In December I moved house from Cambridge to Whanganui. Everything was shifted by Christmas, and most of the unpacking has now been done.
My new home, a few kilometres outside of Whanganui
Ollie the cat surveys new territory
The lawn and garden
Another view of the lawn and garden
Our two horses, Dancer and Penny, in their new home
Looking out at the back of the property towards the sea, a kilometre away
However, the shed I will use for tumble polishing is still in the process of preparation.
Early in January, after the moving and unpacking, I quickly put together a calendar for 2020, using Diamond Photo online. For each month, the Calendar has a set of photos, just slightly smaller than an A4 page, with another A4 size page below it containing the month’s dates. The main internationally-recognised holidays are noted, along with some of the main New Zealand ones.
Front Cover of Calendar – View east along Gemstone Beach, from near the Waimeamea River Mouth
January, Riverton scenes
December, Sam’s stones (Sam is my sister’s grandson)
I was very pleased with the quality of the finished product.
If you want to buy one for yourself, email me at email@example.com. It will cost you $25 (postage included, in New Zealand).
The following are the sets of photos for each month in the Calendar – these are all photos I took in 2019, most but not all of which appeared on TumbleStone blog last year:
January – Riverton scenes
February – Close-ups of Gemstone Beach stones
March – Fossil worm casts in rocks at Tihaka Beach, near Riverton
April – Hydrogrossular garnets
May – Gemstone Beach scenes
June – Fossil worm cast stones
July – Larger stones found on Gemstone Beach and the beach near McCracken’s Rest
August – Polished stones from Gemstone Beach
September – Henderson Bay scenes, Riverton
October – Polished stones, collected by my sister Helen on Gemstone Beach
November – Close-ups of fossil worm cast traces
December – Polished stones, collected by Sam, my sister’s grandson, on Gemstone Beach.
At the end of my first Post in this series, What are Hydrogrossular Garnets?, one of the questions I raised was why hydrogrossular stones are found in New Zealand primarily in two regions more than 400 kilometres apart, in the Nelson area as well as around Orepuki, at the top and bottom of the South Island.
I have come to associate hydrogrossular stones with Gemstone Beach at Orepuki because they are not found anywhere else on the southern coast. I have heard third or fourth hand of someone who found one on a Riverton beach but the point of that story was how unusual this was. I have spent more time on Riverton beaches than Gemstone Beach but have found hydrogrossulars only on Gemstone. Sources of information such as Te Ara The Encyclopedia of New Zealand often mention only Gemstone Beach as where you can find them in New Zealand.
When I undertook more careful research on hydrogrossular garnets it came as a surprise that the area around Nelson was treated as a key source area. In the entry on “Hydrogrossular” in “A Photographic Guide to Rocks and Minerals of New Zealand” by Nick Mortimer, Hamish Campbell and Margaret Low (2011), it is noted that the world’s first occurrence of the rock was described from the Nelson-Dun Mountain area by Colin Hutton in 1943 (more on this in a later Post). It does go on to note that hydrogrossular can be found on Gemstone Beach as well as in the streams and rivers east and south of Nelson city.
Jocelyn Thornton, in “The Field Guide to New Zealand Geology” (2003), reports that “grossular garnet” can be found in the Dun Mountain area, “found in translucent green masses that can be polished” (page 69), as well as in the Roding, Lee and Maitai Rivers in Nelson (see first photo below for Thornton’s geological map of Nelson area). She points out that rodingite, a rock named after the Roding River, a mixture of grossular and diopside or diallage, can also be found in Cascade Creek in the Eglinton Valley near Milford Sound (see the second photo below, from Te Ara The Encyclopeida of New Zealand). “From there the grossular garnet makes its way through the lakes and down the Waiau [River], being tumbled into the rounded pebbles that can be found with a little effort on Orepuki Beach” (page 70). A 1969 research paper by J. G. Williams at the Department of Geology at the University of Otago had identified hydrogrossular in rocks in the area around Cascade Creek in the Eglinton Valley. Thornton’s book includes a photo of hydrogrossular from the Nelson and Orepuki areas (Plate 1E, opposite page 136 – see third photo below). Riverton Museum has a sample of hydrogrossular from the Eglinton Valley (see fourth photo below). In a 2018 “Southland Times” column (second item), local journalist Lloyd Ester reports:Southland’s Gemstone Beach has the best assortment of unusual pebbles in New Zealand. A combination of ocean currents, shape of the coast and the proximity of the Waiau River – the source of many of the stones – means that the rarities are concentrated along a short strip of beach. The best known of the “gemstones” are grossular garnets which are distinguished by their gloss and weight.” Nevertheless, the Waiau River is relatively young, in geological time. For instance, Thornton presents a map of what the Western Southland-Fiordland area would have looked like about 50 million years ago, the Waiau Valley being under sea (see fifth photo below). It’s possible that some of the hydrogrossular rock that became pebbles on Gemstone Beach were swept there by ocean tides millions of years ago.
Jocelyn Thornton (2003), “The Field Guide to New Zealand Geology”, page 67
Jocelyn Thornton (2003), “The Field Guide to New Zealand Geology”, Plate 1E
Hydrogrossular garnet from the Eglinton Valley, Riverton Museum
Jocelyn Thornton (2003), “The Field Guide to New Zealand Geology”, page 155 – I have circled in red the approximate location of present-day Orepuki
To return to the question being examined in this Post: Why are hydrogrossular stones found in New Zealand primarily in two regions more than 400 kilometres apart, at different ends of the South Island? This distribution is not random but is in fact part of a much larger pattern. The first map below, found in a Google image search, shows that the sequence of terranes in Fiordland, Southland and Otago is repeated in Nelson and Marlborough. A “terrane” is a floating bit of the earth’s crust that butts up against a continent (New Zealand is part of the Zealandia continent, much of which is under water – see second map below). This disjunction in the terranes leads to the idea of there being western and eastern geological provinces making up the South Island (see third map below, from a 2013 article in “Gondwana Research”). New Zealand consists of at least 10 terranes (see pages 41-42 of Peter Ballance’s, 2017, “New Zealand Geology: An Illustrated Guide” for a list and description). But these have been partially rotated and then split by the Alpine Fault. As shown in the map by Jocelyn Thornton (fourth map below), the rocks (terranes) demonstrate that Northwest Nelson and Fiordland were once together. The two sources of hydrogrossular garnets may be 400 kilometres apart now but they are in the same terrane and 25 million years ago were in the same locality.
Jocelyn Thornton (2003), “The Field Guide to New Zealand Geology”, page 34
The Alpine Fault runs for about 600 kilometres along almost the entire length of the South Island. It is a segment of the boundary between the Pacific Plate and the Australian Plate (see map below, from Wikimedia). The Southern Alps have been uplifted on the fault over the last 12 million years in a series of earthquakes. However, most of the motion on the fault is sideways, with the Tasman district and West Coast moving north and Canterbury and Otago moving south.Horizontal movement of the Alpine Fault is about 30 metres every 1,000 years, which is very fast by global standards. Each time it has ruptured, it has also moved vertically, lifting the Southern Alps in the process. GNS Science estimates that in the last 12 million years the Southern Alps have been uplifted by 20 kilometres, and it is only the fast pace of erosion that has kept their highest point below 4,000 metres. Glaciers and rivers have removed the rest of the material and spread it out across the lowland plains or onto the sea floor (and beaches!). The Alpine Fault was not recognised until 1941 because the area was rugged and isolated, and earlier generations of geologists did not have the advantage of having an aerial view.
In conclusion, hydrogrossular garnets are found primarily in two regions in the South Island more than 400 kilometres apart because of the way that the Alpine Fault has split whole terranes of rocks and moved them sideways, south to north.
The next Post in this series looks at how hydrogrossular garnet is one of 13 minerals that were first described in New Zealand.
Information on the four photos at the top of this Post:
First photo: A sample of hydrogrossular garnet from the Eglinton Valley, in a drawer of rock samples in the Riverton Museum, Te Hikoi. Second photo: A compartment of hydrogrossular stones and at least one quartz stone, from my own collection of polished stones. Third photo: Handful of hydrogrossular garnets from Gemstone Beach, on page 155 of “The Story of Murihiku/Southland – A synopsis: An overview of Southland’s Heritage” by Russell Beck, Cathy Macfie and Lloyd Esler, April 2007 https://docplayer.net/57903929-The-story-of-murihiku-southland.html Fourth photo: Rodingite rock sample, from Te Ara The Encyclopedia of New Zealand https://teara.govt.nz/en/photograph/9077/rodingite
These four photos are of hydrogrossular garnets discovered on Gemstone Beach, near Orepuki (the last two photos are of the two sides of the same stone). They don’t look like garnets, do they? We are used seeing garnets like this:
The word “garnet” comes from the 14th‑century Middle English word “gernet”, meaning “dark red”. It is thought to be derived from the Latin “granatus”, from “granum” (meaning “grain, seed”), and possibly a reference to “pomum granatum”, the pomegranate, a plant whose fruits contain abundant and vivid red seed covers which are similar in shape, size, and color to some garnet crystals. In fact, garnets can be found in a wide range of colours, although red is the most common.
In a previous Post, What are Hydrogrossular Garnets?, it was noted that the chemical composition of hydrogrossular garnets is hydrous calcium aluminium silicate and that hydrogrossular garnets hardly ever occur as good crystals but rather as dense masses. This Post aims to explain why hydrogrossular garnets are called garnets and why they are not crystals.
Stones are made up of at least one mineral, often more. A mineral is a solid chemical compound that occurs naturally in pure form. These ideas have led to the view that rocks are to be officially named and identified by means of a scientific description of, among other things, the chemical make-up of their constituent minerals. (Other things that help to identify minerals include hardness, “specific gravity” meaning density, “lustre” or how it reflects the light, “cleavage” meaning its tendency or not to break along smooth planes parallel to zones of weak bonding, and colour.) So what is a “garnet” or not depends on its chemical make-up. I am not a chemist so my understanding, as presented here, is based on what sense I can make of a range of material, some of it quite technical.
In the section on “Garnet” in Geology.com, it is noted that Garnet is the name used for a large group of rock-forming minerals: These minerals share a common crystal structure and a generalized chemical composition of X3Y2(SiO4)3. In that composition, “X” can be Ca, Mg, Fe2+ or Mn2+, and “Y” can be Al, Fe3+, Mn3+, V3+ or Cr3+.
Click on the Table below to see how different types of garnets have different chemical compositions in which the X and Y position for chemical elements vary:
The Note on this Table (above) points out that “the compositions listed…are for end members of several solid solution series”. In other words, there are gradations in the series of stones we call garnets and when we group them we are breaking up a continuum, creating “end members” at the start and end of each group. “Garnets are a related group of minerals whose chemical compositions can vary continuously from one to the other” (p. 160 in Busbey et al., 2013, “Rocks & Fossils“). There is actually a wide range of types of garnets that are grouped in various ways. Wikigempedia reports that the garnet family is divided into two sub-groups, Pyralspite which is Calcium-free garnet and Ugrandite which is Calcium-rich garnet. Almandine (iron-aluminium silicate), Pyrope (magnesium-aluminium silicate), and Spessartine (manganese-aluminium silicate) are the main members of Pyralspite sub-group, and Grossular (Calcium-aluminium silicate), Andradite (calcium-iron silicate) and, Uvarovite (calcium-chromium silicate) are the members of Ugrandite sub-group.
The excellent article on Garnet in Wikigempedia presents a table which summarises the common properties of all garnets as well as the variations in chemical formulae for three types of garnets:
Hydrogrossular is a calcium aluminium garnet series (formula: Ca3Al2(SiO4)3−x(OH)4x, with hydroxide (OH) partially replacing silica (SiO4)). The endmembers of the hydrogarnet family (grossular, hibschite, and katoite) depend on the degree of substitution (x): grossular: x = 0 hibschite: 0.2 < x < 1.5 katoite: 1.5 < x < 3.
The latter part of this statement is in effect saying that there are slightly different types of hydrogrossular caused by varying degrees of replacement of silica by hydroxide in the stone.
The Geology.com article points out that garnets in general have a “vitreous [glassy] luster, a transparent-to-translucent diaphaneity [transparency], a brittle tenacity, and a lack of cleavage.” Furthermore, they can be found as individual crystals, stream-worn or beach-worn pebbles, granular aggregates, and/or massive occurrences (“massive” meaning lacking internal crystalline structure).
Wikipedia explains that hydrogrossularis “found in massive crystal habit, sometimes grown in with idocrase [another mineral]”. In terms of its transparency, hydrogrossular is “translucent to opaque”, and can be found in the colours of green to bluish green, pink, white, and gray. The cause of the green color is chromium, and possibly iron. Pink hydrogrossular is caused by the presence of manganese. Hydrogrossular may also have dark gray to black small inclusions.
I am unsure why hydrogrossular garnets are not clear translucent crystals like other garnets can be. I have not found any explanation for that. My suspicion is that it has to do with their water content, the “hydroxide” component. This might “cloud” the stones in their composition.
I first came across reference to hydrogrossular garnets when gathering information online about the stones that could be found on Gemstone Beach, Orepuki, on the south coast of New Zealand’s South Island. Some brief descriptions of Gemstone Beach include comments along the following lines: “Semi-precious gems such as garnet, jasper, quartz and nephrite can often be found on the beach. A few hours beachcombing could easily yield gems such as hydrogrossular, jasper, fossil worm casts and the elusive sapphire” (quoted from information about the Riverton–Aparima South Coast Heritage Trail. See also Nature’s Edge: Tuatapere and the mindat.org entry on Gemstone Beach.) The source of this description could be a Heritage Trail sign that, to my knowledge, is no longer at Gemstone Beach (I don’t recall having seen it on any of my visits).
Calcium-rich garnet is called grossular. A red form, found in South Westland, is known as hessonite. Another variety, containing some water, is called hydrogrossular and was first identified at the Roding River near Nelson. It is also found on the beach near Orepuki in Southland. Rounded lumps of pale green hydrogrossular take a good polish and have been used for jewellery. Hydrogrossular pebbles, being heavy and exceptionally hard, were used by Southland Māori as hammer stones for the making of stone implements
In many ways, all this information raised more questions for me than it answered. What does a hydrogrossular garnet look like? Is it a valuable gemstone, like other garnets? Why is it a garnet? How often can these stones be found on Gemstone Beach?
For a while, I mistakenly referred to them as “hydroglossular” (“…gloss…” not “…gross…”), thinking that because they were likely to be shiny they would be glossy. On my first few trips to Gemstone Beach, I decided that there was a particular stone that was probably a “hydroglossular”, even thought it seemed quite dull. It was a grey stone that was kind of a dull quartz-like thing, looking like there was water within its fabric (the “hydro” part).
For a brief time, I thought this sort of stone could be a hydrogrossular
I collected some and tried polishing them but they were very unremarkable stones.
So what does “hydrogrossular” mean? Wikipedia gives an answer that refers to the physical-chemical make-up of the stone:
Hydrogrossular is a calcium aluminium garnet series (formula: Ca3Al2(SiO4)3−x(OH)4x, with hydroxide (OH) partially replacing silica (SiO4)). The endmembers of the hydrogarnet family (grossular, hibschite, and katoite) depend on the degree of substitution (x): grossular: x = 0 hibschite: 0.2 < x < 1.5 katoite: 1.5 < x < 3. Hydrogrossular is a garnet variety in which a Si4+ is missing from a tetrahedral site. Charge balance is maintained by bonding a H+ to each of the four oxygens surrounding the vacant site.
So the “hydro”refers to “hydroxide”. Consulting Wikipedia again, hydroxide consists of an oxygen and hydrogen atom held together by a covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. (I will comment on other aspects of the above physical-chemical description in the next Post in this series.)
If you look up “grossular” in Wikipedia, you discover that “the name grossular is derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia”.
It wasn’t until someone I met on Gemstone Beach about a year ago showed me some hydrogrossular stones he had just picked up, and actually gave me a couple,that I realised what they looked like and, just as importantly, what they felt like. They feel waxy, not the same kind of smoothness as a quartz stone, which is cool to the touch in contrast to the more “warm” feel of a hydrogrossular.
The Riverton Museum, “Te Hikoi”, has a small room which displays stones from the area, linked to an exercise of stone collecting set up for children on holiday. It includes a few hyrdrogrossular stones. With the permission of Museum staff, I took some photos of the display and of the drawers of rock samples in this room.
Riverton Museum, children’s rock collection display
Riverton Museum, children’s rock collection display
Hydrogrossular garnet stones in Riverton Museum, children’s rock collection display
Riverton Museum, drawers of rock samples
In one of the drawers in the rock display room, Riverton Museum
Hydrogrossular garnet, Riverton Museum
The Southland Museum in Invercargill, now closed due to problems with Earthquake strengthening, had a Minerals display which included a non-smoothed rock of hydrogrossular garnet.