The Colorado Springs Mineralogical Society (CSMS) celebrated 2022 by holding their annual (except for a Covid postponement) Pikes Peak Gem, Mineral, and Jewelry Show. As best I could count (a little tough after 10 fingers), 56 vendors were present at the Norris Penrose Event Center. In addition, ~14 exhibits displayed a variety of minerals, fossils, and gems. The Colorado Gold Panners had a couple of panning troughs filled with sand, gold, and nifty minerals that attracted a variety of participants from 2-100 years old or so. The local Florissant Fossil Beds National Monument represented the Bureau of Land Management, the U.S. Forest Service, and the National Park Service. David and his crew of workers spent three days trying to attract new members into the CSMS Pebble Pups and Earth Science Scholars programs by displaying everything a kid might be interested in, including dinosaurs. And finally, the Silent Auction, the most popular table in the Show, sold hundreds and hundreds of specimens to children and adults alike—everyone went away from this activity happy. Congratulations to Show Chair Lisa and Prez John who recruited a great cast of volunteers.
My prior commitments kept me out of the Show except for a couple of short stints; however, the attendance seemed pretty amazing. People are tired of staying home due to Covid and what better activity to “get them out of the house” than visiting displays of fantastic rocks, minerals, fossils, and gems.
I did find a few specimens that made it home to my cabinet of curiosities, but nothing that a curator/curatrix would display in the Natural History Museum.
I decided to increase my collection of minerals from the Northern Peninsula of Michigan—Yooperland as I noted in several other postings. One of the more interesting acquirements from D’Angelos down in Trinidad was an amphibole known as grunerite. The first question that may come to a reader’s mind is, “what is an amphibole.” My answer is pretty vague as identifying an “amphibole” in my beginning Physical Geology class was confusing to say the least. Amphiboles have now been elevated to Supergroup status by the International Mineralogical Association and contain two Groups divided into several Subgroups and a plethora of individual minerals. Amphiboles are silicate minerals containing SiO4 tetrahedra with cations of iron and/or magnesium—plus a variety of other minor constituents. Individual minerals often form prismatic or needle-like crystals and may crystallize in either the Monoclinic or Orthorhombic Crystal Systems. In addition, amphibole minerals may form in either igneous or metamorphic rocks; hornblende may be the “best known” mineral.
My confusion in Physical Geology, and later Mineralogy, was usually due to the confusion of amphibole minerals with the similar looking pyroxene minerals such as augite. Competent mineralogists will gently explain that amphiboles contain, in their atomic structure, a double chain of tetrahedra while pyroxenes have a single chain, and that amphiboles contain either hydroxyl (OH), fluorine (F), or chlorine (Cl). Try explaining that to a beginning geology student still licking clear specimens trying to identify halite or calcite! So, we were taught to identify minerals of the two groups by examining cleavage angles. Amphiboles cleave at an oblique angle, about 120 degrees, and pyroxene cleavage fragments are about 90 degrees and almost form a "square".
Grunerite is a common mineral if located in metamorphosed iron formations, and occasionally in other metamorphic rocks. It is an iron rich amphibole [(Fe2)(Fe5)(Si8O22)(OH)2] in solid solution with the magnesium (replacing the iron) rich cummingtonite (also a metamorphic mineral). As far as I can determine there are no named minerals between cummingtonite and grunerite. Hawthorne and Oberti (2006) defined the boundary as Mg:Fe=1:1.
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Grunerite
from Yooperland occurs as bronze to brown to brownish green to gray metallic
bundles or radiating sprays of acicular or prismatic crystals. It is difficult to determine but crystals
have a hardness of around 5-6 (Mohs), a vitreous luster but a colorless streak.
Rockhounds usually identify grunerite as the radiating sprays found on Banded
Iron Formations. My specimen came from the Precambrian Bijiki Schist (Iron Formation) near Michigamme Lake in Marquette
County, Michigan, and sprays are intimately associated with black magnetite. I saw minor amounts of quartz in the specimen
but could not locate garnets nor biotite nor chlorite (at times common
associates). Heinrich (2004) described the Marquette Iron Ranges as former siderite-chert rocks now metamorphosed to grunerite-quartz + magnetite.
REFERENCES CITED
Hawthorne, F.C. and R. Oberti, 2006, On the classification of amphiboles: The Canadian Mineralogist Vol. 44.
Heinrich, E.W. (with updates and revisions by G.W. Robinson), 2004, Mineralogy of Michigan: Michigan Technological University, Houghton.
CELEBRATION OF A MINERAL
Beneath the earth’s crust
Hot metamorphic terrain
Grunerite forming
For those readers who might have missed me (no posts for a couple of months), I have been in a state of aestivation, a period of dormancy where my writing has been inactive and has been replaced by the relaxation of reading causing a lowered metabolic rate. Much of the reading has been non-geological in nature in my quest to continue as a lifelong learner. I even brought out one of my ancient lit class assignments (John Keats) and dwelled on the final stanza: