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.
Inventor of names
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.
Ode to a rock hound
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.