Gemstones
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What is phosphorescent?
if the luminescence continues after the exposure to UV radiation * due to impurities and defects in the crystal lattice
Where are gemstones found in casework?
• counterfeits • simulants
How can gemstones be tested to see if they are real or counterfeit?
• luminescence • fluorescence • phosphorescence • birefringence • refraction • dispersion • pleochroism • hardness • specific gravity • magnetism • thermal conductivity
What is fluorescence?
gemstones are exposed to the short or long wave UV radiation, they emit the visible light
What can X-ray diffraction determine?
• lattice parameters - by indexing the position of the peaks • phase composition of the sample - given by the relative amounts of overlaid diffraction patterns • crystal structure - by refining the whole defraction pattern • crystallite size - by looking at peak broadening • identify impurities and defects in the crystal lattice
What does the scherrer equation do?
enables the average size of nanocrystals to be calculated if no microstrain
What is XRD used for?
to establish the arrangement of atoms within a crystal structure and how they stack together
What is the simplist model to understand what conditions are required for diffraction?
Bragg’s Law
What is Bragg’s law?
nλ = 2dsinθ * n = integer (often 1) * λ = x-ray wavelength * d = interplane spacing * θ = angle between plane and beam
When occurs when Bragg’s law is satisfied?
for parallel planes of atoms, with a space (d) between the planes, constructive interference occurs
What do we control in Bragg’s law?
x-ray wavelength and angle between plane and beam
How is almandine garnet identifiable from x-ray diffraction patterns?
from diffraction peaks, but inclusions not visible (can see them in microscopy)
How is quartz identified in x-ray diffraction patterns?
aventurine effect but inclusions not visible
How is glass identified from x-ray diffraction patterns?
doesnt have crystalline lattice so will have a broad peak
How are isotropic substances analysed with optical analysis?
• looking at the refractive index • illuminted with a normal incidence to the crystal face, all light will pass through without deviation • velocity will change but not the direction of light because of Snell’s law
What are some examples of isotropic substances?
• diamonds: cubic structure • opals: amorphous
How do we analyse uniaxial substances with optical analysis?
• light incident on the cleavage face of the crystal will break into two separate rays as the light travels through the crystals • the wave of the two rays vibrate in planes perpendicular to each other and travel through the crystal at different velocities
What are examples of uniaxial substances?
tetragonal or hexagonal crystals * quartz
What are the two rays called for a uniaxial substance?
• ordinary ray (O) - slower ray • extraordinary ray (E) - faster ray • vibrate in different planes through the material and have different RIs • E ray emerges ahead and in a different location to the O ray
What is birefringence?
the difference between the two refractive indices of the two rays in a uniaxial substance