Color Many minerals have a unique color But…
Why Color Isn’t So Useful to Describe a Mineral 9 Different Minerals, Right? NOPE! All Quartz!
Why the Different Colors? Trace impurities can color a mineral Example: Corundum (Al2O3) Typical, “pure” corundum Corundum with Chromium impurities RUBY! Corundum with Iron & Titanium impurities BLUE SAPPHIRE! Corundum with Vanadium impurities PURPLE SAPPHIRE!
2. Streak No, not THAT Streak! • The color of a mineral in its powdered form • Commonly tested on a streak plate (a piece of unglazed porcelain)
Is Streak Useful? Yup. • Unlike color, which can vary significantly, a mineral’s streak will always be the same (or very similar) – Not enough impurities to appreciably change the color of a streak • Must be careful that mineral is softer than streak plate otherwise you’ll get no streak AND a scratched plate!
3. Luster • How light reflects off a mineral Examples of luster: Pearly Mica Adamantine Diamond Metallic Native Gold Glassy Quartz And Many More!
4. Crystal Form • IF allowed to form slowly and with plenty of room, minerals form crystals of distinctive shapes, or forms • Depends on how atoms bond and pack together • ~ 48 different crystal forms
Examples of Crystal Forms CUBIC ◄ Fluorite Halite► Hexagonal ◄ Quartz Jumbo Quartz!►
Not So Useful Usually minerals form too quickly and have too little space. Result: Tiny crystals of irregular shape, virtually impossible to identify by their form What minerals are in this rock??? A Closeup of Granite, containing quartz and feldspar crystals
5. Hardness • How easily a mineral is scratched • This one IS useful • If you rub two minerals against each other, the harder one “wins” and scratches the softer one. The harder one is not scratched.
How “hard” is “hard”? • Geologists measure hardness using the “Mohs Scale” • Based on 10 minerals assigned a hardness of 1 to 10, 10 being the hardest • A mineral will scratch any other mineral that has a hardness less than itself
Using the Mohs Scale • Which of the minerals on the Mohs scale could you scratch with your fingernail? • Which could you scratch with a knife? • A certain mineral can scratch apatite, but not quartz. What mineral is it? • Dolomite has a hardness of 3.5. Which minerals can dolomite scratch on the Mohs scale?
The Special Case of Diamond • Hardest mineral on Earth • 4 times as hard as next-hardest, corundum (of ruby and sapphire fame) • But not impervious (see “Cleavage” later) • Best heat conductor – 5 times better than silver • Highest melting point ~3000oC • Pure carbon (other forms include graphite, buckyballs, and amorphous carbon)
6. Cleavage • No, not THAT kind of cleavage (no video for this one) • When a mineral breaks smoothly to form a flat crystal face • Cleavage occurs where bonds between atoms are weakest, therefore depends on crystal structure and atom arrangement
Examples of Cleavage ◄ Mica – Cleaves into sheets Halite – Cleaves at 90o angles ► ◄ Quartz – Does not cleave (Si-O bonds too strong; crystals break instead) Calcite – Cleaves at 75o angles ►
A Return to Diamond • Each carbon atom bonded to four other carbon atoms in a tetrahedral: Result – Diamond cleaves in 4 directions (and fairly easily)
A Diamond in the Rough • This “easy cleavage” is great for cutting diamonds World’s 15th Largest Diamond Weight: 603 carats Cost: Only $12.4 million (Not the same diamond)
World’s Biggest Rough Diamond • The Cullinan • Weight: 3,106 carats (size of a bowling ball) • Cut diamond currently in Britain’s Imperial Sceptre and is part of the Crown Jewels
Part of the Crown Jewels ◄ The Imperial Sceptre, with Cullinan I The Imperial State Crown► with Cullinan II
7. Fracture Some minerals don’t cleave when you strike them, they break (or fracture) Examples: Quartz (again…) Asbestos
8. Density • Good ol’ Density = Mass / Volume • A given mineral will always have the same density • Densities of most minerals between 2 and 5 g/cm3 • Among most dense is gold (19.3 g/cm3)
Examples of Other, Unique Properties Graphite, C - Feels greasy Talc, Mg3Si4O10(OH)2 - Feels soapy Magnetite, C - Magnetic Metals – Malleable (easily shaped) Carbonates – Produce CO2 gas when acid added Calcite, CaCO3 – Double refraction (translucent and produces TWO images) ►
In Summary… Some characteristics ARE USEFUL to identify minerals: 2. Streak 5. Hardness 8. Density
And Finally… Other characteristics are NOT so useful to identify a mineral, but still important: • Color 3. Luster 4. Crystal Form 6. Cleavage 7. Fracture