What Exactly do I have - Dolomite, Limestone or Dolostone?

by MichaelG

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The other night, I was discussing Eddie Murphy with a friend. Murphy, whose career has been up and down over the past 10 years had a bit of a comeback in 2019 with the film Dolemite is My Name. This is a comedic-bio-pic Netflix file is about blaxploitation film-maker Rudy Ray Moore. Nothing about the movie, except part of the name, Dolemite (which actually refers to a film that Rudy Ray Moore, played by Murphy, made in the 1970s of the same name) has anything to do with geology. But once I started thinking about dolomite, I went down the rabbit hole.

I thought I knew what dolomite. After all, I have some of it in my collection. It’s that creamy mineral you find in Indiana right? It turns out the history of the name and it applies to is a bit torturous and complicated. The name dolomite begins in the 18th century. A french geologist named Dieudonné Sylvain Guy Tancrède de Gratet de Dolomieu usually known as Déodat de Dolomieu, discovered in the old city of Rome, and later during a visit to the Alps of northeastern Italy, a calcareous rock which, did not effervesce when exposed to weak hydrochloric acid. This was unlike limestone. Dolomieu published this discovery in the journal Journal de Physique in March of 1792. The calcareous mineral he described was eventually named after him, as dolomite. In addition, a mountain range in northeastern Italy, part of the southern limestone alps, was named The Dolomites, also after Dolomieu.

Put-in-Bay Dolomite South Bass Island, Lake Erie,Ohio

Ohio Dolostone

_{Photo By James St. John – Put-in-Bay Dolomite over Tymochtee Dolomite (Upper Silurian; South Bass Island, Lake Erie, Ohio, USA) 6, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=82969360}

But what exactly was it that Dolomieu discovered? Limestone had been used as a building material for thousands of years. The ancient Egyptians used limestone to build the pyramids of Giza and the Great Sphinx. The Maya of ancient Mexico used it for carving. The ancient Athenians used it to build the Parthenon on the Acropolis in Athens during its golden age. The Romans used it to build the Colosseum in Rome. Jerusalem stone (limestone) was used to build ancient Jerusalem from mines located near the neighborhood of Yemin Moshe. But this new mineral, despite some similarities clearly wasn’t limestone. The stronger resistance to HCL proved that.

Both limestone and the new mineral, it turns out, were typically sedimentary carbonate minerals. Limestone is typically composed of calcite and aragonite, both forms of crystallized CaCO₃. The new mineral turned out to be an anhydrous form of CaMg(CO₃)₂, and it turns out that the magnesium makes all the difference. For a long time the new mineral, named dolomite in Dolomieu’s honor, was applied to both the crystal and rock formations of the mineral. Confusion about the term Dolomite, which now referred to three distinct things, persisted until 1948 when geologists changed the name of the rock formation of calcium magnesium carbonate to dolostone. Dolomite from that point on only refers to the mineral and not the rock formation. The “Dolomites” is used to refer to the mountain range.

Sphalerite on dolostone Millersville Quarry, Sandusky County, Ohio

Sphalerite crystals atop sucrosic dolostone from Sandusky County, Ohio

_{Photo by James St. John – https://www.flickr.com/photos/jsjgeology/31282767801/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=101721070}

Even though the mineral was discovered in Europe, it is found across the world include Spain, China, Brazil, and Ohio among other places. Spain used to be the source of the finest dolomite crystals, but now Brazil and China also produce world class specimens. Dolomite also appears as a bed on which showier crystals display – see the spakling example below of Dioptase from Tsumeb mine, Namibia, formerly of my collection.

Dioptase with Calcite Crystals on Dolomite Tsumeb Mine Otjikoto Region Namibia DIO3-#DIO3-12

Beautiful example of dioptase with calcite crystals on dolomite from the Tsumeb mine, Otjikoto region, Namibia. This piece was in our inventory previously and is now sold.

Click here to check out our selection of Dolomite for sale.

_{Main photo by Rob Lavinsky, iRocks.com – CC-BY-SA-3.0, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10148460}