Difference Between Orthoclase and Plagioclase

The key difference between orthoclase and plagioclase is that orthoclase appears in green-yellow colour, whereas plagioclase appears in white colour.

Orthoclase and plagioclase are important minerals. Both these forms are members of the feldspar group. A feldspar mineral is an abundant rock-forming mineral typically occurring as colourless or pale-coloured crystals.

1. Overview and Key Difference
2. What is Orthoclase
3. What is Plagioclase
4. Side by Side Comparison – Orthoclase vs Plagioclase in Tabular Form
5. Summary

What is Orthoclase?

Orthoclase is an important mineral in the feldspar group, and the chemical formula of this mineral is KAlSi₃O₈. The mineral is a component in igneous rocks, and it is a tectosilicate mineral as well. Moreover, this is a type of k-feldspar or potassium feldspar. This appears as a green-yellow solid.

The crystal system of this mineral is monoclinic. The Mohs scale harness of orthoclase is 6.0. It has a pearly lustre on cleavage surfaces. However, the mineral streak of orthoclase is white. Moreover, this mineral is mostly transparent or translucent.

Moreover, this mineral is a common component in many granite forms. When considering the uses of orthoclase, it is important in the manufacture of some glass and ceramics, e.g. porcelain. Furthermore, it is an important component in scouring powder. Also, this mineral is important as a gemstone; for example, moonstone, which mainly contains orthoclase, is an important gemstone.

What is Plagioclase?

Plagioclase is an important mineral in the feldspar group, and the chemical formula of this mineral is NaAlSi₃O₈– CaAl₂Si₂O₈. It is also a tectosilicate mineral. The crystal system in this material is triclinic. It appears in white to grey colour.

The Mohs scale hardness of this mineral may range from 6.0 to 6.5. It has a vitreous lustre and the mineral streak is white. The material is either transparent or translucent.

What is the Difference Between Orthoclase and Plagioclase?

Orthoclase is an important mineral in the feldspar group, and the chemical formula of the mineral is KAlSi₃O₈. Plagioclase is an important mineral in the feldspar group, and the chemical formula of this mineral is NaAlSi₃O₈– CaAl₂Si₂O₈. The key difference between orthoclase and plagioclase is that orthoclase appears in green-yellow colour, whereas plagioclase appears in white. Moreover, the crystal system of orthoclase is monoclinic while in plagioclase, it is triclinic.

You can find more details on the difference between orthoclase and plagioclase in the below infographic.

Summary – Orthoclase vs Plagioclase

In summary, the orthoclase is an important mineral in the feldspar group, and the chemical formula of the mineral is KAlSi₃O₈while plagioclase is an important mineral in the feldspar group, and the chemical formula of this mineral is NaAlSi₃O₈– CaAl₂Si₂O₈. The key difference between orthoclase and plagioclase is that orthoclase appears in green-yellow colour, whereas Plagioclase appears in white colour.

Reference:

1. “Orthoclase Feldspar.” Geology, Available here.

Image Courtesy:

1. “Orthoclase-t07-43b” Von Rob Lavinsky, iRocks.com (CC-BY-SA-3.0) via Commons Wikimedia
2. “PlagioclaseFeldsparUSGOV” (Public Domain) via Commons Wikimedia