All visitors to the Crater of Diamonds State Park find calcite crystals.
The diamond mine contains lots of small calcite crystals for two reasons.
First, the lamproite lava and ash that erupted from the Crater volcano contained between seven percent and 10 percent calcium. This amount is the third highest percentage of an element after silica (30-35 percent) and magnesium (20-25 percent).
Second, calcium is extremely soluble in water, especially hot water. When the hot water cools calcite crystals, made of calcium and carbon dioxide, easily form in any cracks in the volcanic ash and lava.
Today, out on the diamond search area we often find lamproite ash pieces containing numerous calcite veins. When the ash weathers, the calcite crystals are released into the dirt that visitors search for diamonds. Finally, the milky white calcite crystals are heavy enough to end up in the heavy concentrates, which visitors produce using the wet sluicing mining method.
It is rare, however, for visitors to find big calcite crystals. That is because calcite is a fairly soft mineral, with a Mohs scale hardness of only three. This property means that calcite crystals can be easily broken or scratched with anything metal, like a pocket knife, tweezers, or car keys. As trained park personnel identify visitors’ finds, they use this simple scratch test to sort the calcite crystals from the quartz crystals and diamonds.
Another simple test for calcite is to scratch a white powder from a crystal and then add a drop of acid, particularly hydrochloric or muriatic acid. The acid breaks down the bond between the calcium and the carbon dioxide. The carbon dioxide then bubbles away, or fizzes.
Calcite is the second most common mineral on the Earth’s surface. Two common rocks that are made of calcite are limestone and marble. Because many animals in the ocean use calcite to make their shells, ocean fossils are preserved in limestone rocks.
Limestone can also be formed by the direct precipitation of calcite crystals out of the ocean water, which naturally contains a high percentage of dissolved calcium.
We use calcite or “lime” in an amazing number of products. Calcite is the hardener in concrete and cement. It makes up chalk, whitewash, the scouring agent in scouring powder, and is the antacid part of Tums and Rolaids. We feed calcite to chickens and cows so they can produce eggshells and calcium rich milk. Finally, we use calcite in many manufacturing processes including making steel.
The mining and production of calcite is big business, with over 19.3 million tons produced in the United States in 2011 with a value of $2.2 billion. China produces the most calcite of any country, with 200 million tons each year.
We obviously don’t have enough calcite crystals at the diamond mine to be valuable. However, if you find large or nicely faceted calcite crystals, they make excellent souvenirs.
Crater of Diamonds State Park
Crater of Diamonds State Park is one of the 52 state parks administered by the State Parks Division of the Arkansas Department of Parks and Tourism.
Most recent significant rainstorms: July 26, 2012, 1.7 inches
Total diamonds found in 2012: 302
Operating Hours: Visitor Center/Diamond Discovery Center is open 8 a.m.-5 p.m., extended summer hours
Admission: $7; Children ages 6-12, $4; children under 6 years, free; admission is good for the entire day—you may come and go throughout the day on the same admission fee
Location: From Murfreesboro, take Arkansas 301 and go 2.5 miles southwest to the park
Address: 209 State Park Road, Murfreesboro, AR 71958
Phone: (870) 285-3113
Please Note: This is the tenth in an on-going series of stories on Crater of Diamonds, an Arkansas State Park
Lost, yesterday, somewhere between sunrise and sunset, two golden hours, each set with sixty diamond minutes. No reward is offered for they are gone forever.