Hydrothermal growth of aquamarine crystals.

 

JV Tairus, Novosibirsk, Russia

 Sergey P. Demin, Victor G. Thomas, Olga A. Kozmenko

Institute of Mineralogy and Petrography Siberian Branch of Russian Academy of Science, Novosibirsk;

 

Oriented visible-range and IR spectra on 1-mm-thick plates were measured by Rudolf Mashkovtsev (UIGGM SB RUS).

 

This work was undertaken to solve the task of the growth of the crystals of intense blue colored aquamarine. This type of aquamarine is one of the most rare and expensive. Properties of different natural and hydrothermal synthetic aquamarines, ranged from near colorless to deep blue, were studied. Complete chemical composition of the investigated samples was determined. The special technique of Fe2+ determination, which prevents the oxidation of Fe2+ during the analysis, has been developed.

The optical spectra of the studied aquamarines contain two absorption bands. The lines can be approximated by a combination of two Gaussians with centers about 620 and 810 nm. Our modeling shows that the color of aquamarine is determined generally by the intensity and maximum wavelength of the first one. The result of such approximation of the spectrum is displayed in graphical chart (fig.1). Results of modeling are plotted on CIE – diagram (fig.2).

   It is obvious that the band at 810 nm produces the least contribution to the aquamarine color even at high band intensity and width. This band is the only one in the spectra collected perpendicularly to the c – axis of the crystals. Thus the crystals have a very pale blue color in this direction.

The spectra and CIE – diagrams (fig.3)

illustrate that the blue hue becomes deeper with increasing Fe2+ at the constant Fe3+ content. Nevertheless, the increase of Fe3+ content at constant Fe2+ content results in an increase of blue color intensity. It is necessary to remark that beryls that contain only trivalent iron are colorless. Those facts allow us to conclude that the intensity of blue color of beryl is mainly determined not the bulk concentration of Fe3+ - or Fe2+ - ions but the quantity of Fe3+-Fe2+ -pairs and cased by the charge transfer Fe3+-Fe2+. It was established that increment of Fe2+-ions bulk quantity causes abrupt descent of aquamarine quality. Therefore, we suggest enhancing aquamarine color by changing Fe3+-ions concentration under the small bulk content of bivalent iron.

 

 

 

Fig.3. Concentration of pairs of Fe³⁺-Fe²⁺ in beryl (this concentration assures absorption in 620 nm) is determined by concentration of  Fe³⁺ - ions, as well as  Fe²⁺ - ions.

           We found that the simultaneous changing of Po2 and the alkalinity of the solutions may be realized this way. On the fig.4 one can see results of experiments of aquamarines grown in the Cs – Li solutions. The higher alkalinity of the solution (by the way of enhance of Cs ratio), the higher the intensity of blue color and its hue is moving toward violet spectrum. One can see the center of the absorption band is moving from 680 to 620 nm, due to this enhancement.

 

 

It was found that combined incorporation of iron and alkaline cautions R+ (Li, Na, Cs) don’t agree with the simple model of heterovalent isomorphism Fe2+ + R+ => Al3+. Regression analysis of our results shows that incorporation of Fe2+ in beryl is accompanied by the more complicate substitutions including substitutions between Si4+, Al3+ and Be2+. Those substitutions may essentially influent aquamarine color. It is illustrated by the figures on fig.5 (Na – aquamarines as example).

 

 

The distribution of Fe3+ and Fe2+ - ions between different positions in the beryl structure is a debatable question. It is even impossible now to estimate directly the quantity of Fe3+-Fe2+ -pairs. Let us compare the chemical composition of the samples with equal bulk concentration as Fe3+ soon as Fe2+ - ions, but with different color intensity.

 

  The first example, are aquamarines with zonal distribution of color. The time-varying activity of SiO2 is the reason of this type of zoning in crystals. Difference in the chemical composition of different zones is explained by following substitutions:

3Si4+  +  3  ó  4Be2+ +  2Fe2+

colorless  ó intense blue, crystal’s quality is low.

Other kind of substitutions between main beryl’s components is realized for Li-bearing beryls:

Be2+ + Al3+ ó  Li+ + Si4+

pale  blue       ó  intensive blue, crystal’s quality is good

We believe that those substitutions assist to cooperation single Fe2+ and Fe3+ ions into the pairs.

   The last example illustrates the common tendency: it is possible to produce intense colored aquamarine with low concentration of iron, while preserving the quality, by simultaneously exhorting influence on main beryl component substitution and on iron that is being incorporated into the beryl structure.