First time I saw this rock, I mistook it for another. Both may look similar, but let's have a closer look at both and see what differences there are.
Granite
Granite is an igneous rock (formed by the cooling of magma). It's main compositions are quartz (light coloured), alkali feldspar (pink) and plagioclase (grey). Granites can also contain other minerals, such as biotite and mica (black). I won't bore you with the details, but keep the colours in mind - our goal is to learn about textures.
Orbicular Granite
These rocks have a unique appearance due to orbicules - concentrically layered, spheroidal structures, probably formed through nucleation around a grain in a cooling magma chamber due to rapid physical changes.
Rapakivi Granite
Rapakivi is used as a a textural term. Rapakivi is a Finnish compound of "rapa" (meaning "mud" or "sand") and "kivi" (meaning "rock").
Rapakivi granite type Wiborgite - with well defined plagioclase rims
Rapakivi granite type pyterlite - note the lack of plagioclase rims
What's the difference?
Well they might be circles - but there are 2 huge differences
- Rapakivi granite ovoids usually show one crystal of alkali feldspar. Orbicular granite orbicules contain many crystals of various minerals.
- The rims of rapakivi granites are thinner and are a layer of plagioclase. Orbicular granite rims are thicker and contain many layers of various minerals
Logging Tasks
The location takes you to a corner store where its facade is the Earthcache of interest. Answer the following in the message center. Try your best, but Q3 will definitely prove your visit.
- Describe the rock texture in front of you. Colours, width, size of minerals, rims etc. As detailed as you can.
- Based on your answer, is the rock orbicular granite or rapakivi granite.
- If rapakivi, is it wiborgite or pyterlite?
- Attach a photolog to prove your visit. A photo of the surroundings will suffice, try not to show the rock as much as you can.