Coal petrology?

“Black is black”: although this sentiment of heartache from the 1966 Los Bravos song implies some sort of absolutism, when it comes to coal, coal utilization byproducts, or coaly sedimentary organic matter, there is more variety than what casually meets the naked eye. I was once challenged by an organic chemist on microscopically discriminating between coal and bottom ash (residue after burning coal in a furnace or boiler), but contrary to the Rolling Stones’ lyrics “No colors any more . . . I wanna see it painted, painted black . . . black as coal”, there is a diversity in morphology, texture, luster or reflectivity, and gray-scale when particulate organic-bearing rocks or products are examined under the microscope. Even in hand-specimen, lumps of coal can reveal details of their stratigraphy and back-story. On a 1992 field trip to an open-pit anthracite mine in Pennsylvania, the dull sooty surface layer of a small slab of coal I picked up displayed large flat pieces of porous charcoal, relics of an ancient forest fire, compared to the millimeter-scale glassy black layers of mostly vitrinite, formed from gelified ancient wood: stacked chapters in the geohistory of a paleo-swamp.

I use coal petrology to solve geologic problems. (Non-geologists: "Petrology" means the study of rocks- "petro" comes from the Greek for rock; "petroleum" means oil from rock; "petrified" means turned into rock; the name "Peter" has the same root.) So what does a coal petrologist do? Four weeks ago, I did the renewal microscope exercise for my coal petrologist accreditation through the International Committee on Coal and Organic Petrology (ICCP), and the two parts of this exercise illustrate primary techniques of coal petrology.

(First as technical background, coal petrologists use reflected light microscopy under oil immersion: instead of light passing through translucent materials on a glass slide from underneath, the light is bounced off the highly-polished surface of a piece of coal. Metallurgists and geologists who study opaque economic minerals like gold, copper sulfides, steel, etc. also use reflected light microscopy. The immersion oil, with a specified index of refraction, forms a meniscus between the specimen and the microscope objective lens; it increases the contrast among the various coal macerals, much like that tasty field geologist practice of licking a rock.)

So the first segment of the accreditation exercise is maceral analysis, counting and calculating percentages of the three major maceral groups. Maceral? Besides being a frequent word in the national spelling bee, a maceral is a microscopically recognizable constituent of organic matter in coal, first defined by Marie Stopes in 1935 (https://coalandcarbonatlas.siu.edu/coal-macerals/coal-macerals-tutorial.php). SAT analogy: maceral is to coal as mineral is to rock. The three general groups of macerals are vitrinite (woody organic matter), inertinite (~fossil charcoal and is non-reactive in some industrial processes), and liptinite (waxy plant parts like spores, leaf cuticles, resin, algae). In addition, there is mineral matter in coal, aka ash, the part of the whole coal that will not combust. The relative proportion of macerals in a coal (or organic-rich sedimentary rock) can give an indication of ancient climate conditions (lots of fossil charcoal= conditions ripe for forest fires), groundwater conditions in the coal swamp (soggier vs. drier), distance of a point in a lake or ocean from the shore line and land plant input. Industrially, the maceral proportions help predict the behavior of the coal in processes such as coke making in the steel industry.

The second part of the accreditation exercise is vitrinite reflectance. With increasing temperature the reflectance or reflectivity of vitrinite (maceral derived from woody plant matter) increases due to chemical and physical changes. The percent of incident light reflected from vitrinite is measured, like maceral counts, on a polished piece of coal or vitrinite-bearing sedimentary rock using oil-immersion reflected light microscopy, plus associated photomultiplier or digital measurement equipment and calibration to reflectance standards. (Standardized procedures are described by ASTM and ISO.) Vitrinite reflectance is a diagenetic to very-low-grade metamorphic indicator and is one of the markers for rank (i.e. lignite, high- and low-volatile bituminous, anthracite) in coals and degree of petroleum generation in organic-bearing sedimentary rocks. It is useful data for basin or regional thermal history studies, resource characterization, and industrial utilization. Vitrinite reflectance can be related to the maximum temperature experienced by a rock or coal through modeling algorithms that combine burial temperature history and vitrinite reaction kinetics.

This blog will cover a broad array of topics under the big umbrella of carbonaceous geologic or earth materials. As the blog summary in my Introduction sidebar describes, the organic carbon here will be mostly particulate and/or combustible, not dissolved in fresh or marine waters or bound in the carbonate of limestone or skeletons. Sometimes the connection of a blogpost to geologic carbon may be tenuous, and the carbonaceous “Bacon number” may be high!

(Addendum: My December 12, 2017 post on "Why there will be no #MaceralCup" describes, with some photos, numerous maceral types within the three major maceral categories. The June 20, 2015 blog post lists relevant online bibliography and photomicrograph atlases, coal and organic petrology-related scientific societies and books.)