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 colour 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:
We are now able to make a little more sense of the Wikipedia entry on hydrogrossular, mentioned in the previous Post, where it states:
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 hydrogrossular is “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.
Wikipedia mentions that hydrogrossular garnet has similarities to jade. This can perhaps be seen in the way in which it can be carved – see Russell Beck’s carvings in Te Ara The Encyclopedia of New Zealand.
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.
The next Post in this series, Why are Hydrogrossular Garnets found in Nelson and Orepuki?, will consider why hydrogrossular garnets can primarily be found in two main regions in New Zealand, the Nelson area and the south-west of the South Island, some hundreds of kilometres apart.
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