This is a slight expansion of my letter “R” contribution to the current weekly Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “R is for Riverton Rocks Red Rock”.
R is for Rhyolite – Spherulitic Rhyolite from Slope Point
I found this stone in April 2023 on a small Slope Point beach, one which has a lone holiday home perched above it. It’s about a kilometre west of Slope Point itself and the tourist carpark. A range of types of rhyolites can be found in this area (some good examples can be found in this Post). Many of them are colourful and contain interesting patterns, such as flow-bands, with small crystals in them. One kind of rhyolite here is spherulitic, also known as “chrysanthemum rhyolite”.
Rhyolite has a very high silica content and is the volcanic equivalent of granite. It is formed from magma that is extruded from a volcanic vent to cool quickly on the surface rather than slowly below ground. Generally fine-grained in texture, rhyolite may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matrix. According to John Farndon’s (2015) book, “Illustrated Guide to Rocks & Minerals”, spherulitic rhyolite originates in “trapped gas pockets of volatile vapours” which develop into “balls of radiating needle-like crystals of quartz and feldspar” (page 64) . Useful online sources on rhyolite are Geology.com and Alex Strekeisen while Mindat has photos of a variety of rhyolites, including spherulitic ones.
The next Post in this Series is “S is for Scoria”. An Index for this Alphabetical Series is here.
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