What is a facies in mineralogy ?

It is a very general term which designates the category in which we can classify a mineral or a rock according to its characteristics. It can refer to the aspect and be descriptive (brecciated, acicular facies, etc...) or have a genetic connotation (metamorphic, volcanic facies, etc.). In mineralogy, the facies designates the external appearance of a mineral, it is a synonym of habit.