The Chemistry of Minerals: Demonstrations and Hands-On Experiments

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The Chemistry of Minerals Demonstrations and
Hands-On Experiments

• Fernando J.A.S. Barriga
• Ecsite, Budapest
• May 29, 2008

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Minerals
Minerals in Museum activities with the public are
usually inspected and identified on the basis of
physical properties only (lustre, hardness,
streak colour, cleavage, crystal shape, colour,
etc)
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Identifying minerals - stage 1

• Suspect the most likely mineral first
• Luster
• Metallic
• Non-metallic
• Density versus luster
• Hardness versus luster
• Color is tricky. Allo- and idiochromatic minerals

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Mineral Identification at MNHN

• At MNHN in Lisbon we are reintroducing bench
  chemical testing of minerals as a highly
  entertaining and educational activity based on
  methods that were at the cutting edge of science
  in the 19th century and in use by prospectors and
  university students up until the mid 20th century

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Main chemical procedures

• Nearly all minerals are soluble in fluxes at high
  T (1200ºC), through fusion in a bead (borax, salt
  of phosphorus, Na2CO3)
• Closed and open tube testing,
• Reaction over charcoal,
• The blowpipe is the key instrument for production
  of a high temperature flame

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Wet chemical procedures

• Adopted after Arrhenius ionic theory
• Solubility in acids (HCl, HNO3, H2SO4)
• Effervescence (in widespread use to identify many
  carbonates)
• Production of H2S (hydrogen sulphide)
• Chlorine detection
• Separation of an insoluble residue

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Equipment for blowpipe analysis (very
inexpensive)

• Flame
• Blowpipe or gas torch
• Oxidising and reducing flame
• Forceps
• Platinum wire
• Charcoal
• Glass tubes
• Hammer, pliers, magnet, mortars, steel file

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Blowpipe spirit lamp
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Blowpipe technique
Not very easy, but can be replaced by jewellers
torch
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Agate mortar
The most expensive piece of equipment (about 250
euro) A binocular microscope is also highly
desirable (may cost from 200 euro)
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Blowpipe tests

• Forceps
• Fusibility (Kobell scale)
• (1. stibnite, 2. natrolite, 3. almandine, 4.
  actinolite, 5. orthoclase, 6. bronzite)
• Flame coloration
• Open and close tube
• Sublimates
• Acid vapours
• On charcoal
• Smell (sulphur, arsenic, etc)
• Sublimates
• Magnetic residue
• Metal globules
• Platinum wire beads
• Borax (Na2B4O5(OH)48H2O)
• Na2CO3
• salt of phosphorous (HNaNH4PO44H2O)

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Fusibility of almandine garnet
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Decomposition of arsenopyrite FeAsS

• In the closed tube (made from laboratory standard
  tubing)
• Yellow sulphur sublimate
• Gray/black elemental arsenic (including As
  mirror)
• Magnetic residue

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Closed tube, arsenopyrite
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Galena (PbS) on charcoal
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Hepar reaction for sulphates (sulphides too)

• First reported by James Smithson in 1819
• Fusion in the reducing mixture (Na2CO3
  charcoal)
• Brownish mass transferred to wet silver surface
• Black tarnish forms
• Pros
• Easy to perform
• Easy to explain chemically
• Very sensitive
• Con might be produced by Se or Te

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Hepar reaction (barite, BaSO4)
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Borax bead technique
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Iron, reducing flame
red, hot
cooling
green, cold
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Borax bead, CuO, oxidising flame
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Future

• Educational activities _at_ the Museum
• Manual
• Lisbon instrument kit
• Collection of powdered standards

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• Practise makes perfect!