Tertiary volcanic rocks occur in an elongate belt up to 8km wide, stretching southwest from the coast at Table Cape approximately 5Km west of Wynyard, northwest Tasmania. The present outcrop belt of volcanics is the remnant of a once extensive sheet that was extruded on to a surface of moderate relief and with a northwest drainage direction (Gee, 1971).

Rising 170m above sea level and standing about 70m above the general level of the surrounding basalts, Table Cape is the prominant topographical feature in this area. It is a flat topped, more or less circular volcanic plug of dolerite containing olivine, titanaugite, labradorite, sanidine, analcite, and rare nepheline.

In Chambers Bay, immediately west of the volcanic neck, a layer 18m thick of scoriaceous and amygdaloidal lava underlies the main pile of lavas. Vesicles, vughs and veins of aragonite, prehnite and zeolites were reported as common by Gee.

Calcite (above)

Natrolite (below)

Prehnite (above)

Calcite, Natrolite & Chabazite (below)

Calcite (below)

Analcite

Analcite is a common zeolite usually forming masses of trapezohedral crystals lining cavities in the basalt. It is commonly associated with natrolite.

Aragonite

Only one vesicle containing aragonite was found, with prismatic transparent crystals to 1.5cm in length.

Calcite

The following forms have been noted:

* scalenohedral colourless crystals to 5mm in length overgrowing natrolite/thomsonite and chabazite,
* prismatic dark brown ferroan calcite to 8mm in length associated with chabazite and natrolite, and
* as minute colourless rhombs perched on natrolite or ferroan calcite, and sometimes at the centre of 'bow-ties' of natrolite.

The latter form is particularly aesthetic when viewed under the microscope.

Chabazite

Most of the chabazite occurs as the phacolitic form, often associated with drusy phillipsite, and usually as small colourless crystals to 6mm across. Although not common, the brilliant lustre of these specimens make them visually very attractive. Chabazite also forms as simple rhombohedral crystals or interpenetrant twins less than 1mm, usuallly perched on natrolite.

Montmorillonite?

A blue-grey clay mineral, possibly montmorillonite, occurs as linings to some vesicles.

Natrolite

Radiating natrolite occurring in amygdales and in irregular cavities was recorded by Gee (1971). It is an easy mineral to collect, being one of the most common zeolites at this location. Large vesicles containing colourless or pure white coatings of natrolite crystals, as well as yellowish (iron stained) or pink, can readily be found. Sprays with crystals up to 1cm in length occur and may be associated with either analcite or phillipsite and chabazite.

Phillipsite

Phillipsite occurs generally as white or colourless simple twinned 'fourlings', usually less than 3mm in length. It is often found as a drusy coating of vesicles.

Prehnite

Orange coated crystalline spheres of prehnite to 3mm were found associated with phillipsite in boulders on the eastern side of Chambers Bay.

Thomsonite

The presence of thomsonite was confirmed by XRD as one of the minerals occurring as a chalky white fibrous material at the base of natrolite sprays. The results indicated the presence of natrolite and thomsonite, probably as an intergrowth. Thomsonite has since been visually identified as thin prismatic crystals and is actually quite common. Most specimens are intergrowths with natrolite and occur rarely as individual sprays on phillipsite.

Todorokite?

Not enough material has been studied to determine a complete paragenetic sequence, however, observations show that analcite, phillipsite and the phacolitic form of chabazite appear to have formed early, followed by natrolite/thomsonite, then calcite, and as a late stage, the rhombohedral form of chabazite.

Acknowledgements

Access to Chambers Bay is through private property, Capelands farm. Permission must be sought from the manager, Mr Nigel Wade to whom thanks are given.

Thanks also go to Mark Cochrane for leading the trip and to Ralph Bottrill for testing specimens.

References

Anon, 1969: Catalogue of the Minerals of Tasmania. Geological Survey Record, Tasmania Department of Mines, 9, 110p.