The facade of this building instantly caught my eye - it took some research but I found out that the rock used for this facade is a schist! The minerals present here are extremely cool. Let us break it down. What you can see here is schist - a metamorphic rock form by heat and pressure on a protolith (primary rock) called shale.

Metamorphism of Shale

Figure 1 showing the degree of metamorphism occuring on shale

1. Chlorite - Green
2. Muscovite - White, grey, silvery
3. Biotite - Black
4. Garnet - Commonly red
5. Staurolite - reddish brown to black, mostly opaque
6. Sillimanite -  yellow, brown, and blue
7. Quartz - various
8. Feldspar - white or buff to gray

Rock Texture

The schist here that you see is a medium-grained rock - if you look closely (or even zoom in with your phone), you can see each individual crystal. Some crystals may be larger than others. Let's have a quick look. 

Figure 2: Foliation in gneiss. Note the 'layers'

Figure 3: Porphyroblastic texture - the large crystals, called porphyroblasts stand out against the smaller crystals. 

Index Minerals

Geologists use index minerals that form at certain temperatures and pressures to identify metamorphic grade. These index minerals also provide important clues to a rock’s sedimentary protolith and the metamorphic conditions that created it. Chlorite, muscovite, biotite, garnet, and staurolite are index minerals representing a respective sequence of low-to-high grade rock.

Some metamorphic rocks are named based on the highest grade of index mineral present. Chlorite schist includes the low-grade index mineral chlorite. Muscovite schist contains a slightly higher grade muscovite, indicating a greater degree of metamorphism. Garnet schist includes the high-grade index mineral garnet and indicating it has experienced much higher pressures and temperatures than chlorite.

Garnets

Clockwise from the top left: red almandine, green tsavorite, yellow mali, orange spessartite, pink malaya, green merelani mint, red pyrope, green demantoid, purple rhodolite, and orange hessonite

Logging Tasks

Send the answers to the following via the message center. Please try your best, and even if you answered wrongly it is not a big deal as long as you can prove your visit to the site with a photo and a nice log about your experience. 

1. The rock you see here is schist. Based on figure 1, what minerals can you expect to be present in the rock?
2. Look at the minerals in the facade - what colours can you see?
   • Match the minerals to the colors
   • Draw a line in figure one based on the minerals you see.
   • What is the grade of metamorphism?
3. Can you see any foliation occuring? Which minerals do you see not foliating?
   • Is this mineral mentioned a porphyroblast? Why?
4. What index mineral can you identify here?
   • Based on the text in index mineral section, what name would you give this schist?
   • What does the presence of these index minerals tell about the metamorphic grade?
5. What is the kind of garnet can you see here?