philip

22nd Jan 2017 16:43 UTCUwe Kolitsch Manager

Click on the bold header "Not slag".

22nd Jan 2017 16:49 UTCJolyon Ralph Founder

Note that 'slag heap' can also be a generic term to define ANY mining-related waste tip, regardless of whether the rock is natural or processed.

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