1 of 23

Minnesota Rocks and Minerals�Part 1 - Mineral Identification

Rick Ruhanen

Minnesota Minerals Education Workshop

June 18, 2013

2 of 23

WHAT IS A MINERAL?

  • Unique Chemical Composition

​

  • Crystalline Solid

​

  • Inorganic

​

  • Occurs in Nature

And, Rocks are composed of Minerals!

3 of 23

Mineral Properties Outline

  • Luster
    • Metallic
    • Non-Metallic
  • Hardness
    • Moh’s Scale
      • Relative, 1 – 10
  • Cleavage
  • Streak
  • Fracture
  • Crystal Form
  • Color

​

​

4 of 23

LUSTER

Non-Metallic

Metallic

Plagioclase

Galena (top); Specular Hematite

5 of 23

Luster II

Common Rock-Forming Minerals

Non-Metallic Minerals

Feldspar – Vitreous

​

Hornblende – Vitreous, fibrous

​

Olivine – Vitreous, glassy

​

Augite – Vitreous

​

Quartz – Vitreous

​

Mica – Silky, pearly,

Metallic Minerals

Pyrite – Pale yellow-bronze

​

Chalcopyrite – Yellow

​

Galena – Lead grey

​

Sphalerite – Black, resinous brown

​

Pyrrhotite – Brass/bronze

​

Hematite – Dull red, yellow, silver, black

​

Magnetite – Steel grey

6 of 23

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.

7 of 23

Cleavage

Cleavage is defined by a mineral’s tendency to break along definite planar surfaces.

Cubic cleavage – Halite (salt)

8 of 23

9 of 23

10 of 23

Streak

Streak is produced when a metallic mineral is rubbed on a piece of non-glazed 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

​

11 of 23

Fracture

  • Mineral breaks which do not follow cleavage planes
  • Quartz has a conchoidal fracture

12 of 23

Crystal Form

Galena - cubic

Quartz - hexagonal

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

13 of 23

Some Common Crystal Forms

14 of 23

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

15 of 23

16 of 23

Minnesota Rocks and Minerals�Part 2 – Rock Identification

​

Minnesota Minerals Education Workshop

June 18, 2013

​

Rick Ruhanen

17 of 23

Rock Types

  • Igneous
    • “Born of Fire”
  • Sedimentary
    • Clastic (Erosion and Deposition)
    • Chemical (Limestone, Chert)
  • Metamorphic
    • “Change in Form”

18 of 23

Igneous Rocks�Formed from a Magma

  • Intrusive (Plutonic)
    • Granite
    • Gabbro
  • Extrusive (Volcanic)
    • Rhyolite
    • Basalt

Crystallize beneath crust – Slower cooling of magma - larger crystals

Crystallize at surface – faster cooling – individual crystals difficult to see

Granite

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.

19 of 23

Igneous Rocks

Intrusive (Plutonic)

Extrusive (Volcanic)

Gabbro

Basalt

Granite

Rhyolite

20 of 23

Sedimentary Rocks

  • Clastic
  • Chemical

-Sandstone

​

-Shale

​

-Conglomerate

-Limestone

​

-Chert

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.

21 of 23

Sedimentary Rocks II

Sandstone

Conglomerate

Fossiliferous limestone

Calcite limestone

22 of 23

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

23 of 23

Metamorphic Rocks II

Schist

Slate

Gneiss

Marble