Any rockhound worth their salt knows that the best place to hunt for interesting minerals is in the void spaces in rock. Void spaces come in two types; vugs and veins. Vugs are usually found in igneous rock and result from trapped gas bubbles. Veins, on the other hand, can be found in any type of bedrock.
Veins are fractures, that have been plugged with minerals, typically by precipitation from circulating water. The above picture was taken in the Ron Coleman quartz mine, near Hot Springs, Arkansas. The near-parallel white streaks that riddle the sandstone are quartz-filled veins. The fractures resulted from the intense deformation of the Ouachita Mountains, by plate tectonic forces, around 300 million years ago. That deformation opened up space for quartz to grow in, and the tremendous heat and pressure from the mountain-building generated the mineral-rich fluid that deposited the crystals.
Even though they might look like it, those crystals in the picture above didn’t come out of a dog’s mouth. They are crystals of dogtooth calcite. Calcite (CaCO3) is the primary mineral that makes up limestone. It occurs in several crystal shapes. The two most commonly found in Arkansas are 6 sided rhombohedrons and the scalenohedral shape you see above. When it forms in this scalenohedral crystal structure it is called “dogtooth spar”.
Calcite is a very common mineral, but this particular crystal form of the mineral is typically only found in Arkansas in conjunction with the minerals sphalerite (zinc ore) and galena (lead ore) in the lead and zinc districts. Calcite is also a polymorph, like the mineral brookite from a previous geo-pic. This means calcite has “sister” minerals with the same chemical composition, but differing crystal structures. The three polymorphs of CaCO3 are: calcite, aragonite, and vaterite.