The Law of Constancy of Interfacial Angles, also known as the First Law of Crystallography, states that the angles between corresponding faces of crystals of the same substance are constant, regardless of the size or shape of the crystal.

In simpler terms, this means that if you take two crystals of the same mineral, even if one is large and well-formed and the other is small and misshapen, the angles between their corresponding crystal faces will be the same. This principle is fundamental to crystallography and the identification of minerals.

This law was first discovered by Nicolas Steno in 1669 when studying quartz crystals.

• Key Implications:

  • Mineral Identification: Allows for the identification of minerals based on their consistent crystal angles.

  • Crystallography: Forms a foundational principle in the study of crystal structures.

  • Understanding Crystal Growth: Provides insights into the consistent internal structure of crystalline materials despite variations in external appearance.