What is Hopper Crystal?

What is Hopper Crystal?

A hopper crystal is a form of crystal characterized by its stepped or pyramidal shape, resembling a miniature staircase or pyramid.

Hopper crystals are formed during rapid evaporation of a solution containing dissolved minerals. As the solvent evaporates, the mineral molecules start to arrange themselves into a crystalline lattice, forming the crystal. However, under rapid evaporation conditions, the edges of the crystal grow faster than the center due to the higher concentration of mineral molecules at the edges. This accelerated growth at the edges leads to the formation of the stepped or pyramidal shape, with the center of the crystal remaining hollowed out.

The edges of hoppered crystals are fully developed, but the interior spaces are not filled in. This results in what appears to be a hollowed out step lattice formation, as if someone had removed interior sections of the individual crystals. In fact, the "removed" sections never filled in, because the crystal was growing so rapidly that there was not enough time (or material) to fill in the gaps.

The interior edges of a hoppered crystal still show the crystal form characteristic to the specific mineral, and so appear to be a series of smaller and smaller stepped down miniature versions of the original crystal.

Hopper Halite crystal from Searles Lake, Trona, California. Photo: Crystalminer

Hoppering occurs when electrical attraction is higher along the edges of the crystal; this causes faster growth at the edges than near the face centers. This attraction draws the mineral molecules more strongly than the interior sections of the crystal, thus the edges develop more quickly. However, the basic physics of this type of growth is the same as that of dendrites but, because the anisotropy in the solid–liquid inter-facial energy is so large, the dendrite so produced exhibits a faceted morphology.

Hoppering is common in many minerals, including lab-grown bismuth, galena, quartz (called skeletal or fenster crystals), gold, calcite, halite (salt), and water (ice).

Hopper Galena. Photo: Quebul Fine Minerals

Hopper halite from Sieroszowice mine in Poland. Photo: Spirifer Minerals

Bismuth. Photo: Gem_stallion