Greenschist

Hardness: Variable, typically 2.5-3 (chlorite content). Color: Green to dark green, often grayish-green. Luster: Dull to sub-vitreous, sometimes schistose sheen. Crystal structure: Foliated, platy texture due to alignment of micaceous minerals. Cleavage: Good schistosity due to parallel alignment of mineral grains. Specific Gravity: 2.7-3.0.

Formed from low-grade regional metamorphism of mafic igneous rocks (basalt, gabbro) or some sedimentary rocks (shale, greywacke). Occurs under conditions of relatively low temperature (300-500 °C) and medium pressure, typically associated with mountain-building processes (orogenesis) and subduction zones. Original rocks are subjected to heat and directed pressure, causing recrystallization of minerals without significant melting. Geological age is highly variable depending on the specific metamorphic event.

Limited industrial use due to its relative softness and friability once weathered. Sometimes used as a decorative stone in landscaping or for local construction where more durable materials are scarce. Primarily of interest to geologists for studying metamorphic processes and understanding tectonic history.

The green color is primarily due to the abundance of chlorite and epidote, which are characteristic minerals of the greenschist facies. It represents one of the lowest grades of regional metamorphism. The term 'schist' refers to the strong foliation (schistosity) which allows the rock to be easily split into thin layers. This rock often forms from oceanic crust materials as they are subducted.

Identify in the field by its distinct green color, often with a fine-grained, shimmering, foliated texture. It feels relatively soft and can sometimes be scratched with a knife, especially if chlorite-rich. Common locations include areas of past or present mountain ranges (e.g., Alps, Appalachians, Scottish Highlands) and ancient shields where regional metamorphic events have occurred. Tips for collectors: Look for samples with clear foliation and distinct green color. Be aware that it can weather easily, so fresh exposures are preferable.