What are Hydrogrossular Garnets?

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).

The Encyclopedia of New Zealand mentions hydrogrossular garnets in the following terms:

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. 

Hydrogrossular garnet, Minerals display, Southland Museum, Invercargill

Minerals display, Southland Museum, Invercargill

Southland Museum, Invercargill. Source: https://www.stuff.co.nz/national/102942792/southland-museum-and-art-gallery-building-closed-because-of-earthquake-risk

Tuatara statue, with petrified wood in background, sited outside the entrance to the Southland Museum, Invercargill

On page 34 of the excellent “A Photographic Guide to Rocks and Minerals of New Zealand” by Nick Mortimer, Hamish Campbell and Margaret Low (2011) is an entry on “Hydrogrossular”: 

Five key points made in this entry are:

1) Its chemical composition is hydrous calcium aluminium silicate.  

2) Hydrogrossular garnet hardly ever occurs as good crystals but rather as dense masses.

3) It can be found in the Nelson area as well as around Orepuki.

4) It is one of 13 minerals first described in New Zealand. 

5) It was first identified by Colin Hutton in 1943.

In the next Post in this series, Why are Hydrogrossular Stones Called Garnets?, I will look at the first two points. Later Posts include Why are Hydrogrossular Garnets found in Nelson and Orepuki? and What is Hydrogrossular’s Place Among the Thirteen Minerals First Described From New Zealand?  Also see Hydrogrossular Garnet on TumbleStoneTwo. Three significant later Posts on hydrogrossular garnets are February-March 2022 Fossicking Trip: Stone of the Day #15, Hydrogrossular Garnet from Gemstone Beach and February-March 2022 Fossicking Trip: Stone of the Day #19, Big Gemstone Beach Hydrogrossular Garnet and January 2022, Stone of the Day #20 – Brown Hydrogrossular Garnet from Gemstone Beach.  

Author: tumblestoneblog

Retired Academic, male, living in the New Zealand countryside near Whanganui with his wife as well as Jasper the dog, Fluffy the cat, Dancer and Penny, the horses, and a shed half-full of stones. Email john.tumblestone@gmail.com. View all posts by tumblestoneblog