Minnesota Rocks and Minerals
Part 1 - Mineral Identification
Rick Ruhanen
Minnesota Minerals Education Workshop
June 18, 2013
WHAT IS A MINERAL?
• Unique Chemical Composition
• Crystalline Solid
• Inorganic
• Occurs in Nature
And, Rocks are composed
of Minerals!
Mineral Properties Outline
• Luster
– Metallic
– Non-Metallic
• Hardness
– Moh’s Scale
• Relative, 1 – 10
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•
•
•
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Cleavage
Streak
Fracture
Crystal Form
Color
LUSTER
Non-Metallic
Plagioclase
Metallic
Galena (top); Specular
Hematite
Luster II
Common Rock-Forming
Minerals
Non-Metallic Minerals
Metallic Minerals
Feldspar – Vitreous
Pyrite – Pale yellow-bronze
Hornblende – Vitreous, fibrous
Chalcopyrite – Yellow
Olivine – Vitreous, glassy
Galena – Lead grey
Augite – Vitreous
Sphalerite – Black, resinous brown
Quartz – Vitreous
Pyrrhotite – Brass/bronze
Mica – Silky, pearly,
Hematite – Dull red, yellow, silver, black
Magnetite – Steel grey
Hardness
Moh’s Scale of Relative Hardness
Softest = Talc
Hardest = Diamond
Everyday items useful for hardness tests:
Fingernail 2, copper penny 3, knife blade
5, window glass 5.5.
Cleavage
Cleavage is defined by a mineral’s tendency to break along definite planar surfaces.
Cubic cleavage – Halite (salt)
Streak
Streak is produced when a metallic mineral is rubbed on a piece of nonglazed porcelain. This will produce a fine powder of a certain color.
Common streak colors
•Hematite = Red, Reddish Brown (Shown)
•Magnetite = Black
•Galena = Lead Grey-Black
•Pyrite = Black-Grey
•Chalcopyrite = Greenish-Black
•Sphalerite = White to Yellow-Brown
Fracture
•Mineral breaks which do not follow cleavage planes
•Quartz has a conchoidal fracture
Crystal Form
Galena - cubic
Perfect crystals only form if the
mineral “grows” without constraint;
quartz in the lower photo is
massive as it had no room to form a
nice crystal
Quartz - hexagonal
Some Common Crystal
Forms
Color
•Many minerals occur in different colors
•Color is helpful BUT not very reliable
Note the colors of the
three Fluorite specimens
Specimens
courtesy of the A E
Seaman Museum
Houghton MI
Minnesota Rocks and Minerals
Part 2 – Rock Identification
Rick Ruhanen
Minnesota Minerals Education Workshop
June 18, 2013
Rock Types
• Igneous
– “Born of Fire”
• Sedimentary
– Clastic (Erosion and Deposition)
– Chemical (Limestone, Chert)
• Metamorphic
– “Change in Form”
Igneous Rocks
Formed from a Magma
• Intrusive (Plutonic)
– Granite
– Gabbro
Crystallize beneath crust –
Slower cooling of magma - larger
crystals
• Extrusive (Volcanic)
– Rhyolite
– Basalt
Crystallize at surface – faster
cooling – individual crystals
difficult to see
Granite and Rhyolite – More potassium, sodium, aluminum, silica; generally lighter
colored. Mineralogy = orthoclase, plagioclase, hornblende; + - quartz, mica.
Gabbro and Basalt – More calcium, iron, magnesium; less aluminum, silica; generally
darker colored. Mineralogy = plagioclase, augite, olivine; + - sulfides, mica.
Granite
Igneous Rocks
Intrusive (Plutonic)
Granite
Gabbro
Extrusive (Volcanic)
Rhyolite
Basalt
Sedimentary Rocks
•Clastic
•Chemical
-Sandstone
-Limestone
-Shale
-Chert
-Conglomerate
Sandstones are just that, composed of sand grains. Shales are very fine grained rocks
formed from mud, clay or silt. Conglomerates have larger clasts – pebbles to cobbles in
size.
Limestone is composed primarily of calcite. It may be precipitated directly from water
but most limestones are the result of organic activity as shelled animals die and settle
to the bottom. Limestones are often fossiliferous. Chert is very fine, or
cryptocrystalline, quartz.
Sedimentary Rocks II
Sandstone
Fossiliferous limestone
Conglomerate
Calcite limestone
Metamorphic Rocks
•Schist is a foliated rock with laminations generally formed by micas
•Gneiss is a coarsely foliated, or “banded,” high grade metamorphic rock
•Slate formed from shale; is a very fine grained rock with unique, “slatey cleavage”which may
or may not follow original bedding
•Quartzite is formed from sandstone; the grains are fused together
•Marble is a metamorphosed limestone; recrystalized calcite
Metamorphic Rocks II
Schist
Gneiss
Slate
Marble