Pictured above is a mineralized vug (approximately 3 inches long) in chert. A vug is a void or open space in a rock. Many vugs are filled with minerals after water that is saturated with a certain mineral flows through the rock. This mineralization can happen in multiple stages. The vug above was initially filled with silica-rich fluid therefore quartz precipitated out of solution and lined the walls of the vug. Afterwards calcite precipitated, as is evident from the larger crystal on the interior left of the vug.
This vug is present in a section of ornamentally banded chert. Chert is a sedimentary rock made up of microcrystalline quartz. It can be a variety of colors or banded and quite beautiful. The chert above is Devonian age (416-359 million years ago) from northwest Arkansas.
Pictured above is the mineral cinnabar (red) on a nest of quartz crystals. Cinnabar is the mineral we extract mercury from. Cinnabar in Arkansas, which was commercially mined in the early 1930’s, is located in a linear band approximately 20 miles long in Pike, Howard, and Clark counties in the Ouachita Mountains of western Arkansas. It is in a linear band because it was deposited as fracture fillings in the highly fractured rock of a tightly folded anticline. The intense deformation that created the fold also caused fractures to open in the rock to relieve the pressure. While this folded rock was still deeply buried it came into contact with mineral rich hydrothermal fluids that were circulating deep in the earths crust. These hydrothermal fluids left deposits of mercury-rich cinnabar in the fractures.
In addition to the cinnabar, early prospectors also found fractures and cavities that were filled with free mercury or “quicksilver” as they called it. Thousands of pounds of this quicksilver were extracted from the deposit during World War 2. Unfortunately, due to poor health and safety measures at that time, and the toxicity of mercury, many of the miners eventually developed very serious health problems; mining of the cinnabar ended in 1945. The level of mercury in cinnabar is high enough that it’s recommended you handle it cautiously.