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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