Henderson’s Quarry is a major excavation in volcanic rock on the eastern side of Mount Ngongotaha, near Rotorua on the North Island of New Zealand.

Featured image: Henderson’s Quarry, Ngongotaha

The quarry is made up of several rock types including pumice, obsidian and rhyolite. Crystalline minerals occur in lithophysal cavities in the rhyolite zone.

One of the field trips conducted as part of the 2007 New Zealand Micro Mineral Symposium and Joint Australian Mineralogical Societies Seminar was to Henderson’s. On this trip, many of the minerals previously identified were found, including some very nice specimens of the rare silicate osumilite-(Mg).

Field Trip 2007

Collecting took place in a number of areas in the quarry, but the most interesting finds, including the osumilite, were from boulders on the edge of one of the upper benches.

Collecting is only by permission.


The Minerals

Augite

Found as single tiny needles on tridymite.

Edenite

Most commonly found as thin white flexible hairs, sometimes with hematite balls attached.

Fayalite

Ubiquitous, next to tridymite, the most commonly found mineral. Many are altered. In an extreme case, a few specimens were found with what appears to be micaceous edges and partially coated by hematite balls (see photo).

Hematite: Common as thin or thick tabular hexagonal crystals, as balls on edenite, or as reddish-brown coatings.

Relatively thick hematite crystal
Hematite balls

Osumilite

Found in one boulder only on this trip as dark green hexagonal crystals to 7 or 8mm long.

Phlogopite

Common as orange-brown thin plates.

Pseudobrookite

Uncommon as small prismatic crystals.

Titanite

Rare as tiny yellow-orange crystals.

Tridymite

Abundant as cavity linings of colourless to white hexagonal crystals.

Other minerals recorded: Ferro-edenite, ilmenite, magnetite, quartz and richterite.


References

  • Howard, D.G., Grapes, R. and Thornton, J., Vug Minerals in Rhyolite, Henderson Quarry, Mt Ngongotaha.
  • Sorrell, S.P. (2008), A report on a trip to Henderson’s Quarry last October, Australian Mineral Collector No. 6.