Schist – Not to be Missed!
This Earth Cache is located within the grounds of the 12 hectare National Botanic Garden of the National Botanical Research Institute. The Institute is open on weekdays only, from 08:00 to 17:00. Entry to the Garden is free and it is a wonderful place to relax, explore and marvel at the numerous indigenous plants in the gardens and attached nurseries. It is also the location of the traditional cache GCVTNV Dassie View.
To reach the cache location: take the B6 (Sam Nujoma Drive) heading east and @ S 22 34.263 E 17 05.346 turn left onto Hugel Street (which becomes Orban Street). Follow this for about 550m to the entrance gate of the Institute. You should be able to park securely inside the gate which will be opened for you by the guard.
The word schist is derived ultimately from the Greek word σχίζειν (schízein) meaning ‘to split’ which is a reference to the ease with which schists can be split along the plane in which the platy minerals lie.
Schist is medium grade metamorphic rock (see Rock Cycle illustration), formed by the intense metamorphosis of sedimentary rocks (eg. clays / mudstone / shale), metamorphic rocks or some types of fine-grained igneous rock (eg. basalts & tufts). The metamorphosis is to a higher degree than slate, ie. the rocks have been subjected to higher temperatures and pressures, and consequently greater deformation (see illustration below).
Most schists are derived from clays and muds that have passed through a series of metamorphic processes involving the production of shales, slates and phyllites as intermediate steps.
Schists have medium to large, flat, sheet-like grains in a preferred orientation (nearby grains are roughly parallel) and is defined by having more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar.
The resulting foliation (repetitive layering or planar arrangement from paper thin to centimeters thick where individual mineral grains can be split off easily into flakes or slabs) is coarser and more distinct than that of slate due to the higher degree of crystallisation of mica minerals (biotite, chlorite, muscovite) forming larger crystals (medium-large grained flakes) and is often referred to as schistosity. The individual mineral grains in schist, drawn out into flaky scales by heat and pressure, can be seen with the naked eye
These larger crystals reflect light so that schist often has a high lustre, i.e. it is shiny.
Porphyroblasts (large mineral crystals in metamorphic rock which have grown within the finer grained groundmass) are common in schist, and they provide information on the temperature and pressure conditions under which the rock formed. Also due to the more extreme (de)formation conditions (high temperatures and pressures) schist often shows complex folding patterns.
There are many varieties of schist which are named after the dominant mineral in the rock (eg. mica schist (primarily composed of biotite and muscovite), green schist (green because of high chlorite content), or perhaps unusual mineral constituents (eg. garnet schist, tourmaline schist, and glaucophane schist) – see Gallery for photos. Most schists are mica schists, but graphite and chlorite schists are also common.
Schist is often the host rock for a variety of gemstones that form in metamorphic rocks. Gem-quality garnet, kyanite, tanzanite, emerald (see photo below), andalusite, sphene, sapphire, ruby, scapolite, iolite, chrysoberyl and many other gem materials are found in schist.
Gem materials found in schist are often highly included. This is because their mineral crystals grow within the rock matrix, often including mineral grains of the host rock instead of replacing them or pushing them aside.
Schist may be used as a decorative rock, e.g. walls, gardens etc. The high percentage of mica group minerals precludes its use in the construction and roading industries.
The rocks exposed in the Windhoek Botanical Gardens are mostly composed of mica-rich and quartz-rich schist that has many fractured quartz veins. This is biotite schist with quartzite, the metamorphic product of siliceous parent rock such as sandstone. It consists of recrystallised interlocking quartz crystals, which forms the Windhoek aquifer that has been utilised as a source of relatively clean water.
These rock formations are in the Kuiseb formation of the Damara Sequence, with many summit heights reflecting older land surfaces of Khomas Hochland (a part of the interior plateau of central Namibia) dating back some 700 million years.
See here for comprehensive information on mica.
References:
https://www.wikiwand.com/en/Schist
http://www.landscapesnamibia.org/windhoek-green-belt/geology
Questions:
1) Examine the rocks and stones on the ground around GZ. Describe:
a) their colour
b) their appearance (eg. dull, shiny, other?)
c) their surface texture (eg. rough or smooth)
d) any other notable feature (eg. hard, soft, crumbly, flaky, etc)
2) What is the original rock from which this schist was formed by intense metamorphosis?
3) What is the main mineral in the stones and rocks around GZ?
(This gives the name of this variety of schist)
4) Look at the rock wall next to GZ:
a) What white feature can you see running across/through it?
b) How thick is this feature at its thickest point?
5) Optional: take a photo of yourself or your party at GZ admiring the wonderful schist.