This 20th Post in this Series features nine hydrogrossular garnets I have collected from Gemstone Beach over the past couple of weeks. In general, a variety of stones have some hydrogrossular in them, some being translucent, some opaque. Colours also vary, depending on what other minerals are present. Hydrogrossular garnets usually feel waxy and they shine brightly when wet on the beach. “Grossular” comes from the botanical name for the gooseberry, and gooseberry green is one of the classical colours of a more translucent type.
Hydrogrossular garnet was first identified by New Zealander Colin Hutton in 1943, from Nelson stones. Hydrogrossular is one of 13 minerals first described from New Zealand and accepted as valid by the International Mineralogical Association (see my Blog Post “What is Hydrogrossular’s Place Among the Thirteen Minerals First Described From New Zealand?”) Hutton’s original report of his mineralogical analysis, “Hydrogrossular, a New Mineral of the Garnet-Hydrogarnet Series”, can be found here on “Papers Past”. Hydrogrossular is a type of calcium aluminium garnet, with hydroxide partially replacing what is silica in other garnets. It forms as dense masses, not nicely shaped crystals. That hydrogrossular can be found as far apart as Nelson and Southland can be attributed to the movement of the Alpine Fault and its stretching of “terranes” (floating bits of the earth’s crust that butt up against or into a continent) (see maps below). My series of Blog Posts on hydrogrossular garnets starts here.
To turn to nine of the hydrogrossular stones I have collected on Gemstone Beach recently. These first three have had a torch placed behind them to show their translucence. The first stone, a lovely brown colour, has black/silver inclusions that are common, and I’ve seen diopside and chromite identified as possible such inclusions. The second is a colourful small stone, while the third stone has white clouds within it, which I find attractive.
Among the six other stones, the first is the largest, a grey colour. The next is an opaque white stone, with a light pink blush usually attributed to thulite (manganese). Then another opaque white one is followed by a classic translucent gooseberry green. The second last has some cream colour in it, always attractive, and the last one is what I call a “porcelain type”, having fine blue lines/patches.
The main page for hydrogrossular garnets on TumbleStoneTwo is here.
The 21st Post in the “Southern Sojourn 2023” Series describes the last fossick I had at Gemstone Beach before I returned home for three weeks. The first Post in this Series is here. The Index to the Series is here.
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