Information about Zeophyllite

Zeophyllite is a rare calcium silicate mineral found almost exclusively in a small number of volcanic localities in central Europe. It is a collectors’ mineral known primarily for its attractive white to colourless spherical or hemispherical aggregates of platy crystals, which form small but neat rosette-like clusters on dark volcanic matrix.

It typically occurs as small hemispherical or disc-shaped aggregates of thin, platy crystals radiating from a central point – resembling miniature white chrysanthemums when seen under magnification. Individual crystals are transparent to translucent, with a pearly to vitreous lustre on cleavage faces. It is typically white to colourless, occasionally very pale grey or cream. Most specimens are best appreciated under a loupe or low-power microscope, as individual aggregates rarely exceed a few millimetres across.

Zeophyllite forms in cavities and vesicles within basaltic and phonolitic volcanic rocks – that is, rocks formed from relatively fast-cooling lava with a relatively low silica content. It is commonly associated with zeolite minerals, which share a similar volcanic cavity environment, as well as apophyllite, calcite, tobermorite, and fluorite.

It has one particularly notable physical property: on heating, it swells and ‘intumesces’ (new word on me, that…) –  essentially what that means is, it puffs up/swells in a manner similar to zeolite minerals.

Uses and History

Zeophyllite has no industrial or gemological applications and is too small and fragile for any practical use. It is collected purely as a scientific and aesthetic curiosity.

Its name comes from the Greek words zeo, meaning “to boil,” and phyllon, meaning “leaf” – a reference both to its platy, leaf-like crystal habit and to its tendency to swell and intumescence on heating, in the manner of a boiling or bubbling material. The intumescent behaviour was thought to ally it with the zeolites, though it is not a true zeolite and belongs instead to the phyllosilicate group.

On heating to around 450-600°C, zeophyllite partially dehydrates and converts to cuspidine – a related calcium silicate fluoride mineral – a transformation that is irreversible and provides a useful confirmation of identity in the laboratory, though not a test any collector would wish to apply to a specimen.

Notable localities are few and the mineral is genuinely rare worldwide. The most celebrated source is the volcanic district of the Eifel in Rhineland-Palatinate, Germany, particularly the Schellkopf at Brenk and the Caspar quarry at Ettringen, where white spherical aggregates occur in phonolitic volcanic rocks alongside zeolites and apophyllite.

Occurrences in the Czech Republic are known from Radejčín and Kozí Vrch in the České středohoří volcanic province; from Křížový vrch near Cvikov in the Lužické hory (Lusatian Mountains); and from several other localities in the Bohemian Uplands, where it occurs in basaltic and phonolitic volcanics of Cenozoic age. A further occurrence has been noted in Skåne County in southern Sweden. No confirmed occurrences are currently known from the United Kingdom.

Mineralogy

Hazards and Warnings

No specific health risks have been formally recorded for zeophyllite. Mineral collectors should wash their hands after handling specimens as a matter of good practice.

Almost all rocks, minerals (and, frankly, almost all other substances on earth) can produce toxic dust when cutting, which can cause serious respiratory conditions including silicosis. When cutting or polishing rocks, minerals, shells, etc, all work should be done wet to minimise the dust, and a suitable respirator or extraction system should be used.

Translations

Further Reading / External Links

• https://en.wikipedia.org/wiki/Zeophyllite
• https://www.mindat.org/min-4396.html
• https://www.dakotamatrix.com/mineralpedia/9062/zeophyllite
• https://www.handbookofmineralogy.org/pdfs/zeophyllite.pdf