Kröger–Vink notation is a type of notation used to describe crystallographic defects (parts of a crystal that do not match its "normal" crystal structure), and chemical equations involving those defects. This notation is most commonly used for salt crystals.

The notation uses the form M^C
_S:

• M is the symbol for what is actually found in the defect. This is normally the atomic symbol for a chemical element, but can also be some special species:
  • V or v means a vacancy. Because a capital V is also the atomic symbol for vanadium, the lower-case is preferred, but the capital is still commonly used.
  • e means an electron and h means an electron hole.
• S is the symbol for the site of the defect. This is normally the chemical symbol of the element expected to be in that site, or a lower-case i is used to mark an interstitial site.
• C is the electrical charge of the defect compared to the normal crystal. Dots are is used when the defect has a higher (or less negative) charge, an X is used when the defect has the same charge, and primes are used when the defect has a lower (or more negative) charge.

This is the equation a Frenkel defect in zinc sulfide:

Zn^×
_Zn + S^×
_S → Zn^{${\displaystyle \cdot \cdot }$}
_i + v^{${\displaystyle \prime \prime }$}
_Zn + S^×
_S

In this equation, a zinc ion moves from a zinc site (Zn^×
_Zn) to an empty interstitial site (Zn^{${\displaystyle \cdot \cdot }$}
_i), leaving a vacancy behind (v^{${\displaystyle \prime \prime }$}
_Zn). The sulfide ions do not move. The charge of the zinc ion is +2, so the charge at the interstitial site is increased by 2 (${\displaystyle \cdot \cdot }$) and the charge at the zinc site is decreased by 2 (${\displaystyle \prime \prime }$). The sulfide ions do not change sites in this type of defect.

Amethyst is a type of quartz with iron impurities. The purple colour of amethyst is caused by oxidation of these iron impurities to the +4 oxidation state.^[1] A simplified equation for this is:

Fe^{${\displaystyle \prime }$}
_Si → Fe^{${\displaystyle \times }$}
_Si + e^{${\displaystyle \prime }$}

Before oxidation, the Fe³⁺ substitutional defect has a charge of -1 compared to the Si⁴⁺ that would normally be at that site (Fe^{${\displaystyle \prime }$}
_Si). Oxidation brings the iron atom to the same charge as a silicon atom (Fe^{${\displaystyle \times }$}
_Si) and releases an electron into the crystal (e^{${\displaystyle \prime }$}).