Garnet pigments

Garnets are natural minerals which have been known for very long time for their beautiful colors. They belong to the more complex orthosilicates with general formula А₃В₂(SiO₄)₃ where A are bivalent cations (Са²⁺, Мg²⁺, Мn²⁺, Fе²⁺) and B – trivalent ones (Аl³⁺, Сг³⁺, Fе³⁺). They crystallize in the cubic syngony. They also prone to isomorphic substitutions of bi- and trivalent ions, as well as the acidic radical which widens the possibilities to obtain garnet pigments of rich color palette.

Garnets are formed in nature under high pressure and temperature. They are artificially synthesized at temperatures up to 1200°С in presence of mineralizers. Original colored garnets which cannot be found in nature were prepared by substitution of the bivalent ions with Ва²⁺ or Ni²⁺.

One of the most famous garnets with beautiful green color is uvarovite Са₃Сг₂(SiO₄)₃ or 3CaO.2Сг₂O₃.3SiO₂. It can be obtained artificially and it is well known under the name of “Victoria green”. On the basis of the garnet grossular Са₃Аl₂(SiO₄)₃, a numner of garnet pigments were obtained by complete or partial substitution of CaO with MgO, MnO and FеО, as well as Аl₂O₃ with Сг₂O₃ and Fе₂O₃. Pigments were synthesized containing Со, Ni, Сu, Ва, Zn and partial substitution of SiO₂ with ZrO₂, ТiO₂ and SnO₂. In the search for new and stable pigments, the acid radical [SiO₄]^4- was substituted also with [РO₄]^4- .

Phosphorous containing garnets are very rare in nature, mostly as spessartine which contains manganese and has pink-red to yellow brown color. It is possible to prepare pigments on the basis of solid solutions of garnets and phosphates. Pigments have been obtained from a mixture of garnets and spinels which formed highly stable mixtures in various colors while preserving the structure of the main crystals.

References

1. Technology of Ceramic Products and Materials, ISBN: 9549841154, Publisher: Saraswati, 2003, edited by Prof. Bachvarov