Hendersons Quarry, Mount Ngongotaha, Rotorua District, Bay of Plenty Region, New Zealandi

Regional Level Types
Hendersons Quarry	Quarry
Mount Ngongotaha	Mountain
Rotorua District	District
Bay of Plenty Region	Region
New Zealand	Country

Mt. Ngongotaha is a rhyolite dome with high-temperature gas phase-deposited silicate and oxide minerals present in vesicules.

This is an operating quarry, for aggregate, about 5km north of Rotorua, and access to collectors is very restricted. Matt Henderson has allowed local rock clubs, mineral symposium field trips, and geologists entry (but not individual collectors just wanting to have a look). The location is one of the most represented in local collections for New Zealand. A 2007 visit stated material was found in boulders along the edges of the upper benches. It appears collectors became aware of specimens at the location around 1990, as Hosking (1990) below shows a process of local collectors trying to work out the species. Initially several were mis-identified (elbaite etc) as they struggled with the tiny crystals, and lack of professional equipment.

Mt Ngongataha is a collection of nine rhyolite domes, the quarry situated in the oldest. The quarry shows an upper finely vesicular pale pumice, commonly brecciated. Below this is fractured brecciated banded obsidian. Next down is grey rhyolite, where most of the specimens are found, and mined for aggregate. It contains many cavities from a few mms to several cms across, often complexly interconnected, showing contortion structures along the walls, and lined with tridymite. The overall appearance is pale grey rock, with many snowy white patches of small bladed tridymite crystal lined cavities.

Most specimens are micro to thumbnail. Tridymite is the most common species, and subsequent species should be considered as part of tridymite lined cavities, unless other wise stated. Tridymite is found as tiny clear, hexagonal crystals and plates, often so thin they are shattered. May also be seen as pale-yellow. Some quartz is found on top of the tridymite as clear crystals.

Fayalite is said to be the next most common mineral, orthorhombic, equant crystals to several mms, scattered singularly, with some multiples, opaque brown, possibly showing iridescence. Sometimes blocky amphibole needles are associated but few other species. It may also be found as short stubby crystals, black coloured, but thin crystals show as greenish, or beer bottle brown.

Iron rich edenite as slender needles or hairs, one cm or more long, as sprays (but not matted), monoclinic prisms, diamond shape in cross section, or hexagonal in thicker crystals. White or brown depending on Fe content.

Hematite as lustrous black hexagonal plates as possibly the most common, but seen in various other forms including barrel shaped crystals, rosettes of thin blades, minute balls speared by edenite needles, specularite, or botryoidal. The hematite contains 10% titanium oxide. An unusual growth habit, is hematite next to titanite crystals, will grow around the titanite without touching it, leaving the hematite arching over the titanite crystal.

Titanite as tiny yellow-orange scattered simple prismatic crystals, sometimes impaled on augite and edenite needles.

Phlogopite as thin hexagonal plates, standing on edge, as single long crystals, and rarely in groups. They are colourless, but the Fe content means they often have an orange tint.

Augite as tiny golden yellow scattered eight sided needles or crystals. Sometimes titanite crystals are found attached on the augite crystals.

Pseudobrookite as small spray groups, or singular thin black shiny bladed crystals. The thinnest crystals show as brownish or reddish tints. The crystals terminate as a flat wedge. The crystals are often less than one mm wide, and only a few mms long. Have been observed growing out of hematite crystals.

Mullite or porcelainite forms tiny sprays of acicular needles, 0.01 mms across, several mms long, square in cross section, with flat terminations. The material is rare, a faint lavender colour, associated with hematite, phlogopite, and osumilite.

Osumilite is a rare silicate, usually a lustrous black, but at Hendersons Quarry can be found as blue-green, and rarely pale yellowish, due to its low iron, and high silica content at the quarry. Hexagonal crystals to 3 mms long, nearly equant, simple prisms, translucent, rough surfaces, and flat terminations. The species is found in a narrow central zone in the quarry, rare, as single crystals or sometimes groups. Associated with hematite, mullite, and phlogopite.

Magnetite as small octahedral crystals with titanite, and quartz on thicker tridymite lined cavities. Goethite as glassy black single blocky crystals, dog tooth like crystals, radiating clusters, or iridescent mammillary. Photographs on Mindat indicate rare pyrite and marcasite is found in the quarry.

Other Regions, Features and Areas containing this locality

References