US3473935A

US3473935A - Synthesis of beryl

Info

Publication number: US3473935A
Application number: US482035A
Authority: US
Inventors: Wayne D Wilson; Hubert B Hall
Original assignee: HUBERT B HALL; WAYNE D WILSON
Current assignee: HUBERT B HALL; WAYNE D WILSON
Priority date: 1965-08-18
Filing date: 1965-08-18
Publication date: 1969-10-21
Anticipated expiration: 1986-10-21

Description

Oct. 21, 1969 D MLSON ETAL 3,473,935

SYNTHESIS OF BERYL Filed Aug. 18, 1965 1 Wayne 0. Wiison x a v t ,3 3 Hubert 5. Hall :5 INVENTORSA I. M;

11 BY ATTORNEY AGENT.

United States Patent 3,473,935 SYNTEESIS 0F BERYL Wayne D. Wilson, 2000 Wallace Ave., Silver Spring, Md.

20902, and Hubert B. Hall, 716 Somerset Place, Hyattsville, Md. 20783 Continuation-impart of application Ser. No. 377,781, June 24, 1964. This application Aug. 18, 1965, Ser. No. 482,035

Int. (3]. C041) /44 US. Cl. 106-42 21 Claims ABSTRACT OF THE DISCLOSURE A method of making beryl crystals by compressing beryl powder at 7.5 to 15 kilobars at the melting temperature of beryl. The melting temperature may be lowered by the use of a flux such as water or a fluorine source.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This application is a continuation-impart of application Ser. No. 377,781 filed June 24, 1964, for Synthesis of Beryl, now abandoned.

This invention pertains to the formation of crystalline materials and more particularly to a method for producing beryl crystals.

Beryl crystals have previously been formed by several different methods each of which has its own peculiar disadvantages. The growth of single crystals from a seed crystal suspended in a melt suffers from the disadvantage that it requires too long a time (probably in the order of a week or more to produce a sizeable crystal) and the Verneiul flame fusion process, although requiring less time, suffers from the disadvantage that it is diflicult to maintain the necessary precise control over the feed rate of the constituents employed. Moreover when these processes have been employed for the production of emerald crystals, the emeralds produced have a density of 2.67 or lower, which is considerably lower than the density of natural emerald (2.70 and slightly higher than 2.71 for the purest natural emerald). These synthetic emeralds also have a series of wisp-like inclusions which are visible to the naked eye and such inclusions seriously affect the optical applications of the crystals.

Accordingly, it is an object of this invention to provide a new method for producing beryl crystals.

It is another object to produce clear single beryl crystals in a short space of time.

It is a further object to produce single clear emerald crystals.

It is still another object to produce emerald crystals that have density, hardness and optical properties that are superior to previous synthetic emeralds.

These and many other objects will become more fully apparent from the following detailed description of the invention wherein:

FIG. 1 is a partial cross-sectional view of a steppedcore pressure apparatus for providing the high pressure necessary to the present invention, the capsule containing the material to be crystallized being shown in place; and

FIG. 2 is a cross-sectional showing of the capsule itself.

The objects of this invention are accomplished by forming beryl crystals under pressure from either beryl powder or the constituents of beryl in their proper proportions (3BeO:Al O :6 SiO which combine under the process conditions to form beryl). It is to be understood that the term beryl powder is meant to include all mem- Patented Get. 21, 1969 bers of the beryl family which includes aquamarine (ferrous ion impurity), morganite (lithium ion impurity), emerald (chromium ion impurity), golden beryl (ferric ion impurity) and goshenite (no coloring impurity). Hereinafter, in referring to the starting material, the term beryl will include a proper proportioned mixture of the constituents of beryl and all the members of the beryl family.

In particular the method comprises melting beryl and subsequently allowing it to solidify with the melting and solidification steps being carried out under pressure. It has been found that if clear transparent crystals of beryl are heated to temperatures above 600 C., there is a molecular dissociation and self-diffusion of some of the constituents of beryl through the crystal, said diffusion destroying its optical properties. Consequently, if an attempt is made to form beryl crystals by melting beryl, self-diffusion of the constituents of beryl would prevent the formation of clear, transparent crystals. It has been found that this problem can be obviated by the application of pressure during the melting and cooling steps.

The pressure is only critical in that it must be high enough to prevent self-diffusion, yet not so high that it prevents the beryl from expanding during solidification. Beryl has a negative coefficient of expansion in a direction parallel to the hexagonal crystal axis and thus during solidification it will expand in one direction and contract in the other. If the pressure is so high that this expansion can not occur, no crystals will be formed. Although beryl crystals can be formed at any pressure that is sufficient to both prevent self-diffusion of the beryl constituents through the crystal and allow expansion of the crystal during solidification, as a practical matter the minimum and maximum pressures that are used are dictated by the particular pressure system employed. For example, in the apparatus depicted in FIGS. 1 and 2 which is described more fully below, it has been found that due to the closed capsule employed the pressure should not exceed about 20 kilobars and preferably should not exceed about 15 kilobars. It has been found that good results are obtained in this apparatus if the pressure is maintained between about 7.5 and 15 kilobars.

The temperature employed is any temperature which is sufficient to melt the beryl without decomposing it (when the constituents of beryl are employed they melt and combine to form beryl) and of course this melting temperature is a function of pressure, with the change being about 40 C. per kilobar (beryl melts at 1410 C. at atm. pressure and 1800 C. at 10 kilobars). Once the beryl is uniformly melted, temperature reduction may be started almost instantaneously and this reduction in temperature may be effected abruptly or over a period of time without any significant change in results.

It is also possible to solidfy the beryl after melting by raising the pressure rather than diminishing the temperature. For example, the beryl can be melted at 1800 C. at a pressure of 10 kilobars and then be solidified by raising the pressure, eg to about 20 kilobars while maintaining the same temperature or any temperature that is below the melting point of beryl at 20 kilobars.

The high temperatures used in the method of this invention introduces limitations as to the material employed as the capsule for the beryl. If the capsule material melts at the temperatures employed, it will diffuse through the beryl preventing the formation of a single clear crystal and therefore the capsule must be made from a material that does not react with or diffuse into the beryl, i.e. it must be inert at high temperatures. It has been found that carbon in the form of graphite is an excellent capsule material at all pressures (melted beryl has been held in such a capsule for as long as one-half hour at 15 kilobars without reaction, mixing or dilfusion processes occurring) with carbon lined materials such as carbon lined Vycor glass giving equally good results. Capsules formed from:

platinum and tungsten are not as effective since the melting and solidification must be performed rapidly in order to prevent diffusion. Another means of avoiding the reaction and diffusion problem is to employ one of the constituents of beryl itself, e.g. BeO as the capsule material.

The process of this invention may be carried out in apparatus such as is shown generally as in FIG. 1. This apparatus is of the so-called stepped-core type and is composed of a plurality of concentric binding rings 11-15 inclusive, surrounding a tungsten carbide core 16. Core 16 has a cylindrical bore 17 therethrough, and counterbores 18 and 19 opening into each end thereof. In coaxial alignment with bore 17 are two

press pistons

23 and 24 having tungsten

carbide end elements

26 and 27 configured to cooperate with counter-bores 18 and 19 and bore 17 in order to impose pressure on the sample contained within bore 17. Pistons 23 and 24 are biased toward one another by apparatus (not shown) which may be a hydraulic press as is well known in the art. The particular apparatus used by the inventors is a 300 ton press.

The same container 28 is shown in detail in FIG. 2. The material to :be crystallized in the process is diagrammatically shown at 29 as contained within a closed capsule 31. Capsule 31 is disposed within a pressed talc cylinder 32 which is in turn supported within a carbon cylinder 33 which serves as a heat source when electric current is passed therethrough. Disposed within the tale cylinder 32 at each end of the sample capsule 31 are pyrophyllite plugs 34r and 35 and surrounding the outside of the carbon cylinder 33 is a body of pyrophyllite 36 which is configured to fit within bore 17. Completing the container assembly are a pair of electrically conductive end caps 37 and 38 which are disposed in counterbores in pyrophyllite cylinder 36 and are in electrical contact with the carbon cylinder 33. Electrical contact is made to the outside of

end caps

37 and 38 through the press pistons themselves, current being supplied to the press pistons by

conductors

39 and 40. Counterbores 18 and 19 are also provided with

annular pyrophyllite members

43 and 44, which along with the pyrophyllite in the sample container and the pressed talc cylinder serves to transmit pressure to the sample. With this apparatus it is possible to obtain pressures up to 60,000 atmospheres in the capsule.

The process is carried out in the apparatus by first adjusting the pressure to the proper value. The temperature is then raised until the powder melts by passing an electric current through carbon cylinder 33. The electric current is then abruptly cut off. Since there is a large mass of steel surrounding the sample the temperature is reduced to nearly room temperature in a matter of a few minutes. Under these conditions, the melt fuses into a single crystal of beryl which is removed from the press after the pressure has been reduced.

It is to be understood, however, that the method of this invention is not to be limited to the apparatus described above since the process may be performed by any means which can effect melting and solidification under pressure. It should also be apparent that many modifications can be made in the apparatus described above without departing from its general concept, e.g. some or all of the pyrophyllite members may be replaced by another pressure transmitting material such as boron nitride.

The process of this invention may be employed to form any of the crystalline members of the beryl family (aquamarine, goshenite, emerald, golden beryl and morganite) by adding the ionic impurity that gives the particular member its characteristic color. For example, if it is desired to produce emerald crystals, chromium ions are added to the beryl sample before melting and cooling under pressure, the chromium ions giving the finished crystal the characteristic green color of emeralds. The chromium ions may be introduced by adding an amount of Cr O to the beryl that does not exceed about 2 weight percent, preferably between about 0.5 and 1 weight percent with 1 weight percent giving especially good results.

-- Although the -proeessdescribed above is-Wholly satisfactory for the creation of large clear single crystals. many modifications can be-made without departing from the spirit of the invention. For example, water can be added to the beryl sample to reduce the temperature coeflicient, said reduction effecting 'a lowering of the process temperatures (1 weight percent of water reduces the'melting point of beryl from about 2000 C. to about 1500 C. at 15 kilobars). f i I Although water addition has a beneficial effect on process conditions, an excess of water adversely effects crystallization and from theoretical calculations it has been determined that the total amount of water present should not exceed about 6.5 weight percent. Since beryl may already contain some water (for example, beryl samples used in the form of goshen'ite usually :contain enough water to effect such a reduction without water addition), the water content should not exceed about 2 weight percent preferably not in excess of 1 weight percent.

It has been found that the temperature, of melting can be reduced even further by using fluorine gas as a flux. One way to obtain the fluorine is to use Teflon as a capsule material. At a temperature near 800 C., the Teflon? dissociates to give off fluorine which causes the beryl to melt inthe neighborhood of 1000 C. at 20 kilobars pressure. Another way to obtain the fluorine is to use CrF in place of Cr O as the coloring agent. When this is done, the fluorine gas is given off at 1000 C. at 15 kilobars.

In another alternative embodiment, the pressure is established at 10 kilobars and the temperature is raised to 1000 C., a temperature well below the melting temperature of the beryl and container material. The conditions of temperature and pressure must be held for a much longer time under these conditions in order to obtain sizeable crystals. In one example, the conditions were held for a period of 25 minutes, resulting in small microscopic crystals of beryl, while the major portion of the sample had the appearance of a gnanular rock. Much longer times are required to achieve larger crystals.

Example I Beryl powder (aquamarine) mixed with 2% Cr O and 1% H O was placed in a tungsten capsule and subjected to a pressure of 15 kilobars and a temperature of 1570 C. for two minutes in the apparatus of FIG. 1. After cooling and reducing the pressure, a single clear crystal was removed from the capsule which proved to be emerald.

Example II Similarly to Example I, beryl powder (aquamarine) mixed with 2% Cr O and 1% H O was placed in a tungsten capsule and subjected to a pressure of 15 kilobars and a temperature of 1470 C. for two minutes. After cooling, emerald crystal having a density of 2,716 was removed from the capsule.

Example III Similarly to Example I, beryl powder (aquamarine) mixed with 1% Cr O and 1% H O was placed in a carbon capsule and subjected to a pressure of 15 kilobars and a temperature of 1425 C. for five minutes. After cooling, it was found that single emerald crystal formed throughout the capsule.

Example IV Similarly to Example I, beryl powder (goshenite) mixed with 0.5% Cr O was placed in a carbon lined Vycor glass capsule and subjected to a pressure of kilobars and a temperature of 1535 C. After cooling it was found that single emerald crystal formed throughout the capsule.

ExampleV A mixture was prepared by combining 6.98 g. BeO, 9.485 g. A1 0 and 35.53 SiO To this mixture there was added 2 weight percent C1'2O3 and 2 weight percent H 0 and the mixture was placed in a tantalum capsule.

The mixture was subjected to a temperature of 2015 C. and a pressure of 15 kilobars for two minutes. The temperature was dropped rapidly and a single emerald crystal was formed in the capsule.

Example VI The mixture employed in Example V was heated at 2000 C. and 15 kilobars for one minute in a tantalum capsule. After rapid cooling it was found that emerald crystal formed throughout the capsule.

The method of this invention may be easily performed in any pressure system which is sufiicient to produce a pressure that will prevent diffusion of the beryl constituents during melting and solidification. The opticum conditions employed will vary from system to system and they may be easily determined by those skilled in the art from the teaching of this disclosure that the starting material may be melted and solidified under any pressure that prevents diffusion of the constituents and allows expansion of the crystal.

The method of this invention is extremely effective for producing single clear beryl crystals. The starting materials, beryl in any of its forms or the constituents of beryl, are readily available and any member of the beryl family can be produced by adding the proper coloring agent. Unlike previous processes for forming beryl crystals, the process of this invention is extremely rapid since the crystal can be formed as quickly as the starting material can be melted and solidified under pressure.

The method of this invention is especially valuable for the production of emerald crystals, since this particular form of beryl, as well as being a valuable gem, has particular utility in laser and maser technology. The emeralds produced by this invention have hardness, density, and optical properties that are far superior to any synthetic emeralds that have been produced in the past.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A method for producing beryl crystals which comprises melting a member selected from the group consisting of beryl powder and a mixture of the constituents of beryl in their proper proportions, and solidifying said member to produce beryl crystal, said melting and solidification being performed at a pressure sutficiently high to prevent self-diffusion of the constituents of beryl through the crystals yet not so high as to prevent expansion thereof during solidification.

2. The method of claim 1 wherein said member is beryl powder.

3. The method of claim 1 wherein said member is beryl powder in the form of goshenite.

4. The method of claim 1 wherein said member is beryl powder in the form of aquamarine.

5. The method of claim 1 wherein said member is the constituents of beryl in their proper proportions.

6. The method of claim 1 wherein said member contains an amount of water that does not exceed about 2 weight percent.

7. A method for producing emerald crystals which comprises, melting a member selected from the group consisting of beryl powder and a mixture of the constituents of beryl in their proper proportions, said member containing chromium ions, and solidifying said member to form emerald crystals, said melting and solidification being performed at a pressure sufiiciently high to prevent self-diffusion of the constituents of beryl through the crystal yet not so high as to prevent expansion thereof during solidification.

8. The method of claim 7 wherein said member is beryl powder.

9. The method of claim 7 wherein said member is beryl powder in the form of goshenite.

10. The method of claim 7 wherein said member is a mixture of the constituents of beryl in their proper proportions.

11. The method of claim 7 wherein said member contains an amount of water that does not exceed about 6 weight percent.

12. A method for producing emerald crystals which comprises melting a mixture of beryl powder and Cr O said Cr O being present in an amount not exceeding about 2 weight percent and solidifying said mixture to form emerald crystal, said melting and solidification being performed at a pressure sufficiently high to prevent selfdiflusion of the constituents of beryl through the crystal yet not so high as to prevent expansion thereof during solidification.

13. The method of claim 12 wherein the beryl powder is in the form of goshenite.

14. The method of claim 12 wherein the beryl powder is in the form of aquamarine.

15. The method of claim 12 wherein the mixture contains an amount of water that does not exceed about 2 Weight percent.

16. A method for producing emerald crystals which comprises melting a mixture of the constituents of beryl in their proper proportions, said mixture also containing Cr O in an amount that does not exceed about 2 weight percent, and solidifying the mixture to produce emerald crystal, said melting and solidification being effected at a pressure sufficiently high to prevent self-diffusion of the constituents of beryl through the crystal yet not so high as to prevent expansion thereof during soldification.

17. The method of claim 16 wherein the mixture contains an amount of water that does not exceed about 2 weight percent.

18. A method for producing emerald crystals which comprises melting a mixture of Cr O and a member selected from the group consisting of beryl powder and a mixture of the constituents of beryl in their proper proportions, said Cr O being present in an amount that does not exceed about 2 weight percent, and solidifying the mixture to form emerald crystal, said melting and solidification being performed at a pressure that ranges from about 7.5 to 15 kilobars.

19. The method of claim 18 wherein the mixture contains an amount of water that does not exceed about 2 weight percent.

20. A method for producing emerald crystals which comprises,

(a) placing a mixture of Cr O and a member selected from the group consisting of beryl powder and a mixture of the constituents of beryl in their proper proportions in a capsule, formed from material inert at high temperatures said Cr O being present in an amount not exceeding about 2 weight percent,

(b) placing said capsule in a pressure vessel having an electric heating element therein,

(0) raising the pressure on said capsule to a pressure between 7.5 and 15 kilobars,

(d) passing an electric current through said heating element to heat the mixture to its melting point under the established pressure, and

(e) solidifying the-melt to produce emerald crystal: 21. The method of claim 20 wherein the capsule is formed from a member selected from the group consisting of carbon and carbon lined materials.

References Cited UNITED STATES PATENTS 3/1969 Gordy 106-42 12/1943 Cooper 10642 6/1960 Wenturf 10642 8/1961 Wenturf 106-42 FOREIGN PATENTS 10/1959 Germany.

OTHER REFERENCES HELEN MCCARTHY, Prin1' u'y Examiner US. Cl. X.R.