What is Parallel Growth?

Mineral Matters #763

In simple terms, parallel growth is when multiple individual crystals grow next to each other, all perfectly aligned in the same crystallographic orientation. Imagine stacking Lego bricks perfectly on top of one another; they all face the same direction. In minerals, this happens because each subsequent crystal follows the same underlying atomic "blueprint" or crystal lattice.

Stepped crystals are the visible result of this process. Instead of forming one large, smooth-faced crystal, the mineral grows as a series of smaller, stacked crystals that create a terraced or "stepped" surface, much like a staircase.

This type of growth doesn't happen under "ideal" conditions. It's typically the result of rapid crystallisation from a supersaturated solution. The fluid (like hot, mineral-rich water) from which the crystal is forming, contains more dissolved mineral-building elements than it can normally hold. This creates an urgent "need" for the crystal to grow and use up these excess elements.

Under these conditions, the crystal grows very quickly. The edges and corners of a crystal are areas of higher energy and are more chemically "attractive" for new atoms to bond to compared to the flat, stable faces. Because growth is happening fastest on the edges, they race ahead of the faces. This creates a sort of skeletal framework. The faces then try to "fill in" behind them. If the growth process slows down or stops before the faces can completely catch up and become smooth, you are left with this beautiful, terraced, or stepped appearance. An extreme case results in hoppered crystals.

This specimen of pyromorphite is a perfect showcase of these features. Look closely at the greenish-yellow crystals in the centre of the image. You can see they are not single, smooth-sided prisms. Instead, they are composed of many smaller hexagonal prisms stacked together along their length. You can see the vertical lines and slight offsets that create a complex, textured surface. This is a classic example of parallel growth.

This growth pattern is found in many other minerals, including:

• Halite: Forms classic cubic "hopper" crystals where only the edges have grown, leaving hollow, stepped faces.

• Bismuth: Lab-grown bismuth crystals are famous for their iridescent, perfectly stepped, square spiral formations.

• Quartz: Especially in "Artichoke Quartz" or "Sceptre Quartz" where many smaller points grow in parallel on a larger crystal.

• Fluorite and Galena: Often show stepped or hoppered faces on their cubic crystals.

Below: Pyromorphite, Quartz, Breitenbenden, Mechernich, Euskirchen, Cologne, North Rhine-Westphalia, Germany. Width of view 9mm.