This article appeared in the November 2026 issue of “StoneSpeak”, the Newsletter of the Southland Geological and Lapidary Club,Newsletter. The two-part version that I have posted on TumbleStone Blog has been edited and expanded. It is a companion piece to an article in the previous Newsletter, “Fifteen Minerals First Identified in New Zealand”. An Index to my reviews and articles for “StoneSpeak” can be found here. The Southland Club’s website is here and its Facebook Page is here.
In last month’s newsletter, I described 15 minerals first named in New Zealand. This month, I will describe four rocks that were given their names in New Zealand. Three of them have been formally accepted internationally – dunite, rodingite and ignimbrite. The other rock named in New Zealand – goodletite – has not been formally adopted though is widely used informally in the country. Both articles have, as their starting point, an entry by Simon Nathan in Te Ara: The Encyclopedia of New Zealand titled “Rock and Mineral Names”. But significantly extra material has been added.
A “rock” is a solid substance made up of one or more minerals. Quartz rock, for example, consists of only one mineral, quartz (silicon dioxide). By contrast, granite (see photo below) is a rock made up of three main minerals, quartz, feldspar and mica. Rocks can be complicated, their composition varying, sometimes quite significantly. In granite, the proportion of quartz in its make-up can vary between 20% and 60%. The feldspar could be white (plagioclase feldspar) or pink (alkali orthoclase feldspar). And there are different kinds of mica that could be present, mainly biotite (dark coloured) or muscovite (light coloured). A granite rock or stone may also contain small bits of other minerals, especially black hornblende.
DUNITE ROCK
Dunite was named by the German geologist, Ferdinand von Hochstetter (1829-1884), in 1859 in Nelson (photo below). In 1857 he had been appointed geologist for the around-the-globe expedition of the Austrian ship “Novara”. When it arrived in New Zealand in December 1858, von Hochstetter decided to stay, and he spent the next nine months in the country. In 1859, the government of New Zealand employed him to make the first geological survey of the country. In his book about his geological explorations (published in 1867), he wrote: “On approaching the harbour of Nelson from the high sea, a bare mountain ridge is seen rising to a height of about 4000 feet which owes its name ‘Dun Mountain’ to the rusty-brown colour of its surface. It consists of a very peculiar kind of rock, of yellowish-green colour when recently broken, but turning rusty-brown on the surface when decomposing. The mass of the rock is olivine, containing fine grains of chromate of iron interspersed; it is distinguished from serpentine, for which it was formerly taken, especially by its greater hardness, and its crystalline structure. I have called it Dunite” (Te Ara, Encyclopedia of NZ).
Dunite is a coarse-grained plutonic rock, forming deep under the surface, consisting of more than 90% olivine, a green mineral. It also contains very minor amounts of other minerals such as pyroxene, hornblende, chromite and magnetite. Dunite is the peridotite with the highest amount of olivine. Peridotites are rocks consisting of more than 40% olivine. Dunite and other peridotites are considered the major constituents of the Earth’s mantle above a depth of about 400 kilometres. Olivine is olive green when fresh but weathers to a dull yellowish-brown or dun colour in temperate climates.
Dunite is one of the world’s major sources of chromium ore. It alters easily to serpentine.
Significant areas of dunite can be found not only in the Nelson area but also in Fiordland and Southland, thanks to the Alpine Fault’s movement. Red Mountain in the very north of Fiordland National Park owes its distinctive red colour to the oxidisation of the heavy metals within its dunite and other peridotite rocks. There is a Dunite Quarry near Bluff, where the dunite is said to be 98% to 99% olivine.
Good sources on dunite include Sandatlas, Alexstrekeisen, University of Canterbury and Geologyin.
RODINGITE
Rodingite was named by the New Zealand geologist Patrick Marshall in 1911. Marshall was born in England in 1869, his family moving to New Zealand in 1876. At Canterbury College he studied geology and biology. He completed a doctoral thesis on the Auckland volcanic field in 1900 and in 1901 became a lecturer in geology at the University of Otago. Marshall was appointed Professor of Geology there in 1908 but resigned in 1916 to go on to other work (more on this below when we look at ignimbrite). During his time at Otago, in 1911, Marshall proposed the term rodingite for a type of rock that had also been found in the Dun Mountain area near Nelson in the 19th century. Unusual coarse-grained bands or dikes had been seen cutting across serpentinite, and the rock in them had been given a variety of names. In New Zealand Geological Survey Branch Bulletin 12 (New Series, 1911), “The Geology of the Dun Mountain Subdivision, Nelson”, co-authored with James Mackintosh Bell and Edward de Courcey Clarke, Marshall suggested the rock be called rodingite, naming it after the district’s Roding River (see pages 31 to 35 in the publication). The term was eventually adopted internationally, and rodingite is found in many places outside of New Zealand, as widely apart as Pakistan and Norway.
Rodingite is described as an extremely hard, tough, homogeneous rock that lacks foliation (banding). The main minerals in rodingite are hydrogrossular garnet, plagioclase feldspar and actinolite. Other minerals that could be present include diopside, vesuvianite, epidote, prehnite, and chlorite. Rodingite arises from the recrystalisation of rocks like gabbro and basalt, and various sandstones and mudstones – these have been saturated with hot fluids that chemically change them. These hydrothermal fluids are calcium-rich and silicon-deficient, often arising as peridotite turns into serpentinite (a process called serpentinisation). When these fluids react with rocks embedded inside the serpentinite, they strip away silica while injecting calcium and aluminium. The texture of the resulting rodingite can vary depending on the initial source. It is thought that the hydrogrossular stones on Gemstone Beach have come from rodingite formed from igneous rocks.
Online information sources on rodingite tend to be limited. Among the most useful are Alexstrekeisen, Mindat, Wikipedia and Facts.net.
IGNIMBRITE AND GOODLETITE
These two other New Zealand rocks are examined in Part Two.
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