Selenite (mineral) was filled with mineral-rich water that drove the crystals' growth. The largest of those crystals weighs 55 tons, is long, and is over 500,000 years old. Gypsum occurs on every continent and is the most common of all the sulfate minerals. Gypsum is formed as an evaporative mineral, frequently found in alkaline lake muds, clay beds, evaporated seas, salt flats, salt springs, and caves. It is frequently found in conjunction with other minerals such as, copper ores, sulfur and sulfides, silver, iron ores, coal, calcite, dolomite, limestone, and opal. Gypsum has been dated to almost every geologic age since the
Vegetation studied plants as taxa and geographers who studied plants as vegetation. The physiognomic approach in the study of vegetation is common among biogeographers working on vegetation on a world scale, or when there is a lack of taxonomic knowledge of some place (e.g., in the tropics, where biodiversity is commonly high). The concept of "vegetation type" is more ambiguous. The definition of a specific vegetation type may include not only physiognomy, but also floristic and habitat aspects. Furthermore, the phytosociological approach in the study of vegetation relies upon a fundamental unit, the plant association, which is defined upon flora. An