WO2011004244A1

WO2011004244A1 - The genesis and identification of a diamond

Info

Publication number: WO2011004244A1
Application number: PCT/IB2010/001663
Authority: WO
Inventors: Stuart Alan Gilder; Maarten Johan De Wit
Original assignee: University of Cape Town
Current assignee: University of Cape Town
Priority date: 2009-07-06
Filing date: 2010-07-06
Publication date: 2011-01-13
Anticipated expiration: 2012-01-06
Also published as: ZA201109252B

Abstract

A method is provided for the determination of the genesis or likely genesis of a diamond wherein the magnetic properties of a diamond are measured at a higher temperature that is above the Verwey transition temperature, typically a temperature of about 300 Kelvin, and at a lower temperature that is below the Verwey transition temperature, typically about 4 Kelvin. The measurements are compared or processed in order to develop an indication as to characteristic inclusions in the diamond. Generally, measurements are made at a series of different temperatures between the higher temperature and the lower temperature. The magnetic measurements may include the determination of a magnetic hysteresis loop and the magnetic moment of the diamond. Measurements may be carried out with the diamond in different positions in order to determine its anisotropy. The magnetic field may have a maximum field strength of from 3 to 7 Tesla. A diamond is also provided that has an identification or certification record comprising data that includes, or is developed from, higher temperature and low temperature magnetic properties of the diamond.

Description

THE GENESIS AND IDENTIFICATION OF A DIAMOND

FIELD OF THE INVENTION

This invention relates primarily to the determination of the genesis of a diamond, or at least the determination of a likely genesis of a diamond, with a view to establishing or indicating its likely origin with possibly differing degrees of certainty. The invention also relates to the categorization of diamonds by means of particular properties thereof that may, depending on the particular properties concerned, also provide for the unique identification of a particular diamond.

BACKGROUND TO THE INVENTION

The illicit trade in conflict (blood) diamonds, typically rough diamonds, has been recognised as a source of funding of illegitimate armed forces. It is accordingly desirable that the origin, or at least the likely origin, of diamonds entering the commercial and processing chain be determined so that steps can be taken to combat the generation of such illicit funds. In this regard it would be desirable to categorize diamonds either individually, or according to common properties, so that they can be identified should they appear in the commercial and processing chain.

Diamond geochemistry is relatively well known, and unfortunately the bulk or trace element compositions of diamonds, per se, are of little value to reveal their origin. Geochemical methods are largely destructive, thus obliterating the diamond sample. A non-destructive technique that can identify these precious stones individually, or at least their likely origin, is presently not available. In any event, bulk geochemical analyses of diamonds would typically be blind to the growth mechanism or strain state acting when the diamonds formed. Such processes are fundamentally important in controlling the way in which inclusions grow and become oriented with respect to the diamond's crystallography.

Thus, to potentially learn something about the genesis of a diamond, a technique should be sought that is non-destructive and can yield information concerning their strain state and inclusion geometry.

Generally in the diamond field, US patent number 6,281 ,680 to Matthews et al describes the use of changes in magnetic fields occasioned by the introduction of a diamond into the physical realm of the magnetic field for the purpose of distinguishing between natural diamonds and synthetic diamonds. This is simply done on the basis that synthetic diamonds contain significantly more iron inclusions and therefore affect a magnetic field to a greater extent.

US patent number 4,125,770 to Lang describes a method of identifying individual diamonds using x-ray topography without any reference whatsoever to the origin of the diamond.

OBJECT OF THE INVENTION

It is an object of this invention to provide a method for the determination of the genesis or likely genesis of a diamond. It is another and independent object of the invention to provide a method of identifying unique characteristics of a particular diamond for purposes of individual identification down the line. It is another object of this invention to provide a method of identifying a class of diamonds having characteristics indicating common origin. SUMMARY OF THE INVENTION

In accordance with one aspect of this invention there is provided a method for the determination of the genesis or likely genesis of a diamond characterized in that the method comprises measuring the magnetic properties of a diamond at a higher temperature that is above the Verwey transition temperature; measuring the magnetic properties of the same diamond at a lower temperature that is below the Verwey transition temperature; and comparing or processing the two measurements in order to develop an indication as to characteristic inclusions in the diamond.

Further features of this aspect of the invention provide for the higher temperature measurement to be carried out at generally ambient temperature, especially at about 300 Kelvin; for the lower temperature measurement to be carried out at a temperature in the region of the boiling point of helium, namely at about 4 Kelvin; for the measurements to be carried out at a series of different temperatures between the higher temperature and the lower temperature; and for the magnetic measurements to include the determination of a magnetic hysteresis loop or the magnetic moment of the diamond, or both.

Still further features of the invention provide format least the higher temperature and lower temperature measurements to be carried out with the diamond in different positions in order to determine its anisotropy; for the magnetic field to have a maximum field strength of from 3 to 7 Tesla and preferably of the order of 5 Tesla; and for the method to be preceded by an initial room temperature measurement of the magnetic hysteresis loop in order to determine diamagnetic, paramagnetic and ferromagnetic contributions to the magnetic properties.

In accordance with a second aspect of the invention there is provided a diamond characterized in that it has an identification or certification record comprising data that includes, or is developed from, higher temperature and lower temperature magnetic properties of the diamond.

It will be understood that the nature or state of the inclusions in a diamond under examination can be compared with characteristics of a variety of different diamonds each of which is known to have a particular origin or to come from a particular deposit. Any correlation or similarity with the characteristics of diamonds known to have a particular origin or come from a particular deposit can be used to suggest that the diamond under examination has, or is likely to have, a particular origin.

In order that the invention may be more fully understood a further more detailed description there are fathers with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:-

Figure 1 is a plot of the magnetisation or magnetic moment of a sample with magnetite over a range of temperatures;

Figure 2 is a plot of room temperature SIRM (saturation isothermal remanent magnetization) data indicating that the magnetite may be oxidized;

Figure 3 is a plot of the magnetisation or magnetic moment of a sample with hematite over the range of temperatures;

Figure 4 is a plot of room temperature SIRM data indicating that the hematite may not be oxidized; Figure 5 is a plot of the magnetisation or magnetic moment of a sample with pyrrhotite over the range of temperatures;

Figure 6 is a plot of initial room temperature measurements of the magnetic hysteresis loop a diamond that exhibits only diamagnetism;

Figure 7 is a plot of initial room temperature measurements of the magnetic hysteresis loop a diamond that exhibits only paramagnetic magnetic qualities; and,

Figure 8 is a plot of initial room temperature measurements of the magnetic hysteresis loop a diamond that exhibits both paramagnetic and ferromagnetic qualities.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

Solid inclusions are virtually never spherical in diamonds. This means that the solid inclusions inherently have shape anisotropy. Moreover, the inclusions may be aligned according to the growth pattern of the diamond, such as along zones or certain crystallographic axes. A non-invasive way to quantify the geometrical alignment of an assemblage of magnetic inclusions is to study the magnetic properties and anisotropy of the diamond. This can be done in two ways.

One is via the anisotropy of magnetic susceptibility. This method yields the eigenvalues of the susceptibility tensor and thus defines the shape of the anisotropy ellipsoid.

Another way is via the anisotropy of magnetic remanence. This method is much more time consuming (typically requiring of the order of one and a half hours per sample), but potentially more revealing because the technique can be applied under different imposed magnetic field conditions. What this means is that the anisotropies of specific magnetic species and the specific grain size distribution of those species can be analyzed.

If a diamond underwent a multiple growth history, with each growth period having a slightly different magnetic composition or characteristic grain size, these differences can be studied individually and thus the generations of growth can be better understood. Thus a diamond's genesis may be uncovered by exploiting the fact that magnetic characteristics are highly sensitive to composition, oxidation and strain.

Paleomagnetic laboratory equipped with a Kappabridge may be used to measure the anisotropy of magnetic susceptibility. A three axis superconducting magnetometer together with coil systems that can measure the anisotropy of magnetic remanence may be used. This magnetometer is fitted with a low temperature insert and a pulse magnetizer that can measure the full magnetic vector of materials to 4 Kelvin.

Like a fingerprint or iris scan, a non-destructive technique has been developed to unambiguously identify the unique magneto-genetic code of diamonds based on the minute inclusions within them. This primarily exploits the multivariate signatures of the magnetic properties at low temperatures. This is because most minerals undergo marked phase transitions the characteristics of which are sensitive to abundance, particle size, composition, oxidation state and strain history. Slight differences in these properties from one diamond to the next makes magnetic characterization a unique tool.

A method for the determination of the genesis or likely genesis of a diamond according to the invention is thus based on measuring the magnetic properties of a diamond, generally including the determination of the magnetic hysteresis loop and the magnetic moment of the diamond. The applied magnetic fields preferably have an intensity of about 5 Tesla.

The measurements are carried out both at a higher temperature that is above the Verwey transition temperature which is about 120 to 125 Kelvin and that is preferably at approximately ambient temperature of about 300 Kelvin; and at a lower temperature that is below the Verwey transition temperature and preferably a low temperature such as about 4 Kelvin. A series of measurements of the magnetic properties is made, in addition, at multiple temperatures in between. These measurements may be made upon cooling or warming to or from 4 Kelvin.

The phase change known as the Verwey transition in magnetite occurs at around 120 K and will actually take place at a temperature, and have a shape in magnetic moment vs. temperature space, depending on the oxidation state, the grain size, the composition and the internal stress of the magnetite.

An example of the plots of the magnetisation or magnetic moment for a sample with magnetite over the range of temperatures indicated above is shown in Figure 1. The one series of the determination was carried out in a 5 Tesla induced magnetic field [indicated by the letters FC] and the other series in a zero induced magnetic field [indicated by letters ZFC].

Oxidation smears out the transition while small additions of aluminum, titanium, etc., will lower the temperature of the transition. When cooled through the Verwey transition temperature in the presence or absence of an external magnetic field, large, multi-domain magnetite grains exhibit different responses from those of small single domain grains. Thus, much can be shown by the response of the magnetic moment at low-temperatures. Even a few nanometer-sized grains produce enough signal to be measurable. Further, and with reference to Figure 2, it can be seen from the absence of magnetic memory in the room temperature SIRM (saturation isothermal remanent magnetization) data that the magnetite may have been oxidized.

Figure 3 illustrates the behavior of hematite over the range of temperatures indicated above both in a 5 Tesla field [indicated by the letters FC] and in a zero induced magnetic field [indicated by the letters ZFC]. Further, and with reference to Figure 4, it can be seen that in this instance a magnetic memory is present, as indicated by the room temperature SIRM data shown.

Figure 5 illustrates the magnetic moment of pyrrhotite over the range of temperatures indicated above both in a 5 Tesla field [indicated by the letters FC] and in a zero induced magnetic field [indicated by the letters ZFC].

The measurements can then be analysed or otherwise processed in order to develop an indication as to characteristic inclusions in the diamond and also, if more data is developed, then the location of the inclusions can be determined and this information can be used to uniquely identify a particular diamond in the manner of a fingerprint.

As a general rule, the method outlined above is preceded by initial room temperature measurements of the magnetic hysteresis loop in order to determine the diamagnetic, paramagnetic and ferromagnetic contributions to the magnetic properties of the diamond.

If a diamond exhibits only diamagnetism, as indicated graphically in Figure 6, then no more magnetic measurements are practical as the inclusions or impurities are insufficient to react adequately according to the invention and the diamond is of a generally high purity. The results of this preliminary measurement can be used as a quantifiable value to assign all diamonds a purity index. In the event that the diamond exhibits only paramagnetic magnetic qualities then the graphical representation of the results will be of the general nature illustrated in Figure 7.

If a diamond exhibits both a paramagnetic and a ferromagnetic component, then the graphical representation of the results will be of the general nature illustrated in Figure 8.

In the event that a diamond is to be further investigated according to the invention, then the diamond is subjected to a series of measurements at room temperature in order to develop magnetic hysteresis loops on the same diamond in different positions with a view to developing the room temperature magnetic anisotropy.

In the event that a ferromagnetic phase is identified, then the hysteresis loops of the diamond are measured from 300 Kelvin to 4 Kelvin.

In the event that no ferromagnetic phase is identified then a determination is made as to whether the sample has superparamagnetic phase(s) in it and the frequency response of the material from 300 Kelvin to 4 Kelvin is measured. This step may also be carried out if ferromagnetic material is present.

The nature or state of the inclusions in a diamond under examination can be compared with characteristics of diamonds which are known to have a particular origin or to have come from a particular deposit. Any correlation or similarity in characteristics can be used to suggest or even positively confirm that the diamond under examination has, or is likely to have, a particular origin. Alternatively, or in addition, using the principles described above, data may be developed that can be used for unique identification or certification records.

It will be understood that numerous variations may be made to the procedure described above without departing from the scope hereof.

Claims

CLAIMS:

1. A method for the determination of the genesis or likely genesis of a diamond characterized in that the method comprises measuring the magnetic properties of a diamond at a higher temperature that is above the Verwey transition temperature; measuring the magnetic properties of the same diamond at a lower temperature that is below the Verwey transition temperature; and comparing the two measurements in order to develop an indication as to characteristic inclusions in the diamond.

2. A method as claimed in claim 1 in which the higher temperature measurement is carried out at generally ambient temperature.

3. A method as claimed in claim 2 in which the higher temperature measurement is carried out at about 300 Kelvin.

4. A method as claimed in any one of the preceding claims in which the lower temperature measurement is carried out at a temperature in the region of the boiling point of helium.

5. A method as claimed in any one of the preceding claims in which the measurements are carried out at a series of different temperatures between the higher temperature and the lower temperature.

6. A method as claimed in any one of the preceding claims in which the magnetic measurements include the determination of a magnetic hysteresis loop or the magnetic moment of the diamond, or both.

7. A method as claimed in any one of the preceding claims in which measurements are carried out with the diamond in different positions in order to determine its anisotropy.

8. A method as claimed in any one of the preceding claims in which the magnetic field has a maximum field strength of from 3 to 7 Tesla.

9. A method as claimed in claim 8 in which the magnetic field has a maximum field strength of the order of 5 Tesla.

10. A method as claimed in any one of the preceding claims in which the method is preceded by an initial room temperature measurement of the magnetic hysteresis loop in order to determine diamagnetic, paramagnetic and ferromagnetic contributions to the magnetic properties.

11. A method as claimed in any one of the preceding claims in which the nature or state of the inclusions in the diamond are compared with characteristics of different diamonds known to have a particular origin or to come from a particular deposit so as to indicate that the diamond under examination has, or is likely to have, a particular origin.

12. A diamond characterized in that it has an identification or certification record comprising data that includes, or is developed from, higher temperature and low temperature magnetic properties of the diamond.