When is a cube not a cube?

Mineral Matters #616

Analcime, formula Na(AlSi₂O₆)·H₂O, is a member of the Zeolite Group, and is part of the isometric (cubic) crystal system. The most common form is a trapezohedron but it can form as modified cubes, or rarely as elongated or sharply pointed crystals (eg: at Mont Saint Hilaire).

But it can have a lower symmetry such as orthorhombic, tetragonal, monoclinic and triclinic. Why is that?

According to Mindat, analcime occurs in multiple crystal systems, but the aluminosilicate framework of the crystal structure does not change in topology at all. The reduction to various lower symmetries occurs because of slight changes in the ordering of Si and Al atoms and slight crumpling of the framework. Macroscopically, the crystals always look pseudocubic, apart from the very slight nonzero birefringence and fine lamellar twinning visible in the polarising microscope, because the ordering and crumpling happens in different directions in different lamellae, and over the whole crystal it averages out. Hence, the differences are far too slight to merit multiple species names, and analcime is a common example of a number of minerals which occur in multiple crystal system/space group varieties. The crystal structure topology and maximum possible symmetry of the idealised structure are what really matters.

The Mont Saint Hilaire book (Horváth, L., Gault, R.A., Pfenninger-Horváth, E., Poirier, G. (2019) Mont Saint-Hilaire: History, Geology, Mineralogy - Special Publication No. 14. The Canadian Mineralogist) suggests, although not yet proven, that some prismatic, and sharp pointed analcime crystals could form that way due to their symmetry.

I have an analcime from the Croft Quarry, Leicestershire, UK, that has misshapen, slightly elongated trapezohedral crystals. They could be due to a lower symmetry (XRD would be required), but equally could be caused by any one or a combination of a number of other factors such as:

• Crystal Growth Conditions: The conditions under which the crystal grew, such as temperature, pressure, and the presence of other minerals, can influence its shape. Variations in these conditions can lead to distortions in the crystal structure.

• Stress During Formation: Tectonic stress or pressure from surrounding rocks during the crystal's formation can cause it to grow in a distorted manner. This stress can elongate the crystal along certain axes.

• Chemical Composition Variations: Slight variations in the chemical composition of the analcime, particularly in the ratio of silicon to aluminium, can affect the crystal structure. These variations can lead to differences in the ordering of atoms, resulting in a distorted shape.

• Presence of Impurities: Impurities or inclusions within the crystal can disrupt the regular arrangement of atoms, leading to elongation or distortion. These impurities can be other minerals or elements that were present during the crystal's formation.

• Post-Formation Deformation: After the crystal has formed, it may undergo deformation due to external forces, such as tectonic activity or metamorphic processes. This can cause the crystal to become elongated or distorted.

• Growth Interference: The presence of nearby crystals or obstructions during growth can interfere with the normal development of the crystal, leading to distortions. This is common in environments where multiple crystals are growing in close proximity.

Below: Analcime, Croft Quarry, Croft, Blaby, Leicestershire, England, UK. Width of view 6mm. Click on the image for a higher resolution version.