Granodiorite

In short

Granodiorite is a coarse-grained rock formed when magma cools slowly deep in the crust. It looks very like granite; it differs in holding more plagioclase than orthoclase feldspar and usually more black minerals (biotite, hornblende), so it looks a little darker. The average composition of the upper continental crust is close to that of granodiorite. In Türkiye it occurs in the Uludağ pluton in Bursa and the Kozak pluton in Bergama, and in the trade it is often called “granite”.

Photos

How to recognise it

1. 1

   Visible grains of quartz (glassy grey), plagioclase (white, milky), a little pinkish alkali feldspar and black biotite and hornblende interlock.

2. 2

   It usually looks darker and two-toned than granite, because black minerals are more abundant and pink alkali feldspar is scarcer.

3. 3

   It scratches glass, cannot be scratched with a steel knife, and does not fizz in vinegar or dilute acid.

4. 4

   Granite and granodiorite are hard to tell apart by eye; a firm answer comes from the proportions of the feldspar types.

Myths and misconceptions

• MythThe Rosetta Stone is made of black basalt.

  FactThe Rosetta Stone is granodiorite. Protective coatings applied in the 19th century made it look like black basalt; a cleaning in 1999 revealed the rock's dark grey colour and crystalline structure.

• MythGranodiorite is just another name for granite.

  FactThey look alike, but in granodiorite plagioclase feldspar outweighs orthoclase, while in granite alkali feldspar stands out. Granodiorite usually looks darker and is often sold as “granite”.

Did you know?

Where is it found?

• Aswan and Gebel Tingar, Egypt · Historic sourceDark grey granodiorite was quarried at Aswan alongside granite. The Rosetta Stone is thought most likely to come from a small granodiorite quarry at Gebel Tingar.
• Mons Claudianus, Eastern Desert, Egypt · Historic sourceA granodiorite quarry worked in Roman times, from the 1st to the mid-3rd century AD. The columns of the portico of the Pantheon in Rome came from here.
• Little Cottonwood Canyon, Utah, USA · Known localityThe Little Cottonwood stock, about 30.5-29 million years old, consists mostly of granodiorite, quartz monzonite and granite; the stone of the Salt Lake Temple was quarried here.
• Sierra Nevada Batholith, California, USA · Major depositA vast mass of plutonic rock about 640 km long, largely formed 105-85 million years ago, made of rocks such as granodiorite and granite.
• Mount Kinabalu, Sabah, Malaysia · Known localityThe 4,095-metre mountain is a pluton of granodiorite that intruded into sedimentary and ultrabasic rocks; the granodiorite cooled about 10 million years ago, and the mountain is still rising about 5 mm a year.
• Uludağ Massif, Bursa, Türkiye · Major depositThe elliptical Uludağ granitoid, about 14 × 7 km, is mostly granodioritic and Upper Oligocene in age. Where it meets marble, the skarn deposit of the now-closed tungsten mine formed.
• Kozak pluton, Bergama, İzmir, Türkiye · Known localityAccording to MTA studies the Kozak pluton is a light-coloured, heavily jointed hornblende granodiorite; stone quarried here was registered as a geographical indication, “Bergama Graniti”, in 2017.

Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →