Phillipsite Twins

Mineral Matters #778

The mineral phillipsite is a common zeolite. However, it often harbors a secret. Even crystals that look like just, well, crystals, are twinned!

First, it's important to understand that phillipsite is a monoclinic mineral. However, its crystal lattice parameters are very close to being orthorhombic. This is called pseudo-orthorhombic symmetry. This "near-symmetry" is the primary reason it twins so readily and in such specific, repeated ways. The crystal "wants" to achieve a higher symmetry, and twinning is a way to do that on a macroscopic scale.

1. The Marburg Twin Law

This is the most famous and visually striking twin law for phillipsite.

• Twin Type: Contact Twin. The crystals are joined along a single, shared plane (the composition plane).

• Twin Plane: The twinning occurs on the {001} plane (the basal pinacoid).

• Resulting Appearance: This law produces the classic cruciform (cross-like) twins.

  • Simple Twin: Two crystals join to form a simple cross.

  • Fourling (Quadruplet): More commonly, four individual crystals intergrow, all twinned on the {001} plane, radiating from a central point. This creates a distinctive shape that can look like a Roman cross (if the arms are at 90°) or a St. Andrew's cross (if the angles are oblique). Sometimes known as a drillbit habit.

These Marburg twins are what give phillipsite its most recognisable habit, especially in volcanic vugs where they have room to grow.

The pseudo-symmetry of phillipsite leads to even more complex phenomena:

• Mimetic Twinning: This repeated, microscopic twinning makes the overall monoclinic crystal mimic a higher symmetry. This is why a phillipsite "crystal" that looks like a single orthorhombic or tetragonal prism is, in fact, a complex intergrowth of multiple, smaller monoclinic crystals. This is the source of its prismatic habits and the formation of radial, spherulitic aggregates.

Below: Marburg Twin Law phillipsite with pseudocubic chabazite, Cairns, Queensland. Width of view 3.75mm.