No.11 January 2012 (Updated May 2015)

World of Magnificent SpinelsProvenance and Identification

VietnamBurma

Inclusions in Spinel

Cornflower-Blue Spinel over10ct from Vietnam

Online release pages 1-2, 269-282

Fig. 1 Magnificent cornflower blue Vietnamese Cobalt spinel over 10 carats in size. This type of colour is considered to be the most expensive. ©Yavorskyy, Bangkok

WORLD OF MAGNIFICENT SPINEL: PROVENANCE AND IDENTIFICATIONAdolf Peretti (1) Anong Kanpraphai-Peretti (2) and Detlef Günther (3)(1) GRS Gemresearch Swisslab AG, Sempacherstr. 1, CH-6003 Lucerne, Switzerland(2) Anong Kranpaphai-Peretti, GRS (Thailand) Co LTD, JTC Building, Silom 19, Bangrak, Bangkok 10500 (Thailand)(3) Laboratory of Inorganic Chemistry, ETH Hönggerberg, HCI, G113, CH-8093 Zurich, Switzerland

INTRODUCTION

The ancient gemstone spinel has experienced an enor-mous shift in popularity over the last decade. For centuries, the transparent stone was coveted by royalty, especially when found in large sizes like the famous ‘Samarian Spinel’ from the Iranian Crown Jewels, considered to be the largest of its kind at 500 carats. Several pieces of the British Crown Jewels include vivid red spinels, such as the famous ‘Black Prince’s Ruby’, which were erroneously named because there were no scientific methods for separat-ing similarly coloured gemstones at the time of their discovery. Spinel may have also occurred in deposits where ruby was found, further complicating its identifi-cation in earlier times.

Attributes of its value

Like any precious stone, there are traits it needs to sustain its appeal. Beauty, rarity and durability of the stone are key factors, but colour is also critical for its wide acceptance. Fortunately, spinel occurs in a variety of colours besides the red favoured by royalty. Spinel is one of very few gemstones that are found in a vast array of colours, such as blue (ranging from greyish-blue to cornflower blue), reds (from vibrant pinkish-red to vivid red), oranges and all kinds of fancy colours. Besides its rainbow of colours, another winning feature of spinel is that it can be found in large sizes, often combined with good clarity. With a hardness rating of 8 on the Mohs scale, spinel is a very durable material. Gem-quality spinel, often found in the same deposits alongside ruby or sapphire, is a very rare occurrence today.

Regions determine colour

Geology plays an important role in the location where certain colours of spinel are found. The most sought-after colours in spinel are vivid red and cornflower blue. The vivid red varieties are found principally in the Mogok Valley in Burma (Myanmar). On the other hand, the cornflower blue material derives mainly from Vietnam, but it can also be found in Sri Lanka and Tanzania. This blue variety has become known in the trade as Cobalt Spinel.

In the last decade, the discovery of spinel deposits in Vietnam, Tanzania and northern Burma’s (Myanmar) Namya region took the gem world by surprise with the production of incredibly beautiful colours such as the

vibrant pinkish-red spinel from Namya. Similarly striking colours have also surfaced in the Mansin mines located in the Mogok valley. Beside the vivid red spinel varieties, the attractive pinkish-reds received the highest prices for spinel around the world. Along-side the more recently discovered deposits, the classi-cal and historically known spinel mining sites in the Pamir mountains of Tajikistan have resumed produc-tion, unearthing important large stones over 40 carats in size. However, not all spinels are recovered from mountainous mining sites. Spinel are also found in ancient river sites known as alluvial deposits in Tanza-nia, Vietnam, Madagascar, Sri Lanka and Burma (Myanmar). Primary marble deposits in Burma (Myanmar), Vietnam and Tanzania as well as olivine veins within marble deposits in Tajikistan are also a source of spinel.

Undisclosed enhancements

Corundum (ruby and sapphire) had to deal with a shat-tering blow in recent years upon revelations of non- disclosed product enhancements such as beryllium treatment, lead-glass treatment, high-temperature treatment with fissure healing and the resurgence of conventional diffusion-treatments. This has resulted in a loss of consumer confidence that has yet to be fully

restored. Spinel by contrast has remained free of such disgrace since it has not yet been subject to similar covert and widespread treatment. Recently, co-diffusion and heat-treatment has appeared in spinel but fortunately it can be easily detected.

Impact of JADE Act

Before the new generation of African ruby was discov-ered in Tanzania (Winza) and Mozambique, spinel was considered a highly desired alternative to the ever increasingly rare ruby emerging from the classical Burmese (Myanmar) origins. Back in 2008, the ban on Burmese ruby, called the ‘Tom Lantos Block Burmese JADE (Junta's Anti-Democratic Efforts) Act of 2008’, banned all importation of Burmese ruby and jade into the US in protest of human rights violations occurring in that country. With that embargo in place, the interna-tional gem- trade scrambled to fill the demand for top-quality ruby by looking for close alternatives like fine spinel. This refocus actually provided a big pay-off in bringing interesting new colours to the fore espe-cially where couture high-jewellery was concerned.

Price hike

The rise in popularity of spinel is of course reflected in its price surge as shown in the chart above. In the last five to ten years, spinel prices have increased more than 10-fold depending on its colour and origin. Today, spinel prices are comparable to those of unheated sapphire; this is particularly true with cornflower blue spinel from Vietnam or vivid red spinel from Mogok, Burma (Myanmar) and vibrant red spinel from Mahenge, Tanzania. The principal attribute of spinel’s surprising discovery to the gemmological world is its immense variability by colour and geographic traits particular to specific regions. This is an enormous help to researchers in separating spinel by origin and identifying treatments in the microscope.

Introduction

01

Fig. 2 A set of three faceted Sri Lankan spinels ranging from 12 to 16 carats in size. A typical colour variety of this origin is the pastel purple to violet ‘lavender’ colours. © Yavorskyy, Bangkok

Fig. 3a-b (Left) A tray of faceted Burmese spinel at the gem market in Mogok. (Right) Vietnamese spinel with star effect formed by inclusions of silk arranged in three different directions. © GRS Gemresearch Swisslab

INTRODUCTION

The ancient gemstone spinel has experienced an enor-mous shift in popularity over the last decade. For centuries, the transparent stone was coveted by royalty, especially when found in large sizes like the famous ‘Samarian Spinel’ from the Iranian Crown Jewels, considered to be the largest of its kind at 500 carats. Several pieces of the British Crown Jewels include vivid red spinels, such as the famous ‘Black Prince’s Ruby’, which were erroneously named because there were no scientific methods for separat-ing similarly coloured gemstones at the time of their discovery. Spinel may have also occurred in deposits where ruby was found, further complicating its identifi-cation in earlier times.

Attributes of its value

Like any precious stone, there are traits it needs to sustain its appeal. Beauty, rarity and durability of the stone are key factors, but colour is also critical for its wide acceptance. Fortunately, spinel occurs in a variety of colours besides the red favoured by royalty. Spinel is one of very few gemstones that are found in a vast array of colours, such as blue (ranging from greyish-blue to cornflower blue), reds (from vibrant pinkish-red to vivid red), oranges and all kinds of fancy colours. Besides its rainbow of colours, another winning feature of spinel is that it can be found in large sizes, often combined with good clarity. With a hardness rating of 8 on the Mohs scale, spinel is a very durable material. Gem-quality spinel, often found in the same deposits alongside ruby or sapphire, is a very rare occurrence today.

Regions determine colour

Geology plays an important role in the location where certain colours of spinel are found. The most sought-after colours in spinel are vivid red and cornflower blue. The vivid red varieties are found principally in the Mogok Valley in Burma (Myanmar). On the other hand, the cornflower blue material derives mainly from Vietnam, but it can also be found in Sri Lanka and Tanzania. This blue variety has become known in the trade as Cobalt Spinel.

In the last decade, the discovery of spinel deposits in Vietnam, Tanzania and northern Burma’s (Myanmar) Namya region took the gem world by surprise with the production of incredibly beautiful colours such as the

vibrant pinkish-red spinel from Namya. Similarly striking colours have also surfaced in the Mansin mines located in the Mogok valley. Beside the vivid red spinel varieties, the attractive pinkish-reds received the highest prices for spinel around the world. Along-side the more recently discovered deposits, the classi-cal and historically known spinel mining sites in the Pamir mountains of Tajikistan have resumed produc-tion, unearthing important large stones over 40 carats in size. However, not all spinels are recovered from mountainous mining sites. Spinel are also found in ancient river sites known as alluvial deposits in Tanza-nia, Vietnam, Madagascar, Sri Lanka and Burma (Myanmar). Primary marble deposits in Burma (Myanmar), Vietnam and Tanzania as well as olivine veins within marble deposits in Tajikistan are also a source of spinel.

Undisclosed enhancements

Corundum (ruby and sapphire) had to deal with a shat-tering blow in recent years upon revelations of non- disclosed product enhancements such as beryllium treatment, lead-glass treatment, high-temperature treatment with fissure healing and the resurgence of conventional diffusion-treatments. This has resulted in a loss of consumer confidence that has yet to be fully

restored. Spinel by contrast has remained free of such disgrace since it has not yet been subject to similar covert and widespread treatment. Recently, co-diffusion and heat-treatment has appeared in spinel but fortunately it can be easily detected.

Impact of JADE Act

Before the new generation of African ruby was discov-ered in Tanzania (Winza) and Mozambique, spinel was considered a highly desired alternative to the ever increasingly rare ruby emerging from the classical Burmese (Myanmar) origins. Back in 2008, the ban on Burmese ruby, called the ‘Tom Lantos Block Burmese JADE (Junta's Anti-Democratic Efforts) Act of 2008’, banned all importation of Burmese ruby and jade into the US in protest of human rights violations occurring in that country. With that embargo in place, the interna-tional gem- trade scrambled to fill the demand for top-quality ruby by looking for close alternatives like fine spinel. This refocus actually provided a big pay-off in bringing interesting new colours to the fore espe-cially where couture high-jewellery was concerned.

Price hike

The rise in popularity of spinel is of course reflected in its price surge as shown in the chart above. In the last five to ten years, spinel prices have increased more than 10-fold depending on its colour and origin. Today, spinel prices are comparable to those of unheated sapphire; this is particularly true with cornflower blue spinel from Vietnam or vivid red spinel from Mogok, Burma (Myanmar) and vibrant red spinel from Mahenge, Tanzania. The principal attribute of spinel’s surprising discovery to the gemmological world is its immense variability by colour and geographic traits particular to specific regions. This is an enormous help to researchers in separating spinel by origin and identifying treatments in the microscope.

Fig. Sp3a Fig. Sp3b

Introduction

02

Page 3 to 268and 283-293coming soon

In order to understand the potential for colour enhancement of spinels by heat-treatment, we have performed heat-treatment experiments. For this purpose, we have acquired a heat-treatment furnace in Mogok and used the local heat-treatment techniques available in the Mogok area (Fig. 298a-l). 60 Spinel samples were also acquired in Mogok area. The stones were cut in half, perpendicular to the table (Box. 7). One half of the stone (sample A) was kept as an unheated reference sample. Comparing spinel before and after heat-treatment (Fig. 299c-d), the major colour shifts observed were removal of brownish and orangey hues to become more red, with corresponding changes in the UV-VIS spectrum. Heat-treatment of spinel has been analyzed by Photoluminescence spectroscopy as shown in Fig. 303-306 and by Raman spectroscopy (Fig. 306c). The results clearly showed that we could differentiate heated from unheated spinel by this method as well as spinels produced by different synthetic production methods. For the understanding of the structural changes in heat-treated spinel’s, we analyzed the corresponding unheated (sample A) and heated-treated spinel (sample B) for structure analysis at the University of Berne, Switzerland (March 2012).

CRYSTAL STRUCTURE ANALYSISMETHODSWe have measured two crystals, sample A and sample B by single-crystal X-ray analysis (Bruker Apex2 CCD two-circle diffractometer, graphite monochromated MoKα radiation). One crystal was little bit larger allowing shorter exposure time and the second was relatively smaller. One of the major problems with gem spinels is the high crystal quality with very low degree of mosaicity. Such crystals tend to show strong extinction effects and multiple diffraction phenomena (Renninger effect). Due to nearly perfect periodicity of the crystals (low mosaicity) the diffracted X-ray beam acts as an additional “primary beam” and this beam is diffracted again. A typical observation in such cases is decreased intensity of strong reflections and increased intensity of weak reflections. To reduce the effect of extinction and multiple diffraction, small crystals are superior. However, there is the disadvantage that for small crystals, low intensity reflections are always associated with high standard deviations.

RESULTSThe structure of a spinel grown at low temperatures (such as in nature typically at about 500-600 °C) is

altered when heated at a high temperature due to intra-crystalline diffusion. This has been verified by X-ray single-crystal structure analysis of a gem spinel before and after heating. In its natural state, the investigated spinel (spinel A) had the cell dimension a = 8.0875(1) Å. After heating the (spinel B) cell dimension decreased slightly to a = 8.0850(1). Most significantly, however, the oxygen parameter u = 0.26326(3) changed to 0.26157(12). Oxygen parameters > 0.2625 characterize a spinel structure in which the tetrahedral site (T) has a longer T-O distance than the M-O distance of the octahedral site (M). The opposite holds for u < 0.2625. This structural change with heating was interpreted to mean that in the untreated (natural) spinel the larger Mg ion preferentially resides at the tetrahedral site (T-O: 1.9368(4) Å) whereas the smaller Al concentrates at the octahedral site (M-O: 1.9206(2) Å). After heat treatment, Mg had diffused to the octahedral site (M-O: 1.9323(9)) whereas some Al was substituted at the tetrahedral site (T-O: 1.9124(17) Å). A corresponding behaviour has been shown before for synthetic MgAl O spinel (Andreozzi et al., 2000) and a red spinel from Myanmar heat-treated under laboratory conditions (Widmer et al, 2014). A more direct analysis of intracrystalline diffusion by X-ray diffraction methods is hampered by the similarity of X-ray scattering factors for Mg (12 electrons) and Al (13 electrons). The intracrystalline diffusion due to heat treatment of gem spinel may be interpreted as a transformation from a “normal” spinel MgIVAl VIO towards inverse spinel AlIV(MgAl)VIO4. The low concentration of red chromophores (Cr3+) has no significant bearing on this behaviour. In inverse spinels, two different cationic species (Mg, Al) are disordered at the octahedral site. Because of the significant size difference (radius Mg2+ = 0.72 Å, radius Al3+ = 0.535 Å, Shannon, 1976) inverse spinels are also characterized by enlarged smearing of oxygen atomic displacement-parameters. This is due to a statistical overlay of short Al-O and long Mg-O distances where the position of the cation is fixed by symmetry. For the (untreated) natural spinel (A) the isotropic (equivalent) atomic displacement parameter of oxygen converged to 0.00580(7) Å2 whereas the corresponding value for the heat-treated spinel (B) was almost twice as high (0.01134(51) Å2). This behaviour is consistent with the assumed cation distribution. The temperature of the heat-treatment experiment was estimated on the basis of the X-ray analytical data as approximately 1000 oC and this estimate is in general agreement with the experimental data by Widmer et al. (2014).

GENERAL FEATURES OF THE SPINEL STRUCTURE

In the spinel structure of cubic space group Fd3m, cation positions are fixed by symmetry and the only variable structure parameter, apart from anisotropic atomic displacements, is the so called oxygen parameter u defining the oxygen coordinates at x = u, y = u, z = u. The centre of the tetrahedral site is at 1/8, 1/8, 1/8 and the centre of the octahedral site is at 1/2, 1/2, 1/2. Thus the metrical relationships of the spinel structure are fully explained by the cell dimension a and the oxygen parameter u.

HEAT-TREATMENT OF SPINEL Adolf Peretti (1) Ngwe Lin Tun (2)Thomas Armbruster (3)(1) GRS Gemresearch Swisslab AG, Sempacherstr. 1, CH-6003 Lucerne, Switzerland(2) GRS-New Aurora Lab, Mogok, Myanmar(3) Mineralogical Crystallography, Institute of Geological Sciences, University of Bern, CH-3012 Bern, Switzerland

Spinel Crystal Structure

269

AB2O4 “Normal Spinel” MgAl2O4

Fig. 297a-b The spinel structure is shown in Fig. 297a as a polyhedral model. It consists of octahedral and tetrahedral polyhedra. Each corner is occupied by an oxygen anion and in the centre of these polyhedra is either empty or occupied by a cation in their centres. In the different types of spinel, only particular cations can be found in the octahedral and in the tetrahedral sites, respectively, and moreover, only a certain portion of the tetrahedra and octahedra are occupied. This depends on the type of spinel (See Box 6). The unit cell of the mineral spinel contains 8 formula units of MgAl2O4. On the right side a layer parallel to the (111) face is shown. In this special view of the crystal structure of spinel, the connection and arrangement of octahedra and tetrahe-dra are clearly seen. Only Al occupies half of the octahedra and Mg occupies only 1/8 of the tetrahedral positions. When a normal spinel is transformed in an inverse spinel during heat-treatment, Mg from tetrahedral position moves into octahedral positions and Al moves from octahedral to tetrahedral positions (Arrow).

NORMAL SPINEL All occupied octahedra (B-position) contain only Al All occupied tetrahedra (A-position) contain only Mg 1/8 of all tetrahedral positions in a closest packed arrangement of X anions are occupied by Mg and 1/2 of all octahedral postions are occupied by Al. This results in the formula: A(BB)X4 or for example MgAl2O4 (Spinel), CoAl2O4, ZnAl2O4, Mn2+Mn3+2O4 (hausmannite) The following cations of 2 positive charges can be found in the tetrahedral position of A: Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd The cations of 3 positive charges can be found in the octahedral positions of B: Al, Ga, In, Ti, V, Cr, Mn, Fe, Co, Rh Other cations with other valences of 1, 4, and 6 are also found in spinel, depending on their ion radius.

INVERSE SPINEL 50% of all magnesium (Mg) and 50% of all alumina (Al) ions are found in the octahedral positions (centre of the blue octahedra of Fig. 297a) 1/8 of all tetrahedral and 1/4 of octahedral positions are occupied by Al and another 1/4 of all octahedral positions are occupied by Mg. This results in the formula: B(BA)X4 e.g. MgFe2O4, Fe3+(Fe2+Fe3+)O4 or Fe3O4 (magnetite), CoFe2O4, Fe(NiFe)O4 Magnetite is an example of a natural mineral that is an inverse spinel. In summary, in comparison to the normal spinel, in the case of the inverse spinel, the positions of Mg and Al have changed within the crystal lattice.

DEFECT SPINEL Some of the normally occupied positions in spinel remain empty. For example Mg2+ can be replaced by Al3+ that results in a gamma-Al2O3 structure.

BOX 6 Different Spinel types

Fig. 297a Fig. 297b

TM

O

Unit cell

Spinel Crystal Structure

270

Fig. 298A Fig. 298B

Fig. 298E Fig. 298F

Fig. 298I Fig. 298J

The spinels are imbedded in clay and than filled in the crucible.

The furnace is held at a temperature above 1200 C. After 30-40 minutes the spinels are removed from the oven.

Experimental set up for heat-treatment of spinels in Mogok (Burma, Myanmar) with the following components: Clay, crucible, and charcoal, open air oven.

Experimental Heat-Treatment of Spinel in Mogok

271

Fig. 298C Fig. 298D

Fig. 298G Fig. 298H

Fig. 298K Fig. 298L

The furnace is ventilated with a hand-driven ventilator with simple air.

The heat-treated spinels are separated from the clay. About 300 pieces were heat-treated experimentally.

Spinels are cut half for purpose of keeping half of the material untreated. Half of the material is filled into the crucible. Half is kept untreated.

Experimental Heat-Treatment of Spinel in Mogok

272

N H

N H

N H

N H

N H

N H

N H

(A) Less orange and less brown and more toward pink color

Orange Orangy Pink

Orange Purple

Pastel Pinkish Orange Pastel Pink

Pinkish Orange Orangy Pink

Pinkish Orange Pink

Pinkish Orange Purplish Orange

Pastel Orangy Pink Pastel Pink

Orangy Pink Purplish Pink

Brownish Orange Purple Pink

Orangy Pinkish Red (Light Orangy) Pinkish Red

(B) More towards pink and increase in saturation

Pink Pink (10% more saturation)

Pastel Pink Purple Pink

Pastel Pink Pink

Purplish Pink Purplish Pink (10% more saturation)

Blue Blue (10% more saturated)

Pastel Greyish Blue Pastel Greyish Purple (more saturation)

(C) No Change in hue, tone and saturation

Pastel Pink Pastel PinkPurplish Red Purplish RedViolet Violet

Box 7: Color change with heat-treatment

Half heat-treatedHalf unheated

Heat-Treatment Experiments of Spinels: Color Enhancement

273

Fig. 299c Fig. 299d

N H

N H N H

Fig. 299a Fig. 299b

Fig. 299a-h

Fig. 299e

Fig. 299f

Fig. 299a-b Cracks are formed around octahedral negative crystals by heat-treatment during our heat-treatment experiments

Fig. 299c-d Two halves from the same spinel are shown. One half was spared from thermal treatment. The colour shift from orange-red to purplish-red is due to heat-treatment.

Fig. 299e-f Two parts of the same original spinel are shown. One spared of heat-treatment (left side). The black graphite inclusions (without crack in sample on the left) developed a crack around them after heat-treatment (right hand side).

Inclusions in Heat-Treated Spinel

274

665.

6 672.

2 674.

267

5.8

685.7

687.2 707.

4

717.

6

0

100

200

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heated-ref-20503pl405rt-1-284cnoheat-ref-20503pl405rt-1-105csynthetic-ref-2181pl405rt-1-471c

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696.

369

7.9

700.

1

722.6

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0

20

40

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80

100synthetic flux spinel (black)versus heated natural spinel

(red)

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heated-ref-20503pl405rt-1-284ctsynthetic-ref-2181pl405rt-1-471ct

Inte

nsity

Compare_PL405_heat_synthetic_RT

Fig. 301a-b PL spectra (excitation 405nm) of natural spinel recorded at room temperature (RT) for the respective groups: Unheated spinel and synthetic Flux spinel (Fig. 301a) and heat-treated spinel and synthetic Flux-grown spinel (Fig. 301b). The ordinate shows the intensity (counts) in arbitrary units. Note that heat-treated and synthetic spinel cannot be distinguished by this test but unheated spinel can be distinguished from both, heat-treated and synthetic flux-grown spinel at room temperature.

Fig. 300 An insider’s look at the GRS diamond research laboratory instrumentation in Hong Kong and Switzerland. On the left side is the Photoluminescence analysis system with 2 Laser lights at 405nm and 532nm. The right side shows UV-VIS-NIR instrumentation. All measurements are conducted at liquid nitrogen or room temperatures. Results see Fig. 301-306.

Fig. 301a Fig. 301b

Photoluminescence of Spinel

275

wavenumber (cm-1) wavenumber (cm-1)

388.

9

544.

6

386.

8

534.

5

heated-ref-20503-uis-ln-1-284ctheated-ref-20504-uis-ln-0-953ctheated-ref-20505-uis-ln-1-193ctnoheat-ref-20503-uis-ln-1-284ctnoheat-ref-20504-uis-ln-0-820ctnoheat-ref-20505-uis-ln-0-713ct

300 400 500 600 700 800 900 1000

Abs

orba

nce

(arb

itrar

y un

its)

Fig. 302b

300 400 500 600 700 800 900 1000

heated-ref-20505-uis-ln-1-193ctnoheat-ref-20505-uis-ln-0-713ct

Abs

orba

nce

(arb

itrar

y un

its)

687.0

716.

8

671.

367

3.4

674.

9

684.8685.1

686.

568

8.9

695.

5 697.

269

9.4

704.

370

6.6

708.

8 717.

071

9.8

721.

9

heated-ref-20504pl532-ln-0-953noheat-ref-20504pl532-ln-0-820

660 680 700 720 740 wavelength (nm)

Inte

nsity

660 680 700 720 740 wavelength (nm)

heated-ref-20505pl405ln-1-193cnoheat-ref-20505pl405ln-0-713c

Inte

nsity

687.0

673.

367

4.8

684.9

686.

368

8.8 69

5.3 69

7.0

699.

3

704.

1 706.

470

8.6

716.

871

9.7 72

1.8

Fig. 302a-b UV-VIS-near IR absorption spectra recorded at liquid nitrogen temperature (LNT) for the respective groups: Unheated and heated spinel pairs. Important features are highlighted. The ordinate shows the absorbance in arbitrary units. Spinel was cut in half and one piece was heat-treated and the other half-piece remained untreated as a reference sample (see inserted picture).

Fig. 302a Fig. 302b

Fig. 303 spectra (excitation 405nm) of natural spinel recorded at liquid nitrogen temperature (LNT) for the respective groups: Heat-treated and unheated spinel. The ordinate shows the intensity (counts) in arbitrary units. Spinel was cut in half and one piece was heat-treated and the other half-piece remained untreated as a reference sample (see inserted pictures). Red curve represents the heated spinel and blue curve the unheated spinel.

Fig. 304 PL spectra (excitation 532nm) of natural spinel recorded at liquid nitrogen temperature (LNT) for the respective groups: Heat-treated and unheated spinel. The ordinate shows the intensity (counts) in arbitrary units. Spinel was cut in half and one piece was heat-treated and the other half-piece remained untreated as a reference sample (see inserted pictures). Red curve represents the heated spinel and blue curve the unheated spinel.

UV-VIS-near IR absorption spectrocopy and Photoluminescence of Spinel

276

Fig. 305a

Fig. 305b

Fig. 305c

546.

8

580.5

626.8

423.

644

4.6

582.

2

625.

4

0

0.5

1.0

1.5

2.0

2.5

300 400 500 600 700 800

Abs

orba

nce

Blue_compare_Verneuil_Flux_LucYenref-2124-uisrt-blue-flux-spineluisrt-verneuil-spinel-112uisrt-verneuil-spinel-131ref-7524-uisrt-blue-lucyen-spinelsref50332-uisrt-blue-lucyen-spinel

689.0

699.

0

744.

2

640.4

648.

4

665.

6

684.

4

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8 666.

6

686.3

706.

971

6.6

721.

3

734.0

0

20

40

60

80

100

120

600 650 700 750 800 850 900wavelength (nm)

Inte

nsity

Blue_compare_Flux_Verneuil_Natural_PL532pl532lnt-verneuilspinel-112ref-2124-pl532lnt-blue-fluxsref50332-pl532lnt-blue-lucyen-spinel

686.5

707.4

716.5

721.7

515.5

639.

6

689.2

699.

070

9.5

718.

3

744.

1

0

100

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300

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500

600

700

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450 500 550 600 650 700 750 800 850 900wavelength (nm)

wavelength (nm)

Inte

nsity

Blue_compare_Flux_Verneuil_Natural_PL405ref-2124-pl405lnt-blue-fluxpl405lnt-verneuilspinel-112pl405lnt-verneuilspinel-131sref50332-pl405lnt-blue-lucyen-spinel-2

Fig. 305a UV-VIS-near IR absorption spectra recorded at liquid nitrogen temperature (LNT) for the respective groups: Synthetic blue Cobalt-spinel (Verneuil, Flux) and natural blue Cobalt spinel from Vietnam. Important features are highlighted. The ordinate shows the absorbance in arbitrary units. Note the typical absorption characteristics of Cobalt in spinel structure in the range 550nm to 650nm.

Fig. 305b PL spectra (excitation 405nm) of natural spinel recorded at liquid nitrogen temperature (LNT) for the respective groups: Heat-treated and unheated spinel. The ordinate shows the intensity (counts) in arbitrary units. Samples: Synthetic blue Cobalt-spinel (Verneuil, Flux) and natural blue Cobalt spinel from Vietnam. Note the difference in number of lines and their position. The synthetic spinel is interpreted as inverse defect-spinel.

Fig. 305c PL spectra (excitation 532nm) of natural spinel recorded at liquid nitrogen temperature (LNT) for the respective groups: Heat-treated and unheated spinel. The ordinate shows the intensity (counts) in arbitrary units. Samples: Synthetic blue Cobalt-spinel (Verneuil, Flux) and natural blue Cobalt spinel from Vietnam. Note the difference in number of lines and their position. The synthetic spinel is interpreted as inverse defect-spinel.

UV-VIS-near IR absorption spectrocopy and Photoluminescence of Spinel

277

Fig. 306b

Fig. 306c

Fig. 306a

687.4 689.1

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200

250

650 700 750 800 850

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689.1515.5

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100

150

200

300 400 500 600 700 800 900 1000

verneuilspinel-109verneuilspinel-112verneuilspinel-112verneuilspinel-113verneuilspinel-120verneuilspinel-122verneuilspinel-131verneuilspinel-135

wavelength (nm)

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Fig. 306a PL spectra (excitation 405nm) of natural spinel recorded at liquid nitrogen temperature (LNT) for the respective groups: Cobalt spinel produced by Flux method and Cobalt spinel produced by the Verneuil method. The ordinate shows the intensity (counts) in arbitrary units. Note that the 2 methods can be separated by small differences in the position and shape of the peaks.

Fig. 306b PL spectra (excitation 405nm) of natural spinels recorded at liquid nitrogen temperature (LNT) for the respective groups: Heat-treated and unheated spinel. The ordinate shows the intensity (counts) in arbitrary units. 3 spinel pairs are shown. 8 different types of Verneuil spinel have been tested and the same strongest peaks at 689.1 were observed.

Fig. 306c Raman spectroscopy of Cobalt spinel (NIR Raman spectrometer). Note the sharp lines at 402nm of the natural spinel, while synthetic spinel (interpreted as inverse defect-spinel) showed only unstructured broad bands. The heated spinel shows a strongly enlarged FWHH indicating changes in the crystal structure in comparison to the unheated counterparts.

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Raman Spectrocopy and Photoluminescence of Spinel

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