The purpose of a mineralizer is to facilitate the transport of insoluble “nutrient” to a

seed crystal A seed crystal is a small piece of single crystal or polycrystal material from which a large crystal of typically the same material is grown in a laboratory. Used to replicate material, the use of seed crystal to promote growth avoids the otherwi ...

by means of a reversible

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

. Over time, the seed crystal accumulates the material that was once in the nutrient and grows. Mineralizers are additives that aid the solubilization of the nutrient solid. When used in small quantities, mineralizers function as catalysts. Typically, a more stable solid is crystallized from a solution that consists of a less stable solid and a solvent. The process is done by dissolution-precipitation or crystallization process. Hydrothermal growth involves the crystallization of a dissolved solid at elevated temperatures. Often high pressures are involved. Historically, the goal of hydrothermal growth was to grow large crystals. Due to the recent developments in

nanotechnology Nanotechnology, also shortened to nanotech, is the use of matter on an atomic, molecular, and supramolecular scale for industrial purposes. The earliest, widespread description of nanotechnology referred to the particular technological goal o ...

, small nanocrystals are now desired and made by hydrothermal growth with crystal size controlled by mineralizers. Different mineralizers result in crystals of different sizes and shapes. Typical mineralizers are hydroxides (

NaOH Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali t ...

, KOH,

LiOH Lithium hydroxide is an inorganic compound with the formula LiOH. It can exist as anhydrous or hydrated, and both forms are white hygroscopic solids. They are soluble in water and slightly soluble in ethanol. Both are available commercially. While ...

), carbonates ( Na₂CO₃) and halides (

NaF The Naf River ( bn, নাফ নদী ''Naf Nodi'' ; my, နတ်မြစ် ; rki, ) is an international river marking the Bangladesh–Myanmar border, border of southeastern Bangladesh and northwestern Myanmar. Geography The Naf Rive ...

, KF,

LiF Lithium fluoride is an inorganic compound with the chemical formula LiF. It is a colorless solid, that transitions to white with decreasing crystal size. Although odorless, lithium fluoride has a bitter-saline taste. Its structure is analogous to ...

NaCl Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...

KCl Potassium chloride (KCl, or potassium salt) is a metal halide Salt (chemistry), salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous lustre, vitreous crystal appearance. The solid dissolves readily in wa ...

, LiCl).

Cation effects

Although usually the anion of the mineralizer is most active in dissolving the nutrient material, the cation also exerts an influence in some cases. The mineralizer can interact with impurities on the surface of the crystal and increase the growth rate. For example, the growth rate for

sapphire Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, chromium, vanadium, or magnesium. The name sapphire is derived via the Latin "sapphir ...

(Al₂O₃) and

zincite Zincite is the mineral form of zinc oxide ( Zn O). Its crystal form is rare in nature; a notable exception to this is at the Franklin and Sterling Hill Mines in New Jersey, an area also famed for its many fluorescent minerals. It has a hexagonal ...

(ZnO) in potassium-containing solution (KOH, K₂CO₃) is higher in comparison to that in sodium-containing solution (NaOH, Na₂CO₃). This difference is not readily understood, but are attributed the interaction between potassium and an impurity absorbed on the surface.

Applications

Synthesis of quartz

Basic mineralizers such as NaOH or Na₂CO₃ are used in the hydrothermal growth of

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

crystals. The precursor or nutrient is crushed

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

and a solvent. Typical containers are made of air-tight steel cylinders called

autoclave An autoclave is a machine used to carry out industrial and scientific processes requiring elevated temperature and pressure in relation to ambient pressure and/or temperature. Autoclaves are used before surgical procedures to perform sterilizat ...

s that can withstand high temperature and pressure. In the case of quartz crystals, the container is heated at 300 °C (which produces a pressure of 140 MPA). Without the mineralizer, higher temperatures are required to solubilize silica. Hydroxides and carbonates make silica more soluble by forming water-soluble sodium silicates. Simplified equations can be represented as in the equation below :SiO₂ + 2 NaOH → Na₂SiO₃ + H₂O :SiO₂ + Na₂CO₃ → Na₂SiO₃ + CO₂ Anhydrous sodium silicate is a chain polymeric anion composed of corner shared SiO₄ tetrahedra. Hydrates form with the formula Na₂SiO₃•nH₂O which contain the discrete, approximately tetrahedral anion SiO₂(OH)₂²⁻ with waters of hydration. In three-dimensional silica glass, the addition of sodium ions causes oxygen ions formed a bridge, these oxygen ions possess an effective negative charge. The positively charged sodium ions provide partly covalent and partly ionic structure. As the concentration of Na⁺ increases, ionically bound material link and eventually form a network of continuous channels. Once silica is solubilized, components in the nutrient are transferred to the seed crystal, which is held at a cooler temperature than the nutrient, resulting in a high purity quartz crystal.

Synthesis of zeolites

Hydroxide mineralizers are also used to control the alumina/silica ratio of

zeolites Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a metal ion or H+. These pos ...

. A typical recipe for the production of a zeolite includes the mineralizer, the solvent, the seed crystal, a nutrient consisting of silica (SiO₂) and alumina (Al₂O₃), and a template. Templates are cations that direct the polymerization of the anionic building blocks to form a certain zeolite structure. Different templating cations lead to different zeolite structures. Typical templates include tetramethylammonium (TMA), sodium (Na⁺) and potassium (K⁺). Different zeolites can also be formed by changing the ratios of the nutrient source, the type of mineralizer or the temperature and pH of the reaction. At high pH, zeolites with high alumina content are formed, because hydroxides prevent the ability of silica to condense and oligomerize through the reaction shown above. At lower pH, zeolites with high silica content are favored.

References

{{Reflist Chemical synthesis Equilibrium chemistry