Advances in Thermal and microstructural characterisation of Precious opal. Coober Pedy National Opal Symposium 2007 Anthony Smallwood

September 22, 2016 | Author: Samantha Martin | Category: N/A
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1 Advances in Thermal and microstructural characterisation of Precious opal Coober Pedy National Opal Symposium 2007 Ant...

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Advances in Thermal and microstructural characterisation of Precious opal Coober Pedy National Opal Symposium 2007 Anthony Smallwood

Introduction/Background • CSIRO electron micrographs • 1960 theories – – – – –

Weathering model Jones Segnit Darragh Sanders

Precious opal –current theories for opal formation • Weathering theories – Chemical weathering

• Jones, JB., Biddle, J., Segnit, ER. “Opal Genesis” Nature, Vol 210, 1966, 1353-1354.

– Bacterial action and weathering • Watkins and Behr

• Syn-tectonic postulation

– Pecover, SR., “A new Genetic model for the origin of Precious opal in: • The Great Australian Basin” Extended Abstracts No. 43, Mesozoic geology of the Eastern Australia plate conference, Geological Society of Australia, 1996, 450 – 454.

• Mound springs postulation

• Devison, B., “The origin of precious opal – a new model”. The Australian Gemmologist, Vol 22, No. 2, 2004, 50 – 58.

Two Geological Environments • Opal is often named after the geological environment of the rocks in which it is found. – A Sedimentary Environment – A Volcanic Environment

• Opal is deposited after the rocks, in which it is found, have formed.

Are these theories too “field specific” • Does opal form the same way in all fields? • What are the differences in opal from different fields? • Is the geology “really” different in different fields? • Are we asking the wrong question? • Where is it? Rather than, What is it?

Opal is the same? • There are two groups based on the environment – Sedimentary opals – Volcanic opals

• Sedimentary opal is different from volcanic opal. • All sedimentary opal has specific characteristics • All Volcanic opal has a different set of specific characteristics. • Is there a transition? • Does it matter?

The weathered profile Queensland opal fields – Jundah.

No not all opal is the same • Sedimentary opal – Opal –A – Refractive Index 1.42 – 1.45 – Density ~ 2.15 – Less water – 4% – 9% – Structurally amorphous

• Volcanic opal – Opal – CT – Refractive Index 1.40 – 1.42 – Density ~ 2.00 – More water – 9% - 18% – Possibly a more crystalline structure

Requirements for the formation of OPAL • What we are concerned about – – – – –

A source of silica A solution of silica and water The right chemical environment Growth of spheres Structure of spheres

• For the Geologists

– A method for water/solutions to permeate or move about in the strata - mobility. – A depositional site and method for deposition. – Geological stability – A time of formation

A Source of Silica • Weathering theories – the rocks! – Sedimentary – Sandstones – Volcanic – Rhyolites, Tuffs, Volcaniclastics

• Other theories – Artesian or Hydrothermal solutions containing silica – Underground, subterranean solutions

A solution of silica and water Feldspar

Example/Process Silica Solution Dissolution

Colloidal Particles

Precipitation

K2O.Al2O3.6SiO2 + nH2O → Al2O3. 2SiO2.2H2O + KOH +4SiO2 Feldspar

Water

Kaolinite

Alkali

Silica

Chemical Environment

Modified from: Iler, RK. “The Chemistry of Silica: Solubility, Polymerisation, Colloid and Surface Properties, and Biochemistry”. John Wiley and Sons, New York. 1979

Historical Formation of silica spheres • Amorphous silica slowly settles out of a dilute water solution. • Concentrated by progressive evaporation of water. • Primary spheres 30-40nm form first. • Cluster to form larger spheres • Darragh, P.J. Gaskin, A.J. & Sanders, J.V., 1976. “Opals”. Scientific American 234/4,84-95

Silica spheres formation • Dissolution by hydrolysis – Si02 + 2 H2O → Si(OH)4 • Enrichment by evaporation • Nucleation and polymerisation • Sphere Growth – pH Controlled – Accretion – Ostwald Ripening. • Brown, L.D. “Characterisation of Australian Opals” Ph.D thesis, UTS, 2005

Coober Pedy opal 450,000x

A lower magnification300,000x

~400nm

50nm

The reverse – primary sphere or a hole? 175,000x

Two “primary” spheres – 300,000x

Perhaps more or merged spheres?

Mintabie Dark opal

Mintabie Dark opal

200,000x

200,000x

MintptGR– POC07

Mintabie Grey– a07

Specimen preparation • • • •

Fracturing Etching (HF) Coating Orientation

Etching the surface

~400nm

Coober Pedy Light opal – Strong etching 200,000x CPWH10013

~400nm

Coober Pedy Light opal – Light etching 200,000x CpwtPOCPt72

Coating Comparisons

Coober Pedy Light opal Carbon Coated 200,000x CC47

Coober Pedy Light opal Platinum Coated 200,000x Pt 147

Differences between Sedimentary and Volcanic opal possibly due to preferential etching of silica

~275nm

~275nm

Coober Pedy Light Opal 120000x CPLtpt70

Tintenbar Crystal volcanic opal 100,000x Tint18

Some other interesting opal structures

A “healed fracture” in light opal from Coober Pedy 2,500x

The same area showing “Potch” opal filling “colour”.

Fracture occurring before final sedimentation of the opal gel?

20,000x

Other interesting opal Brazil ~275nm

~275nm

~75nm

Brazil volcanic opal

Brazil volcanic opal

200,000x

300,000x

Brazil134

Brazil113

Other interesting opal! Idaho ~240nm

1000nm

Idaho Volcanic opal - Banded

Idaho Volcanic opal - Banded

60,000x

100,000x

Idaho Banded (50)

Idaho Banded (01)

Very interesting structure Idaho Opal

Idaho Volcanic Opal – banded

Idaho Volcanic opal - banded

120,000x

60,000x

Idaho banded (228)

Idaho banded (203)

Characterisation of opal by thermal analysis • How much water • Rate of water loss of the sample • Dimensional change of the sample • Structural change of the sample

Characterisation of opal by thermal analysis • How much water is in the opal structure • What is the nature of the water in the opal structure • The rate of water loss from the structure talks about the porosity of the structure • Dimensional changes talks about the mechanical stability of the opal structure – crazing.

Water content Thermo gravimetric analysis (TGA) • Water content Measured. • Measured by weight loss of specimen. • Heating opal specimens • A constant heating range of 1°C per minute • Range 20°C - 1200°C.

• • • • • • •

Sample preparation What amount used? Milligrams (1000th) Usually 25mg Economic amount Mortar and pestle Powder and larger pieces.

Lightning Ridge Black Potch 102

––––––– –––––––

LR BPlPow3-2.001 LR BPlum p3-2.001

Weight (%)

100

98 6.690% 7.033%

6.690% 7.242%

96

94

92

0

200

400

600 800 Temperature (°C)

1000

1200

Universal V4.1D TA Instrum ents

What is happening – what are the differences? • Sample is loosing water

– Powder 6.69% : Larger pieces 7.24%

• Difference between the two

– A difference in diffusion of water through the structure – The possibility of “free” water unbound in the structure • 0.343% @1000°C • 0.552% @ 1200°C

What does it mean?

– The powder loses water faster – Some water may be lost during the process of powdering the sample

Lightning Ridge Black Potch – differentiating the curves shows the temperature of fastest water loss. 0 .0 3 2 7 4 .4 5 °C

––––––– –––––––

L R B P lu m p 3 - 2 .0 0 1 L R B P lP o w 3 - 2 .0 0 1

1 8 1 .7 6 ° C

Deriv. Weight (%/°C)

0 .0 2

0 .0 1

0 .0 0

-0 .0 1

0

200

400 600 T e m p e ra tu re (°C )

800

1000

U n iv e r s a l V 4 .1 D T A In s tr u m e n ts

Comparison Potch, Sun flash & Matrix – Lightning Ridge 102

––––––– ––––––– –––––––

LRidgeSunFlash.dat LRidgeBlackPotch.001 LRidgeGreyMatrix.001

Weight (%)

100

98 4.093% 96 5.984% 94 6.612% 92

0

200

400 600 Temperature (°C)

800

1000

Universal V4.1D TA Instrum ents

South Australian opal

102

––––––– ––––––– ––––––– –––––––

A n d a m o o k a .0 0 1 C o o b e r1 1 M ile .0 0 1 L ig h tA la n sR ise .0 0 1 B la c k A la n sR ise .0 0 1

Weight (%)

100

98 6.858% 6.615% 7.146% 7.243%

96

94

92

0

200

400

600 800 Tem p eratu re (°C )

1000

1200

U n iv ers a l V 4 .1 D T A In s tru m en ts

Derivative – South Australian opal 0.05

––––––– ––––––– ––––––– –––––––

Andam ooka.001 C oober11Mile.001 LightAlansR ise.001 BlackAlansR ise.001

Deriv. Weight (%/°C)

0.04

0.03 142.04°C 144.18°C 215.75°C 261.28°C

0.02

0.01

0.00

0

200

400

600 800 Temperature (°C)

1000

1200

1400

Universal V4.1D TA Instrum ents

Sedimentary Opal and Volcanic Opal 105 6.610%

Weight (%)

100

––––––– ––––––– ––––––– ––––––– ––––––– ––––––– –– –– – ––––––– ––––

Andam ooka.001 Coober11Mile.001 BlackAlansRise.001 LRidgeBlackPOT.001 LightAlansRise.001 Boulder1.dat TintenbarPOC.001 MexicanPOT.dat EthiopianPOC.001

9.531% 9.523%

95

7.243% 90 17.64% 85

80

0

200

400

600 800 Temperature (°C)

1000

1200

1400

Universal V4.1D TA Instrum ents

Dimensional change of opal structure Thermo Mechanical Analysis (TMA)

Dimension Change (µm)

100

200.53°C 255.56°C 203.42°C 257.70°C 226.03°C

––––––– ––––––– ––––––– ––––––– –––––––

CooberPedyShellPatchl.001 CooberPedy9Mile.001 CooberPedy14Mile CooberPedyOlym pic.001 CooberPedyShellPatch2.001

0

-100

-200

-300

0

200

400

600

Temperature (°C)

800

1000 Universal V4.1D TA Instrum ents

Lightning Ridge Sun flash 600

––– –––– ––– –––– ––– ––––

LR S F02 -2 .001 LR S F02 -3 .001 LR S F02 -4 .001

Dimension Change (µm)

624.73°C 400

623.11°C

200

622.84°C

400°C

0

-200

0

200

400

600

Tem perature (°C )

800

1000 U niversal V 4.1D T A Instrum ents

What is happening? What information do we gain? • • • •

The opal structure is expanding The transition temperature The sample then contracts What does this tell us about the opal structure?

– We know the water is diffusing through the opal structure – The opal sample is sintering – Is the mineralogy changing - unlikely?

The relationship between thermal analysis and SEM

Coober Pedy light opal

Coober Pedy light opal

Platinum coated

Platinum coated

No heating

Heating to 200°C

The relationship between thermal analysis and SEM

Coober Pedy light opal

Coober Pedy light opal

Platinum coated

Platinum coated

Heating to 400°C

Heating to 600°C

The relationship between thermal analysis and SEM

Coober Pedy light opal

Coober Pedy light opal

Platinum Coated

Platinum Coated

Heating to 800°

Heated to 980°

Thanks • To all who assisted with samples – – – – – –

Schellnegger mining Frank Palmer Yanni Athanasiadis Taiyo Adolphous Stuart Jackson Others

– – – –

Dr Paul Thomas Prof Abhi Ray Dr Rick Wuhrer Jean-Pierre Guerbois

• Thanks co-workers who help with equipment and discussion

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