Almost all the rocks and minerals in New Zealand are found elsewhere in the world, and have been given internationally accepted names. But New Zealanders also have names for a small number of rocks that are distinctive or important.
In the chemical sense, a mineral is a naturally occurring inorganic substance with a definite composition and structure. Quartz, with composition SiO2, is one of the most common minerals. All rocks are made of two or more minerals.
New Zealand does not have a designated national rock, but if one was ever chosen it would have to be greywacke. This drab grey stone is found everywhere in New Zealand – on the mountains, in the rivers, on the beaches. It consists of layers of hard, muddy grey sandstone alternating with thinner layers of darker mudstone (argillite). Technically the term greywacke refers to the sandstone (wacke is a German name for a type of sandstone), but it is also used as a general term for the entire rock.
Greywacke (Grauwacke) was first used in the 18th century to describe rocks in the Harz Mountains of Germany. Ernest Dieffenbach, a German scientist who travelled widely in New Zealand between 1839 and 1841, was the first person to use it for local rocks. English geologists regarded greywacke as an uncouth foreign term, but it was adopted in Scotland. Archibald Geikie’s Text-book of geology, published in 1903, gave descriptions of greywacke, and helped persuade New Zealanders that it was an appropriate term for their most widespread rock.
Greywacke is pronounced ‘greywacky’. Children’s author Lynley Dodd, known for her rollicking words and rhythms, used it in a picture book about a cat named Slinky Malinky, who spends his nights hobnobbing with his friends Greywacke Jones, Butterball Brown and the rest of the gang.
In the 1960s some geologists argued that the term greywacke was vague and imprecise. A subcommittee of the Geological Society recommended that it be dropped, but this was widely ignored. The term is possibly used more widely in New Zealand than anywhere else in the world.
The terms papa or papa-rock are used for the widespread soft, blue-grey mudstone or muddy sandstone.
Although the word papa is of Māori derivation (meaning earth), by the mid-19th century it was being used by Pākehā writers referring to mudstone, and in 1905 it was nicely summed up: ‘The Papa Rock, of which many of the cliffs in the bush country of New Zealand are formed, is really a very hard, blue clay … It lies in distinct strata, and when the wet penetrates to one beneath, the surface of this latter becomes as slippery as glass’. 1
The rock known as papa was deposited on the sea floor over the last 15 million years, then later uplifted. It is relatively soft because it has never been deeply buried or compacted. For engineering purposes, papa is classified as ‘soft rock’ or ‘engineering soil’ because it has the physical properties of unconsolidated soil rather than rock.
Pounamu is the Māori name for New Zealand jade (also called greenstone), prized for jewellery, tools and weapons, and widely traded. Although the term greenstone was generally used by Europeans, it is gradually being dropped in favour of pounamu.
In New Zealand, pounamu is found only in the South Island, and the name refers to two different minerals. Nephrite, the more common form, is a calcium magnesium silicate belonging to the amphibole group. It is found in the area close to Hokitika on the West Coast, and other sites. Bowenite, found only at the entrance to Milford Sound, is an iron magnesium silicate that is an unusual, translucent form of serpentine.
This hard, flinty form of argillite, from D’Urville Island and other South Island localities, was valued by Māori for making adzes, and widely traded. The name was originally used by archaeologists.
Adzite occurs along narrow zones between argillite and serpentinite. The argillite has been soaked in hot fluids which have reacted with the rock – a process called metasomatism – to impart the flinty nature to the adzite.
Dunite, rodingite and ignimbrite are rock names used internationally but first recognised and described from New Zealand. A fourth rock, goodletite, has only been recognised within New Zealand.
When he visited the chromite workings near Nelson in 1859, Austrian naturalist Ferdinand Hochstetter recognised that the rock was unusual:
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. 1
Dunite is now the name for rocks composed almost entirely of olivine. It is thought to originate in the mantle, deep within the earth. It occurs only in narrow tectonic zones where mantle rocks have been pushed up into continental crust.
Geologist Patrick Marshall had a talent for identifying rocks and minerals. He proposed the rock names rodingite and ignimbrite as well as identifying a new mineral he called tuhualite. All these names are still in use today.
Geologists who examined rocks in the Dun Mountain area in the 19th century recognised unusual coarse-grained bands or dikes cutting across serpentinite. These were given a variety of names. In 1911 geologist Patrick Marshall proposed the term rodingite (after the Roding River), and the name has subsequently been adopted internationally. Rodingite mainly consists of two calcium silicate minerals: hydrated lime garnet (for which mineralogist Colin Hutton later proposed the name hydrogrossular) and pyroxene.
Marshall considered that rodingite was a distinct rock that crystallised from a magma, but later investigators agree that it is a hydrothermally altered rock.
The origin of the widespread volcanic rocks that blanket the central part of the North Island was long debated. However, early observers recognised that they were composed of ash and pumice fragments. Similar rocks in the western USA had been called ‘ash-flow tuffs’ or ‘welded tuff’. In 1932 Patrick Marshall coined the name ignimbrite – derived from Latin ignis (fire) and imber (shower). This has gradually gained acceptance. He imagined that ignimbrites were deposited from immense clouds of intensely heated ash and pumice, which would today be called pyroclastic flows.
New Zealand scientists have played a major part in studying ignimbrites – deposits of some of the world’s largest volcanic eruptions.
Here is an extract from a student poem about William Goodlet, after whom the ruby rock goodletite is named:
He’s the boy who found out rubies
On the west coast of this island,
Found the new stone,
The green matrix,
Goodletite we’ll always call it.
From Tasmanian exhibitions
Come gold medals to our Wullie,
For his minerals awarded.
2
Rare boulders of a beautiful greenish-grey rock containing ruby and sapphire (corundum), found in glacial gravels near Hokitika, have been informally known as goodletite. Despite numerous searches, the rock has never been found in the place where it was formed. The rubies are not good enough quality to be regarded as gems, but the rock is prized by collectors.
Miners originally discovered the ruby rock. On a visit to the West Coast, around 1892, William Goodlet, a laboratory assistant at the University of Otago, obtained a sample. It was later described by G. H. F. Ulrich, director of the Otago School of Mines. The name goodletite was never formally proposed, but is now in common usage.
Thirteen minerals, all named after local features or people, have been named from New Zealand, and most have since been identified overseas. Seven of the mineral names are derived from Māori place names from where the minerals were first discovered. Four are modifications of existing mineral names, and two are named after local mineralogists.
To avoid duplication, proposed new mineral names need to be approved by the International Mineralogical Association. Proposals are checked to ensure that a mineral is indeed new. The minerals listed below have all been accepted by the association.
In 1886 James Hector (a geologist) and William Skey (an analyst) described taranakite – the first new mineral to be identified in New Zealand. It is a fine-grained, cream phosphate, originally found in veins cutting volcanic rock in the Sugar Loaf Islands near New Plymouth in Taranaki.
Taranakite forms by a chemical reaction between bird droppings (guano) and weathered volcanic rock. It has since been found in coastal localities, in New Zealand and overseas, where bird droppings accumulate.
This natural nickel-iron alloy was originally found as water-worn grains in the Gorge River, South Westland. It was identified by William Skey in 1886, and traced back to serpentinite in the Cascade Valley. It has since been found in serpentinites throughout the world.
The name is slightly misleading as the mineral does not occur in the Awarua River or Awarua (Big) Bay.
The British aviator and yachtsman Francis Chichester joined an expedition organised by Patrick Marshall to look for tuhualite on Mayor Island as cook. His culinary standard was not high, and he recalled:
One day I grilled some steaks from a 150-lb swordfish; I must admit they were pretty tough. We had two professors in the party, and they had a dispute; one of them got so angry that he picked up a loaf of bread to hurl it at the other. He was so furious that when he drew back his hand to hurl the loaf it flew out backwards; when his hand came forwards there was nothing in it. I thought that this was the funniest thing I had seen for years, and doubled up with laughing. Whereupon they both turned on me and said that it was entirely my fault; that my bad cooking had upset their livers. 1
Unique to New Zealand, tuhualite was identified by geologist Patrick Marshall in 1932. It is a rare constituent of the distinctive alkali rhyolites of Tūhua (Mayor Island) in the Bay of Plenty. It has also been recorded in volcanic ash on the Coromandel Peninsula, derived from Tūhua. Tuhualite occurs as isolated, tiny violet to deep purple crystals, up to 0.25 millimetres long.
In 1943 mineralogist Colin Hutton gave the name hydrogrossular to a calcium-rich variety of garnet that is the main constituent of the rock rodingite, also named from New Zealand. As the name indicates, it contains some water. Recent research has shown that most of what was previously described as grossular garnet contains some water, and is better described as hydrogrossular.
Hydrogrossular ranges from yellowish brown to pale green. Rounded pebbles are found on some Southland beaches, and it has been used for carving.
As part of a government-funded search for radioactive minerals in the 1940s, Colin Hutton undertook a detailed survey of beach sands in South Westland. He identified a rare, highly radioactive mineral which he described as a uranium-free variety of thorite. It was recognised as a distinct mineral by A. Pabst, who named it huttonite. It is recognised worldwide as a rare radioactive mineral.
Exploration for geothermal steam in the 1950s led to detailed studies of the minerals and rocks in thermal areas. Alfred Steiner, a pioneer geothermal scientist, discovered and named wairakite in hydrothermally altered drill cores at Wairākei. Wairakite occurs as veinlets and cavity fillings. It has subsequently been found worldwide.
Most wairakite occurs as microscopic crystals, but well-crystallised specimens have been found inside casings in a steam well.
Named after the Wairau Valley in Marlborough, this is a natural iron-cobalt alloy. It occurs as microscopic, scattered grains in serpentinite, often alongside awaruite. The grains rarely exceed 5 microns (0.005 millimetres) in diameter. Wairauite is the first New Zealand mineral to be identified by electron microprobe – the only possible means of analysis.
A manganese-rich patch of chert and carbonate on the south Otago coast, near the mouth of the Akatore River, was found to contain tiny crystals of a fibrous yellow mineral which was hard to identify. In 1971 Peter Read and Tony Reay (geologists from the University of Otago) recognised this as a new manganese silicate mineral, which they named akatoreite. Although originally known only from the locality, it has since been recognised in Sweden, and is likely to occur elsewhere with manganese-rich rocks.
A poorly cemented calcareous rock, informally called ‘beach limestone’, occurs at two places on the shore of Browns Island (Motukorea) in the Waitematā Harbour. In 1977 Kerry Rogers and colleagues from the University of Auckland analysed the fine-grained rock and recognised a new mineral, which they named motukoreaite. It occurs as a boxwork of tiny crystals, each about 3 microns (0.003 millimetres) across.
Motukoreaite has been recognised elsewhere as a low-temperature alteration product of volcanic glass.
A new iron-rich variety of tourmaline found in a coarse-grained granitic rock on Cuvier Island off Coromandel Peninsula, was collected and analysed by J. D. Grice and G. W. Robinson in 1989. This mineral is part of the uvite group, and as it is rich in iron (Fe), it was named feruvite.
A small area of manganese-rich rock at Watsons Beach on the south Otago coast (about 3.5 kilometres south of where akatoreite is found) contains brown patchy aggregates that include tiny brownish-yellow fibrous crystals. This new manganese mineral was described by Japanese mineralogists Teruhiko Sameshima and Yosuke Kawachi in 1991, and named for Professor Douglas Coombs of Otago University.
In the 1990s, crystals from the Hokonui hills, east of Gore, were analysed by electron microprobe, a new technique for microscopic analysis. Previously the crystals had been identified as celadonite, a well-known mineral in altered volcanic rocks. Mineralogists knew that it had varying proportions of magnesium, iron and aluminium. However, the microprobe revealed two new minerals, which were named ferroceladonite and ferroaluminoceladonite. The crystals are extremely small: 1–2 microns thick (there are 1,000 microns in a millimetre).
Grapes, Rodney, and Ken Palmer. ‘(Ruby-sapphire)-chromium mica-tourmaline [goodletite] rocks from Westland, New Zealand.’ Journal of Petrology 37, no. 2 (1996): 293–315.
Railton, G., and W. A. Watters. Minerals of New Zealand. Bulletin 104. Wellington: New Zealand Geological Survey, 1990.
Reed, J. J. ‘Mineralogy and petrology in the New Zealand Geological Survey 1865–1965.’ New Zealand Journal of Geology & Geophysics 8, no. 6 (December 1965): 999–1,087.