It is tough to be a diamond in a rhinestone world! Dolly Parton
I suspect that most rockhounds have a garnet or hundred in their collection. These silicate minerals have been used as gemstones for millennia, as well as having numerous uses in industry. Collectors love garnets since they are usually easy to identify by their red to reddish brown color (or green or black or pink or violet), by their hardness of ~6.5 to 7.5 Mohs, their crystal habit that is often 12 sided rounded dodecahedrons, and their occurrence in high temperature metamorphic rock like schists, igneous rocks such as granite, and detrital sediments such as sands. The more serious, or even semiserious, collectors define the general garnet formula as X3Y2(SiO4)3. This formula is then used to differentiate the 1) Pyralspite Garnets where aluminum fills the Y site: the iron aluminum Almandine, the magnesium aluminum Pyrope, and the manganese aluminum Spessartine and 2) the Ugrandite Garnets where calcium fills the X site: calcium iron Andradite, the calcium aluminum grossular, and the calcium chromium uvarovite. These two major groups are actually solid solution series with intermediate garnet varieties and several mixed cation varieties. Chemists also experiment with several synthetic varieties of garnets where new elements substitute for silicon in SiO2 part of the chemical formula.
The fat, quarter-size crystal I want to describe in this post is not one of the common, shiny, dark red, iron-rich, well-defined dodecahedrons of almandine---although it used to be! The specimen was collected from the Michigamme Mine in Marquette Michigan and that locality provided the clue to its description. Greenish black to black chamosite has pseudomorphed the classic dodecahedral crystal of almandine.
Chamosite is the iron rich member of the chlorite group of minerals, a hydrous iron aluminum silicate (Fe2+)5Al(Si,Al)4O10(OH,O)8. As a sheet silicate it has a laminar shape similar to the mics. These sheets are usually greenish gray to brown to greenish black in color and are translucent to transparent, and soft (~3 Mohs), and flexible. The luster is usually dull to pearly. Chamosite is in solid solution with clinochlore, its magnesium analog, and both commonly occur in sedimentary iron formations.
The chlorites, including chamosite, are part of the “clay minerals”, and as such, have very tiny crystals that are often earthy in appearance. In my specimen chamosite has altered, disrupted, and changed the internal lattice and structure of the almandine; however, the external shape, a dodecahedron, has remained. It remains beyond my pay grade to understand exactly “why” or “what caused” the almandine to be replaced by chamosite.
The most famous of the chamosite ps. almandine specimens have been found in the dump piles of the Michigamme Iron Mine, "Marquette Iron Range," in Marquette County, Michigan. Termed “Black Diamonds” by the miners, these very hard specimens were a pain in the derriere for the ore crushers. However, historical accounts noted that local entrepreneurs sold the black diamonds to visitor and tourists, especially those on the trains stopping at the Michigamme Railroad Depot. A note on my specimen stated it was collected ca. 1900.
Location of the Marquette Iron Range. Public Domain map courtesy of W. F. Cannon, USGS.
The Michigamme Mine, from ~1872 to 1905, produced perhaps, I don’t have good figures, a million tons of iron ore from the Proterozoic Negaunee Iron Formation and the overlying Michigamme Formation. The cherty ore pay rocks were hematite-rich with secondary magnetite and grunerite. I also saw a note that recent Michigan road construction crews have covered the dump piles of the mine with a new road. The reporters were lamenting the loss of again collecting these magnificent black diamonds.
It is more fitting for a man to laugh than to lament over it.
Seneca the Younger
Almandine garnet crystals on gray-green schist. Public Domain photograph courtesy of Didier Descouens.