This is a slightly expanded version of my letter “X” contribution to the current weekly Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils” (though I seem to be the only one regularly participating now). In the 2021 version of this Series I posted “X is for X-Figures When Veins Cross in a Stone”.
X is for Xcellent Xample of an Xtrusive Igneous Stone, a Post made on Xmas Eve
This is an extrusive igneous stone, a rhyolite, that I found on Gemstone Beach, Southland, on 5 March this year. Extrusive rocks are volcanic – they have formed when molten magma breaches the surface of the Earth. Depending on the way that breaching occurs, rhyolite can form as pumice (from a gas-rich frothy material with lots of tiny holes), ignimbrite (from compressed layers of volcanic ash), obsidian (glass-like), flow-banded (from the slow, viscous movement of lava). Stones like this one look like they could be flow-banded.
Rhyolite is the most silica-rich of extrusive rocks. It is generally fine-grained in texture, but may be porphyritic, containing larger mineral crystals in an otherwise fine-grained material. The small light-coloured crystals in this stone illustrate that. The minerals that make up rhyolite are predominantly quartz, sanidine (a high temperature potassium feldspar), and plagioclase feldspar. It is the extrusive equivalent of granite. Granite is intrusive, forming beneath the Earth’s surface, having a similar mineral composition to rhyolite. However, some textbooks point out that rhyolite’s mica is black biotite and granite’s mica is muscovite, and rhyolite’s potassium feldspar sanidine is different from granite’s orthoclase.
I first learned to recognise rhyolite from spending time on Slope Point beaches with a local rockhound, Oliver. He showed me the difference between rhyolite and petrified wood – sometimes the fine-lined flow bands in rhyolite can look like the grain in petrified wood. After that, I began to recognise rhyolite stones on Gemstone Beach (see three further specimens below). In “Gemstones” (page 35), Jocelyn Thornton has a photo of 10 Slope Point rhyolites (see photo below, bottom row, right), one of which looks very similar to this one.
For a tumble polisher like myself, the high silica content and often glassy nature of rhyolite mean it often polishes well, though the occasional tiny gas hole can interrupt the smooth surface. I have come across a small range of rhyolites on Gemstone Beach. I usually see one or two of this orange and brown flow-banded stone each fossick on Gemstone Beach but I have only collected a handful or so of the smoothest specimens.
The next Post in this Series is “Y is for YesterYear’s Yellows at Year End”. An Index for this Alphabetical Series is here.
