Publisher Summary This chapter describes hydrothermal crystal growth. In discussing hydrothermal crystal growth, it stresses experimental results and techniques rather than the theoretical aspects of crystal growth. This may be done for two reasons: (1) the experimental technique of hydrothermal crystal growth differs greatly from other methods, and to provide a practical guide for use of the method, and to encourage its adoption, emphasis on experimental details and results seems justified, and (2) in spite of some progress in recent years, crystal growth theory is still rather crude. The theory of crystal growth from solutions is especially difficult to relate to experimental results and, in its present form, is of little quantitative use to the practical crystal grower. Many of the disadvantages of the hydrothermal technique are those that arise because of the extreme pressures required. They include direct pressure hazards, the need for rather unusual apparatus, and the inability to directly observe the crystallization while in progress. The problems then are principally those of technique, and the authors hope that what has been presented here will facilitate the adoption of the techniques by others.

Hydrothermal Crystal Growth

Hochverdichteter Wasserdampf bei uberkritischen Temperaturen kann Feststoffe wie Alkalisalze, Siliciumdioxyd und andere Oxyde und Hydroxyde in betrachtlicher Menge auflosen. Fluchtige Stoffe wie CO2 und Argon sind oberhalb 500°C noch bis zu mehr als 2000 bar vollstandig mit Wasser mischbar. Bei Dichten von mehr als 0,2 bis 0,3 g/cm3 besitzt uberkritischer Wasserdampf elektrolytische Eigenschaften. Die Dielektrizitatskonstante wird mit Werten zwischen 5 und 20 hoch genug, um die Ionendissoziation geloster Stoffe zu ermoglichen. Entsprechend der niedrigen Viscositat des uberkritischen Mediums erreichen die Beweglichkeiten der lonen darin das Zehnfache der Normalwerte. Das Ionenprodukt reinen Wassers wachst mit steigender Temperatur und Dichte und ist z. B. bei 600°C und 2000 bar etwa 105mal groser als bei 0°C. Hydrolysereaktionen werden dadurch stark begunstigt. Die Bedeutung von Losungen im uberkritischen Wasser fur naturliche Lagerstattenbildungen und technische Probleme wird gezeigt. Dichte, Viscositat, Dielektrizitatskonstante und Losungsfahigkeit uberkritischen Wassers werden quantitativ diskutiert. Leitfahigkeitsmessungen werden beschrieben und Dissoziationskonstanten von Salzen sowie von KOH, HCl, HF und NH3 zwischen 400 und 750°C werden angegeben. Methoden zur Berechnung des Ionenprodukts und von Hydrolysegleichgewichten werden gezeigt.

Überkritisches Wasser als elektrolytisches Lösungsmittel

Frosting of quartz grains is thought to be due in minor degree to mechanical action (1) by wind and (2) by water, but mainly to chemical action (3) by corrosive solutions and (4) by alternate solution and deposition of matter, especially in desert areas. Defrosting under water is likewise a physico-chemical process involving incipient crystal growth or solution. These physico-chemical actions, in contrast with mechanical attack, affect the whole surface of the grain equally, even in deep indentures. The typical circular markings of 3-10 μ diameter described by Cailleux as percussion cones are shown to be diffraction images of the light source. They can be observed also on the etched surface (e.g., by hydrofluoric acid) of crushed quartz, while electron micrographs of frosted grains fail to show any circular pattern of a few microns in diameter. Because of the diffraction effect, the resolution of the microscope is insufficient to distinguish chemical and mechanical action readily. True percussion cones oc...

Experimental Abrasion 5. Frosting and Defrosting of Quartz Grains

Research at the Bell Telephone Laboratories on the problem of growing large single crystals of quartz has now progressed to a point where it is possible to grow crystals weighing more than 1 lb. each in a period of 60 days or less. Equipment now in use includes autoclaves 4 inches in inside diameter and 4 ft. long, weighing about 1150 lb. each. In developing the hydrothermal process used to grow these quartz crystals it has been necessary to solve many problems in the little-known field of high pressure. The results point to the possibility of growing other types of crystals, and the field of usefulness of this process now appears to be much more extensive than was the case at the beginning of the investigation in 1946. Of prime importance is the fact that crystals grown from solution are likely to be better formed and of more perfect quality than those grown from the melt or by other methods. Many of the difficulties inherent in this work have been due to corrosion of steel in alkaline solution. This was a rather unexpected problem, since in high-pressure steam boilers alkali is added in small amounts to prevent corrosion of the boiler tubes. Such corrosion has been shown to be responsible for the appearance of electrical twinning on the growing faces of the quartz crystals. Other causes of such twinning have also been found in the course of this work.

Hydrothermal Synthesis of Quartz Crystals

The use of high temperature hydrothermal reactions to prepare crystals having mineralogically-related structures is described. Complex naturally occurring minerals can have fascinating structures and exhibit important features like low dimensionality, noncentrosymmetry, or ion channels that can provide excellent guideposts for the designed synthesis of new materials. Actual minerals, even though they may have intriguing physical properties, are often unsuitable for study because of the persistent impurities inevitably present in natural samples. Hydrothermal fluids at relatively high temperatures provide access to large, high quality single crystals of structures with mineral-like structures. This enables the study of physical properties like ionic conduction, magnetic spin frustration and non-linear optical behavior. Some fundamental considerations of the hydrothermal technique are discussed in the context of synthesizing mineralogically-inspired materials. The metal vanadates provide a surprisingly rich and diversified range of compounds and are selected to illustrate many of the concepts described here. A series of low dimensional mineral analogs featuring isolated units, chains, and layers have been prepared in the laboratory as large single crystals using a high temperature hydrothermal synthetic methods, and their physical properties are under investigation. The metal silicates are also highlighted as another promising field of exploration, since their hydrothermal synthesis surprisingly lags behind the enormous literature of the natural silicate minerals. The introduction of heteroelements, such as boron to make borosilicates, appears to also open the door to additional new materials. Many of these new materials have direct equivalents in the mineral kingdom, while others have no known analogs but are reminiscent of minerals and can be classified in the same ways. From these initial results there appears to be a very rich vein of synthetic minerals waiting to be unearthed in the laboratory using the high temperature hydrothermal method.

A. C. Walker

Papers

Growing Large Quartz Crystals