Dolomite – A Mineral or Rock?
A MINERAL OR ROCK?
Collectors are familiar with crystallized dolomite which occurs as discrete crystals, tiny associated crystals, and larger more appealing minerals that dominant a specimen. Luckily, dolomite has also formed in large crystals well suited for display.
The word “dolomite” often brings to mind crystals, and may not always bring to mind hard, gray, and massive limestone rock. These limestone formations can extend across landscapes for miles, which can be thousands of feet thick. But there are important mountains in Europe called “the Dolomites.” Therefore, dolomite has two meanings as fine collectible crystals and as huge rock formations that are part of the earth’s crust.
I found this a little confusing, so I looked back in history when it was decided that limestone rock formations needed two names. The darker gray limestone formations are composed of calcium magnesium carbonate, while white calcium carbonate rock is also limestone. Keep in mind that the mineral dolomite is a calcium magnesium carbonate. So, in 1778, dark magnesium-rich limestone formations were given the name dolomite. That made things even more confusing, and the confusion persisted until 1948 when geologists decided to clarify matters. Minerals that are calcium magnesium carbonate should continue to be called dolomite. The large limestone rock formations, previously called dolomite, should have a new name, dolostone, or magnesian limestone. Calcium carbonate limestone is just limestone.
Dolostone is a major sedimentary rock formation in the earth’s crust and joins with other sedimentary formations like shale, sandstone, and limestone. During the same time as the adoption of the name dolostone, the process of fracking came into vogue.
Fracking had been a process known for many decades, but only in recent years has it become a serious oil and gas recovery method. Deep-seated sedimentary formations can release oil and gas when shattered, which the public has become aware of. But why include that process in an article about dolomite? The organic material released by fracking is what remains of the very early life forms that existed in the muds and sands; which developed into rock formations, with the remains of early life forms trapped in the rock structure. Not all sedimentary formations contain such valuable ingredients, but when found in quantity among shale, sandstone, limestone, and dolostone, fracking can be employed.
Fracking has no direct connection to the collecting of minerals, but we certainly do collect examples of minerals in sedimentary rock formations. These formations can show evidence of early life forms that lived in the sediments that became sedimentary rocks.
When collecting quartz crystals around Herkimer, New York, rock hounds will see evidence of ancient organic material as small black inclusions locked in some of the quartz crystals, and in the pocket itself. What is the rock they have to bash and pry loose to find those quartz crystal pockets? Dolostone.
Herkimer dolostone started out as a limey mud millions and millions of years ago. The mud, mainly carbonate dioxide, was invaded by magnesium-rich waters resulting in calcium-magnesium limestone. But that’s not the whole story. As the mud was accumulating, so was a new and unique form of bacteria called cyanobacteria, which was growing the mud.
What made cyanobacteria unique was it could absorb carbon dioxide, a dominant gas in the ancient atmosphere, and produce its own food while releasing oxygen as a waste product.
Some waste product! Cyanobacteria were
Composed of simple cubic forms, this specimen exhibits an exotic, golden honey-yellow color of fluorite, particularly in transmitted light. The lower side of the piece is lightly “dusted” with a number of small, saddle shaped dolomite rhombs of an off-white coloration.
the earliest first life form on earth to use photosynthesis, which had a profound impact by changing life on earth. All this was going on while the limey magnesium-rich mud was altering to calcium magnesium carbonate rock we call dolostone!
As the mud continued to pile up, the cyanobacteria would cultivate in large clusters, or colonies, called stromatolites. It would then gradually die off and new colonies would develop. The dead colonies would be buried creating gaps, or open spaces, in the newly forming dolostone. The organic residue of these dead stromatolite colonies remained in the cavities they created.
Today, as you work the Herkimer area, dolostone is a real challenge because of its hardness. You are looking for the open pockets created by those dead stromatolites. Later, watery solutions containing silica gradually seep through the dolostone and join any silica already in the mud. It slowly develops the lovely quartz crystals we collect. During crystal growth, the quartz would also pick up bits of the cyanobacteria organic residue, and is seen as black organic material we have named anthraxolite, in Herkimer’s quartz crystals.
As interesting as dolostone is, especially when it produces nice crystals, the dolomite we are most interested in is not a huge rock formation, but lovely crystallized specimens that are found in Spain, China, Brazil, and several U.S. localities. Crystallized dolomite is found as an accessory mineral in many ore deposits, and as a common mineral in some hydrothermal deposits as discrete crystals, and excellent collector specimens.
Dolomite, as noted, is a calcium magnesium carbonate so it is relatively soft, just a bit harder than calcite. It develops in the trigonal rhombohederal crystal system and, like calcite, will cleave in any one of three directions. While acids will attack calcite vigorously, dolomite reacts less vigorously. One of the odd features of some dolomite is its tendency to form what are called saddle-shaped curving crystals. These tend to be small, as you see in specimens in the Tri-State area, and especially from Baxter Springs, Kansas.
Dolomite’s color tends to be colorless, white, creamy, and an uncommon pink. China yields an unusual, orange-colored dolomite, but it is rare. Dolomite forms a complete series with ankerite and kutnohorite, both of which are iron, calcium, magnesium, manganese carbonates.
For years, the finest dolomite crystals were harvested in quarries around Eugui, Esterbar, Spain. The dolomite from here occurs in large, sharp, water clear rhombic crystals often in interlocking clusters of crystals sometimes showing penetration twinning. The crystals can be exceptionally large for the mineral, as much as two inches on an edge. The rhombic faces often show small growth patterns that are quite interesting. Though these dolomites are still considered among the world’s finest such dolomite specimens, they are now rivaled
by crystals from China and Brumado, Brazil.
Having been a collector for decades, I recall the early years of my collecting life when dolomites from Eugui, Spain were virtually unknown in America. It was not until the latter quarter of the 20th century that superb Spanish dolomites appeared. This was due in part to the Tucson Show becoming so important that it attracted more and more European dealers and collectors. Another reason came about in the 1970s when an active collector-dealer, Victor Yount, living in Spain at the time, brought superb Eugui dolomite crystals to Tucson. Victor collected far and wide in Portugal, Spain, Morocco, and other European places. He also managed to be an extra in several movies shot in Spain, including the noted war movie, Patton. Victor was also one of the first to bring superb specimens of bright in red hexagonal crystal groups of vanadinite from Morocco. At the time Morocco did not allow exporting of minerals, so Victor rolled his vanadinite specimens up in cheap rugs, and packed them in brass pots, and shipped them back to Spain! His introduction of Eugui dolomite at Tucson caused quite a stir at the time. Since then, fine dolomites have come from quarries in Brazil and China that rival some of the Eugui specimens. These localities have produced marvelous dolomites associated with equally fine crystals of magnesite in Brumado, Brazil. Here in the States, collectors are most familiar with the small creamy dolomite crystals found as an associated mineral with galena, calcite, sphalerite, and chalcopyrite from the Tri-Satte area of Oklahoma, Missouri, and Kansas. When these lead mines operated, specimens were so abundant they were almost ignored because they were so common. Local retired miners in particular collected quantities of specimens which they offered for
A small cabinet sized cluster of dolomite crystals with a number of superb shiny untwined cinnabar crystals growing scattered on it. ~Rock H. Currier
sale in stores, roadside stands, and in front yards. The dolomite crystals were never large, usually under a halfinch, but sharp, slightly curved, and a light cream color. But they played a subordinate role to the larger crystals of galena, fluorite, and even calcite from these mines.
China has really stirred things in the mineral collecting world since opening its borders a few decades ago. As for dolomite, China has two major sources for this lovely mineral. From the mercury complex mines of Wanshan, Tongren,China, the dolomite is found in white opaque, lustrous, and sometimes transparent, colorless crystals. The dolomite occurs abundantly, but generally only as an associated mineral to the more important ore mineral bright red cinnabar crystals. Some of the most attractive and larger cinnabars are found in China. With the snowwhite attractive dolomite crystals for a background, the showy and less common red cinnabar crystal specimens are very appealing and collectible.
The cinnabars are seen as single, and more often, twinned crystals of bright luster which are all the more attractive because of the sharp contrasting white dolomite crystals on which the cinnabar forms. The dolomite crystals are usually under a half inch.
China’s other important source of dolomite crystals has been the Shangbao Pyrite mine, Hunan Province associated with pyrite crystals. The pyrites from here are lustrous and can be quite large, well over an inch or two on an edge. More importantly here, are the dolomite crystals found with other species. They are large rhombic crystals that occur with calcite, some quartz, and fluorite; and the better specimens are with dolomite the dominant species.
More interestingly are the dolomite crystals found that are sharp and exhibit dolomite’s unusual curving saddle-shaped crystals far larger than any we have seen from the U.S.
These larger saddle-shaped clusters reach almost three inches across.
Their faces are sharply steeped in a series of rising crystal edges.
Another surprise from this locality are the dolomite crystals that are richly colored by iron, so they have a good yellow-orange color. The crystals are under a half-inch, but in a nice radiating cluster on which quartz or calcite has developed.
While dolomite, usually lacking a really vibrant color, is not among the more popular exhibit minerals, it does occur in fine crystal groups. As an important companion to many showier species, it also plays an important role. Just remember if it is a rock it is dolostone, and if it’s a crystal it’s dolomite!