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Appearance
Prehnite typically ranges from yellow to light green in color, but it can also be white, blue or clear. Although normally translucent, some specimens may be almost transparent. Its shape is usually stalactitic or botryoidal. Stalactites are tapered mineral formations often seen hanging from the ceilings of caves. The term "botryoidal" means that the surface of the mineral tends to be globular, like a bunch of grapes.
Physical Properties
Prehnite has a rating of 6 to 6.5 on the Mohs scale of hardness, which measures how easy it is to scratch a mineral's surface. Diamond is at the top of the Mohs scale, with a hardness of 10. Quartz has a hardness of 7, and talc has a hardness of 1. Prehnite's rating means that it can scratch minerals with a hardness below 6, and receive scratches from minerals with a hardness above 7. Prehnite has an approximate density of 2.9 grams per cubic centimeter, which is roughly three times denser than water. It is brittle, and fractures unevenly. When scraped across a streak plate, it leaves a white streak.
Occurrence
Prehnite most commonly occurs in veins and cavities of volcanic rocks that are rich in magnesium and iron, such as basalt. It commonly appears alongside zeolites, and is sometimes confused with them. However, prehnite is a phyllosilicate mineral, whereas zeolites are tectosilicates. Simply put, this means that prehnite has a flaky, sheet-like structure, while zeolites have a strong, stable lattice structure (or "framework").
Formation
Prehnite is a low-grade metamorphic rock, meaning that it forms under a lesser degree of heat and pressure than intermediate and high-grade metamorphic rocks. Prehnite is indicative of the "prehnite-pumpellyite facies," which is a mineral assemblage that can form from basalt given a temperature range of 250 to 350 degrees Celsius (482 to 662 degrees Fahrenheit), and pressures between 2 and 7 kilobars. As a comparison, 2 kilobars is roughly two thousand times the atmospheric pressure at sea level, or twice the pressure at the deepest point in the ocean.