The following are my Posts for “M” and “N” in the alphabetical series of a Facebook Group I belong to, “New Zealand Lapidary, Rocks, Minerals, Fossils”. The first Posts in this Series can be found here.
“M” is for “Muscovite Mica” from Joyce Bay, Charleston. Thirty kilometres south of Westport is the town of Charleston and its small twin bays, Constant Bay and Joyce Bay. Constant Bay was named after the ketch “Constant” whose Captain, Charles Bonner, in the mid-1860s managed to squeeze the boat into the tiny bay, bringing supplies for the town that had sprung up due to the discovery of gold.
The rocks that line the shore of these bays contain pegmatites, large veins of quartz with masses of mica in them. A pegmatite is a crystalline igneous rock containing a variety of crystals formed by slow crystallization at high temperature and pressure at depth, exhibiting large interlocking crystals.
When I visited the area in 2018, at the northern end of Joyce Bay I came across some stones from a vein that must have been almost pure mica. I chose this small boulder-sized rock to bring home, and it has sat in the lounge since, one of the very few non-tumbling stones I have kept.
There is so much mica in the rocks around Constant and Joyce Bays that the sea sparkles with it.
The two most common forms of mica are the silvery muscovite and the darker biotite. Mica minerals form large, flat sheets due to their atomic structure. Strong silicon-oxygen bonds extend outwards in two directions while the third direction only has weak bonds with large, low charge atoms in the space. One thin sheet is transparent and almost colourless, feeling like flexible plastic. Sheets of muscovite five metres by three metres have been found in Nellore, India. Technically, muscovite mica is composed of potassium aluminum silicate hydroxide fluoride. The name “muscovite” comes from Muscovy-glass, a name given to the mineral in Elizabethan England due to its use in medieval Russia (Muscovy region) as a cheaper alternative to glass in windows.
Sheets of muscovite have high heat and electrical insulating properties and are used in the manufacture of many electrical components. Muscovite sheets were used for kitchen oven windows before synthetic materials replaced them. Muscovite can form during the metamorphism of argillaceous rocks. The heat and pressure of metamorphism transforms clay minerals into tiny grains of mica which enlarge as metamorphism progresses. Small flakes of mica are often a component of a wide range of part of different rocks. For example, muscovite occurs as isolated grains in schist and gneiss, and it can be abundant enough that the rocks are called “mica schist” or “micaceous gneiss”. Jocelyn Thornton in her “Field Guide to New Zealand Geology” (2003) writes on page 103: “Hidden in the gorse-covered stretch between Charleston and Constant Bay is a pegmatite that was once mined for mica; the outcrop still contains large crystals of muscovite mica.” In “Geology of the Greymouth Area” (2002) by GNI, page 43, it is noted that “a small amount of mica was mined in 1911-12 from a pegmatite near Constant Bay, but the grade is low.”
![Charleston mica mine https://natlib.govt.nz/records/22415334?search[i][subject_authority_id]=-3477&search[path]=items&fbclid=IwAR3hQzJ9Pr6-Aw-UHtczoN8hVmrCPeV7-0mC5bIK9jrG32nRgYBsPo0WTS0#content](https://i0.wp.com/tumblestoneblog.com/wp-content/uploads/2021/08/11-mica-mine-1900.jpg?w=416&h=416&crop=1&ssl=1 "11 mica mine 1900")
For more on muscovite mica and its uses, see here.
“N” is for “November” in the Calendar – Each year I produce a customised wall calendar, using an online site, with stones or beaches or tumble-polishing as the theme. This is the month of November in my 2021 calendar. The month’s theme is “Stones from Leithfield Beach, Canterbury” with close-ups of four un-polished stones. One of the four stones shown here is a red and yellow jasper, collected in August 2020.
November is also usually the month in which I prepare the calendar for the following year.
For the next Post in this Series, see here. For the Series Index, see here.
