Crystal Habits

Whenever we talk about minerals, especially our favorite minerals, we use all sorts of descriptive terms that don’t seem to have anything to do with one of the six crystal systems.

- BOB JONES

Mineral specimens are commonly described as dendritic, acicular, columnar, striated, botryoidal, banded, and prismatic, acicular.

These terms are the language of crystal habits and are an integral part of a conversation when we describe mineral specimens. While using such terms, we seldom think about why a particular term works for us. I doubt we realize these terms are the direct result of two things, the mineral’s internal atomic structure and the role it plays in a mineral’s development and the effects of the environment on a mineral during formation. We already know when a mineral forms, it develops as one of the recognized crystal systems: isometric, hexagonal, monoclinic, triclinic, tetragonal, or orthorhombic. You may also come across a seventh system, trigonal. This is actually a sub-system based on two basic crystal forms in the hexagonal system.

Regular calcite can show an obvious hexagonal form while some calcites develop in rhombic crystals, so they are identified as trigonal crystals. You have undoubtedly seen this in some rhodochrosite specimens, which is another carbonate. This mineral type shows the same two hexagonal systems. When we describe a mineral, we may start by naming its crystal system. But we need to use terms that describe the specimen in far more detail, and that’s when we describe the mineral’s crystal habits. Native copper is an example of this. It is a cubic or isometric mineral.