This is the eighth in this Series reproducing my recent regular Posts in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. The first Post in this Series is here.
Tuesday 25 May 2021: This is the day I made a Post in the Group’s Alphabetical Series – “D” is for “Dendrites” – see this Post.
Wednesday 26 May 2021: Four small quartzite stones from Kakanui, recently tumble-polished. Between 2.5 cms and 3.5 cms in size, varying colours.
Some research over the last couple of days has led me to the following: Quartzite is usually a metamorphosed sandstone. The intense heat and pressure of metamorphism causes the quartz grains to compact and become tightly intergrown with each other, resulting in a very hard and dense quartzite – see photo of quartzite under a microscope below (from https://www.alexstrekeisen.it/english/meta/quartzite.php – the very tightly packed quartz grains in this photo range in colour from white to gray to black depending upon their optical orientation).
Geology.com reports that quartzite is usually white to gray in colour – if stained by iron, it can be pink, red, or purple, while the presence of other minerals can cause quartzite to be yellow, orange, brown, green or blue (see https://geology.com/rocks/quartzite.shtml). In her page on beach pebbles from Moeraki-Kakanui in “Gemstones”, Jocelyn Thornton includes a “Quartzite from Central Otago”, and the photo is of a stone very similar to Stone 3 (see photos above) – see page 34 here. On page 20, “Other Varieties of Silica”, Thornton notes: “Quartz sands cemented by silica make an extremely hard and durable quartzite which is found on some hillsides in Central Otago and washed down rivers and out along the coasts. These quartzites have cloudy patterns and take a very good polish.”
Thursday 27 May 2021: Three small recently tumbled stones from a beach near Slope Point in the Catlins, Southland. 2 cms to 4 cms in size. Jocelyn Thornton writes of Slope Point: “On the Southland coast between Waipapa Point and Haldane there are a few pebbly beaches visited by fishermen and rock collectors. The cliffs contain layers of conglomerates with pebbles which weather out and collect on the beaches. At first glance they appear dark, but closer examination reveals a multitude of subtle colours and patterns” (page 35 here).
Friday 28 May 2021: Small Gemstone Beach (Orepuki, Southland) polished stone, 3.5 cms long, collected earlier this year. Came out of a tumble barrel yesterday. Striking green lines.
[A comment on this Post from a resident of Scotland: “Love those epidote patterns, John. I am amazed that we have conglomerate cobbles from a Mississippi type delta down the Angus coast of Scotland that mirror the ones you post. These originated in Greenland/ N America poles apart”.]
The next Post in this Series is here.
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