Experimental growth and structural-morphological characteristics of co-tourmaline

Abstract

Experiments aimed at investigating opportunities of Co-tourmaline growth onto a seed crystal in aqueous boron-chloride cobalt-containing solutions under temperatures of 400‐500 ° C and pressure up to 1000 bar were performed. Experiments were conducted under thermogradient conditions in titanium autoclaves, and in several cases with the help of a dividing diaphragm. Tourmaline growth onto a seed crystal occurs only with the use of separate tourmaline-forming components (monocrystal corundum and quartz rods) as the melting stock. The layer (up to 600 micron) of Co-tourmaline grown is intensively crimson-colored and occurs only at the face of positive monohedron {+0001}. Along with the monocrystal outgrowth, a great amount of very fine (30‐150 micron in size) tourmaline crystals of selfgeneration nucleation occurred on the seed crystals and in the melting stock. Their composition does not differ from the grown layer composition, and their unit cell parameters ( a = 15.82638 ± 0.00215 A, c = 7.09434 ± 0.00186 A) correspond to intermediate values of the tourmaline group minerals olenite and elbaite. Tourmaline is a complex borosilicate of the general crystal chemical formula XYZ , where X corresponds to large voids filled with Na, Ca, K, or a vacancy; Y corresponds to closely packed octahedrons of Mg, Fe 2+ , Li, Al, Mn, and Cr; and Z corresponds to deformed octahedrons of Al and Fe 3+ . In nature tourmaline rather often occurs in granite pegmatites and pneumatolyte‐hydrothermal and hydrothermal formations. Its presence is not unusual in such granites as well as in metasomatites and metamorphic rocks. This indicates the diversity of the tourmaline formation conditions. Interest in growing tourmaline monocrystals is connected with the task of obtaining new promising piezoelectric and pyroelectric materials possessing enhanced characteristics in comparison with those already known. Moreover, synthetic tourmaline can be used as a substitute for the natural form in the jewelry industry similar to other synthetic analogues of precious stones. There are many questions concerning the tourmaline formation conditions in nature that have remained unclear up until the present despite numerous geological, mineralogical, geochemical, and crystal chemical investigations. Safe and reproducible methods of growing its monocrystals have also not been developed. This situation is connected in general with the tourmaline complex chemical composition resulting in the existence of numerous varieties according to shades of color, which often change even within the single crystal. Obviously, this is a consequence of complex and changing physicochemical crystallization conditions of this mineral during its growth as well as special predisposition of its structure to a wide exhibition of iso- and heterovalent isomorphism. Investigations in the sphere of synthesis and growth