Clue 1 - Minerals
Mystery Detectives - Alien landing - Clue #1
Tuesday, May 01, 2012
9:22 AM
Keep in mind your overall goal:
To identify a mysterious igneous rock and determine the tectonic setting in which it formed so that you can narrow in on the possible places on Earth where  Granny landed on Earth.
To identify your rock, you must first identify the minerals within it, if any.
Name: 
Partner(s):
Date:
Station 1-4:  Minerals that make up most igneous rocks
There are many different kinds of minerals, but just 7 kinds make up most igneous rocks.  If you know these minerals, you'll be able to recognize important igneous rocks.
Common minerals that make up igneous rocks:
•  Feldspar - plagioclase and potassium feldspar varieties
•  Quartz
•  Mica - biotite and muscovite
•  Amphibole
•  Pyroxene
•  Olivine
Tips!
Section headers are in bold orange letters.
Things to do are in bold black letters.
Questions are in fancy blue letters. 

Links are in small green letters.
Station 1:  Orthoclase feldspar and plagioclase feldspar - 
The most common minerals in igneous rocks and the Earth's crust
Interactive mineral  photo gallery
Observe all of the minerals at this station.   They are all feldspars.   Feldspar is the most common mineral in the Earth's crust.  
Observe the minerals labeled "plagioclase feldspar" and "orthoclase feldspar".   These are two important varieties of feldspar.   You will learn how to recognize each of these types of feldspar
Learn to distinguish plagioclase feldspar from orthoclase feldspar.  Distinguish" means to tell them apart.  
Look at the orthoclase feldspars.  
What colors are they?  
Is orthoclase feldspar clear?
Look at the plagioclase feldspars.  
What colors are they?  
Is plagioclase feldspar clear?
Both orthoclase and plagioclase feldspar can be white.  Is the color white a diagnostic characteristic of either potassium or plagioclase feldspar?
Salmon pink is a diagnostic color for orthoclase feldspar.   It can be very light pink or nearly red.  The word diagnostic refers to a characteristic of a mineral that identifies its type.  If it's salmon pink, it's orthoclase feldspar.  Plagioclase feldspar is never pink.  The rock granite is often pink or reddish because it contains crystals of pink orthoclase feldspar.
Note that "orthoclase feldspar" can also be called "potassium feldspar."  
Learn to distinguish plagioclase feldspar by the presence of striations  Plagioclase feldspars are usually shades of white, black and many shades of gray in between.  They are not usually very transparent (clear) except in very thin pieces.   Some orthoclase feldspar and quartz crystals can look like plagioclase feldspar.    
How to distinguish plagioclase feldspar - look for striations.  To the eye, striations are tiny rows of grooved lines across certain faces of plagioclase crystals.  If large enough, you may be able to feel the grooves.  To find them you will need to rotate the sample until the light catches and reflects on them just so.   In a speck of plagioclase feldspar in a rock, you will have to look very carefully to find them.
Not all plagioclases exhibit striations, but when they occur, it is diagnostic.
Look at the plagioclase feldspars until you find the striations.  
Are the striations very straight and parallel or just generally so?
Station 2:  Quartz - Second most common mineral in the Earth's crust
Interactive mineral  photo gallery
Link for 3D model
[link]
Striations in thin section of moon rock.
Observe all of the minerals at this station.   They are all forms of the mineral quartz.   Quartz is the second most common mineral in the Earth's crust, a key mineral in the igneous rocks granite and rhyolite, and in the sedimentary rock sandstone.
Observe the variety of colors of quartz.  
What colors are they?  
Is quartz clear?
Observe the well-formed crystal of quartz.
How many sides does the crystal have?  
Does it form a point?
Quartz is made up of incredibly tiny atoms of silicon and oxygen arranged in a particular pattern.   Atoms are so small that you cannot even see them using a microscope.   
Observe the sample of silicon.
What color is pure silicon?
Is silicon transparent?
Observe the air around you.  Breathe some of the oxygen atoms in that air.
What color is pure oxygen?
Is oxygen transparent?
Look at the online models of quartz that show how the atoms of silicon and oxygen are arranged inside a single crystal of quartz.
When silicon and oxygen are arranged in the pattern found in quartz, is the resulting substance like either of its ingredients alone?
Atoms combine in different ways to produce materials with very different properties than in their pure forms.   You will learn more about how to predict these behaviors in high school when you take chemistry.   If you need a material with certain properties, like something that absorbs sunlight and turns it into electricity,  you would seek a chemist or materials scientist to find or develop just what you need!
Station 3:  Mica - Biotite and Muscovite
Interactive mineral  photo gallery
Online 3D model of biotite mica
[link]
Online 3D model of muscovite mica
[link]
Mica lampshade
Vermiculite
Lepidolite at Pala
Observe the samples of mica.  Mica is easy to recognize in large pieces because it is flaky.   
Distinguish muscovite from biotite.
Which mica is nearly black and dark-colored? 
Which mica is lighter shades of gray and more transparent?
Look at the online models of mica that show how the atoms inside it are arranged.
Micas are flaky and separate into thin layers where the forces that hold the atoms together are not very strong.
The model shows the arrangement of atoms and shows where forces that attach the atoms are strong.   The strong bonds are indicated by sticks.  
Rotate the model until you find the planes through the structure of atoms that has very few bonds.   Those are the flat areas through the mineral along which mica flakes easily.  
Are atoms of silicon and oxygen found in biotite or muscovite? 
Are atoms of gold found in biotite or muscovite?
In hunting for gold, is mica a good source of gold?
Silicon and oxygen are the most common kinds of atoms (elements) found in the Earth's crust and mantle. 
Minerals and the rocks containing certain minerals are the source of many of the elements we need for material objects in our everyday life except for food and most forms of energy and clothing. 
Station 4:  Amphibole, pyroxene and olivine - The dark minerals
Interactive mineral  photo gallery
General image gallery
[link]
Peridot
Green sand beach Hawaii.
3D model.
Observe all of the minerals at this station.   They are 3 of the 7 most common minerals in igneous rocks.
Learn to distinguish by the presence of striations These minerals tend to be dark colors from green to black. 
Look at the olivine.   Olivine is a common mineral in  the Earth's mantle.   Exceptionally large, clear and beautifully colored pieces of olivine are use in jewelry where it is called peridot.
What colors is olivine? 
Olivine can be very light-colored or nearly black.  It usually forms clumps of sandy-looking grains.   Individual crystals can be transparent to cloudy.
Look at amphibole and pyroxene.  Both are commonly black to dark green and difficult to tell apart, particularly as very tiny crystals within a rock.   This is difficult even for many geologists.
Look at the very straight cracks through crystals of amphibole and pyroxene. Those cracks are called "cleavage planes."  Those planes are the areas within each crystal where the atoms inside aren't held together or bonded strongly.   Being weak planes, the crystal exhibits cracks in certain directions. 
The cleavage planes through amphibole and pyroxene are at slightly different angles.  The angles are diagnostic. 
What color is amphibole? 
What color is pyroxene?
Is the color of amphibole or pyroxene diagnostic?
Are the cleavage planes in amphibole and pyroxene very flat or wavy?
Station 5:  How minerals form and grow in an igneous rock
Interactive diagram showing how crystals form from molten rock
Explore the interactive diagram that shows how igneous rocks form.
How do all igneous rocks form?
What is molten rock inside the Earth called?
What is molten rock that has escaped to the Earth's surface called?
There are no mineral crystals in molten rock.
Do mineral crystals add atoms and grow from the center out or the outside in?
As time passes while molten rock cools, do individual crystals grow larger or smaller?
What keeps individual crystals from growing infinitely large or forming perfect shapes?
Ignous rocks that form at the surface cool much faster than those that cool underground.
Which igneous rocks contain large visible crystals, those that formed at the surface or underground?
If an igneous rock is new, it formed right now or very recently.
What does a geologist mean when they say that an igneous rock formed 30 million years ago?
Station 6:  Color, luster and light transmission - How they look and behave with light
Interactive mineral  photo gallery
cts for discussion
One person's aqua might  be another person's blue.  How do scientists describe colors uniformly?
[link]
Do you think that colors on the computer
Pigment
Observe samples placed in three groups labeled - color, light transmission and luster.
What is the color of each mineral?
Mineral
Types of atoms
Cause
Color
Orthoclase feldspar
Potassium, aluminum,
Silicon , and oxygen
Normal  color of mineral
Quartz 1
Silicon and oxygen
Normal  color of mineral
Quartz 2 (aventurine)
Silicon and oxygen
Tiny mica crystals inside
Quartz 3
(amethyst)
Silicon and oxygen
Imperfectly arranged atoms
Observe the way light passes through each mineral  or the "transparency" of each mineral.
Match the type of transparency with each mineral.
Type of
transparency
Description of transparency
Mineral that exhibits this type of transparency
Opaque
No light passes through
Transparent
Most or all light passes through, so appears clear
Translucent
Some light passes through but not all, so appears cloudy
Observe the luster of each mineral sample and match the luster with the sample.
Match the type of transparency with each mineral.
Type of
luster
Mineral
Earthy
Pyrite
Glassy
Opal
Metallic
Limonite
Quartz
Glassy
Station 7:  Hardness and streak - How they behave ...
Slide show of objects for discussion
Diamond
Take each of the minerals in the "streak" sample set and try to scratch the piece of unglazed porcelain tile with it.  If any of the minerals left a mark, the color of the mark left by the mineral is called the streak.  
What is the streak color for the following minerals? 
Mineral
Color of streak
Ability to scratch glass
Quartz  
Orthoclase feldspar
Corundum
Limonite
Hematite
Pyrite
Pencil
Only metallic minerals tend to have a streak.  
Which minerals have a streak?
Mineral
Streak or no streak
Which minerals had a streak?
Opaque
Transparent or translucent
Some minerals are harder than others.  But just how much harder is quartz and gypsum?  In order to make comparisons, geologists  came up with a set of reference minerals called the Mohs Hardness Scale.   
What mineral has a hardness of 7?
What is the hardness of feldspar?
What mineral is the hardest on the Mohs scale?
In the field, outside where you find rocks in their natural setting, it can be difficult to distinguish quartz from feldspar based just on appearances.  What's a geologist to do when faced with a mineral identification problem away from lab equipment and with no money to pay for expensive tests?  
TBA
Expert advice 
Hardness and streak aren't the same thing.
Streak only works on opaque and metallic minerals.
Station 8 :  Density - An important characteristic 
Slide show of objects for discussion
Hold each of the following cubes in your hand.   They are all the same three-dimensional size.  Put them in the order of 
If you can finish the superchallenge, determine the density of water too.
Weigh each of the cubes and write down their weights in the table below.
 Most dense
Weight (grams)
1
2
...  
Water
Observe the minerals labeled "plagioclase" and "potassium feldspar".   These are two important varieties of feldspar. 
How to distinguish plagioclase feldspar from potassium feldspar.  "Distinguish" means to tell them apart.  
Look at the potassium feldspars.  
What colors are they?  
Look at the plagioclase feldspars.  
What colors are they?  
Can you tell them apart by only color?  Explain why or why not?
Expert advice
Station 9:  Crystal form - Second most common mineral in the Earth's crust
Slide show of objects for discussion
Minerals grow into certain shapes.  The shapes of different minerals can be 
Observe the minerals labeled "plagioclase" and "potassium feldspar".   These are two important varieties of feldspar. 
How to distinguish plagioclase feldspar from potassium feldspar.  "Distinguish" means to tell them apart.  
Look at the potassium feldspars.  
Cube
Rhombohedron
Look at the plagioclase feldspars.  
Calcite
Galena
Expert advice