Exhibit Focus: 1

Highlighting objects on display in our Museums

Location: Guernsey Museum & Art Gallery - Main Gallery - Environmental displays	Click image to view enlargement

Identification:	Mineral Specimen - NATROLITE on PREHNITE
Provenance:	Lukis Mineral Collection
Geographic Origin:	Undocumented, but almost certainly from mineral veins in the St Peter Port Gabbro of Guernsey, by virtue of the associated rock matrix and comparison with other known specimens. This rock was extensively quarried during the 19th and early 20th centuries.
General Comments: searchtest	This magnificent specimen is certainly the most spectacular local (Guernsey) item in the Lukis Mineral Collection. It consists of long white crystals of the mineral Natrolite which have grown outwards as radiating bundles from a central point. They rest on a surface which is coated - in various shades of green - with the mineral Prehnite. The long natrolite crystals have a square cross-section and their outer ends are naturally terminated with tiny triangular facets, showing they grew freely into space within the rock. The space was certainly a crack (or 'joint' - to use the geological term) which might have had its origin in any one of several different ways - perhaps cooling and shrinkage of the original magma (St Peter Port gabbro is an igneous rock, cooled from a an original molten state) or caused by later earth movements, long after the rock was solidified. Such cracks or joints form natural conduits for fluids to pass through a rock mass and if these fluids are rich in dissolved minerals, these frequently crystalise on the joint surfaces. It's rather like the limestone scale or 'fur' which develops on the inside of pipes and kettles in hard water areas. The logical conclusion, in the geological setting, is that the joints completely fill up with mineral material and this is indeed how most mineral veins form. What we have here is a small fragment of one side from such a crack or joint. Earlier in its history the surface was first coated with the mineral prehnite (it is likely that the opposite wall of the joint was similarly coated) and then the relative proportions of dissolved 'ingredients' in the water percolating through the joint changed. This initiated growth of the natrolite crystals which adopted their typical radiating growth pattern. Mineral growth stopped before the cavity was completely filled and it was finally exposed to the world by quarrymen in the 19th century. Frederick Corbin Lukis (1788-1871), the original owner of the specimen, clearly had 'arrangements' with local quarry owners and workers. They obviously knew of his interest in any fine or unusual geological specimens brought out of their workings. This has had the fortunate effect of leaving a valuable body of evidence about local geology in his collection, which would certainly be hard to obtain today. Many of the old quarries are now completely filled in or otherwise inaccessible, making resources like the Lukis collection highly valuable reference sources.

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