North Seattle Community College's
@2002 -- The information contained in this document
A Scheme for Identifying Your Minerals
This web page is designed to help you to learn mineral identification by presenting some basic information on mineral properties and then stepping you through the identification of your twelve mineral samples (labeled 1 through 12). Some observations can be difficult for beginning geologists. I find it easier to start with the observations which we can make with more confidence. After sorting our minerals in the easiest ways first, we can work with the more difficult properties to determine the exact identification of our samples.
You will earn about a half a point for each correct answer to the 35 numbered blanks below (in purple). Use the submission form for "Lab Homework Week 3 -- Part 1" in your In Box to turn in these answers.
START WITH A READING ASSIGNMENT
The Critical First Step: Review the description of mineral features in Chapter 2 of your textbook. The easiest basic properties for new geologists to determine are hardness, streak and color. The most difficult basic properties to deal with are luster, cleavage and fracture.
Second, Study Appendix D (pages A-6 and A-7): Become familiar with this identification table in your text book. You don't need to memorize it, but understand how it has been set up. This table is the key to successful identifications.
You might want to mark lightly (with pencil!) the 26 specific mineral descriptions in Appendix D we will work with. They are:
THEN ORGANIZE INFORMATION INTO THE USEFUL TABLE BELOW
Transfer the Information: Print off this page. Then transfer the 26 mineral names (listed above) from Appendix D to the handy-dandy chart below (You do NOT use your actual specimens for this part!! Just work with the information in the Appendix) .
The first "cell" has been filled in for you. The "hard metallic" minerals in the table are magnetite, hematite and pyrite. Which other minerals have obvious metallic lusters according to Appendix D? I found two metallic mineral descriptions which fit into the "average metallic" cell and one mineral description which fit into the "soft metallic" cell. The blank lines represent mineral names to fill in. Now find these other three metallic minerals (out of the 26 listed above) according to the descriptions in the "Usual Color/Luster" column of Appendix D.
Now onto the nonmetallics. NOTE: The hardness scale overlaps. For example, a mineral with a hardness of 5 will be listed in both the "hard" and the "average" cells. For example, I have filled in the "average nonmetallic dark" cell in the table with 6 mineral names. But hornblende has a hardness of 5-6 so it also fits into the "hard nonmetallic dark" cell as well. And biotite has a hardness of 2.5 - 3 which means it also fits into the "soft nonmetallic dark" cell.
Why overlap the mineral hardness categories? It is sometimes difficult to determine the exact hardness of a mineral so it is best to not eliminate any "suspects" too quickly. We will be comparing our actual mineral samples with the suspects in each cell. We are giving ourselves a little "leeway" in determining exact hardnesses by using these cell "extensions".
With this in mind, find the 7 minerals that fit into the "hard nonmetallic dark" cell (knowing that one of them is hornblende) and the 3 minerals that fit into the "soft nonmetallic dark" category (knowing one of them is biotite).
Complete the chart. You will notice that some minerals can be either light or dark so they fit into a couple cells due to this color variation (olivine is one of these). Once you have written all the mineral names onto the 23 spaces in the chart below, you will be able to fill in the first 23 questions on the associated Lab-Part 1 submission form in your In Box. This process will help you learn mineral properties but, more importantly, it will help arrange the minerals in a way that is easier to handle.
NEXT -- TAKE OUT YOUR MINERAL SAMPLES
Look at Your Minerals: Now we are going to look at each of our mineral samples and ask three questions:
Take each of your minerals and determine its hardness using your fingernail, penny, nail, and glass plate and the Hardness Scale on page 34 of your text book. Does it have a metallic luster, that is, does it shine like a metal. It can also look dull like old metal. If it looks glassy instead, is it glassy and dark in color or is it glassy and light in color (including white, pink and light green)? Remember that we have practiced this already as part of our textbook homework.
Now place each mineral in the appropriate cell in the chart above ACCORDING TO ITS HARDNESS, LUSTER AND LIGHTNESS/DARKNESS. You now have placed your twelve minerals into cells with a listing of the most promising "suspects" .
So where do your minerals fit? List their numbers (1 through 12) in the right spots below. Transfer these answers to the same numbered questions on the homework submission form. DO NOT TRY TO NAME THEM -- SIMPLY IDENTIFY THEM BY THEIR NUMBERS AT THIS POINT!
FINALLY -- NAME YOUR MINERALS
Ask Five More Questions: Now we need to ask a few more questions in order to complete our identifications (to actually NAME our minerals). For example, if you have one mineral which fits into the "hard, metallic" cell in the chart, is it magnetite or hematite (the two names you have written in that cell)? This is where the next set of mineral properties comes into play. These are the questions to ask next:
Ask these questions for each mineral, one at a time, as you try to match it up with one of the suspects in its cell. You might want to right down this information on an index card for each sample. When you answer the questions in the second lab exercise, you will need to know this information. You can easily check on your index cards for these properties without needing to determine them again for each mineral. You've already done the work!
Some minerals have distinctive colors as noted in Appendix D. At least one of your minerals is magnetic and at least one of your minerals will effervesce. Some of your minerals have strong streaks (as checked on your porcelain plate). If a mineral feels especially heavy for its size, it has a high specific gravity (abbreviated as "sp. gr." in the lab manual table). A mineral will feel heavy if its specific gravity is greater than 6.0 (as is true for at least one of your minerals).
Check your observations: In each cell of the chart above, compare your observations on your own minerals to the properties of those promising "suspects." Look at the mineral properties described in Appendix D, specifically Streak and Color. For example, if you have two minerals that are in the "hard, metallic" cell, then the promising suspects are pyrite, magnetite and hematite. Pyrite is a pale brass yellow with cubic crystals and a greenish black steak. Magnetite is dark gray to black, has a dark gray streak, attracts a magnet, and is not in our "very heavy" category (its specific gravity is 5.18). Hematite is silver to gray to brown, has a red to red-brown streak, and is also not in the "very heavy" category. Which of these sets of mineral properties best fits with your samples? There are differences in streak, color, and magnetism which will help us distinguish between these two possibilities.
You will probably be able to identify some of your minerals with confidence at this point. Go through your twelve minerals and identify as many as possible. You will likely have about half left for which you still need to make a couple more observations.
Checking on your confidence level:
Fill in the following answer on your homework submission form:
Ask Some Tougher Questions: Other mineral properties are harder for new geologists to determine. These are crystal form, fracture, and cleavage. It may not be easy to unambiguously determine these properties. Crystals and perfect crystal faces are rare in nature -- usually minerals don't have the space to grow into distinctive crystal forms. How minerals break is a more common property to observe.
"Cleavage planes" are flat surfaces which a mineral has "split" or been broken apart. Not every surface on your mineral samples represents a cleavage plane. Look for relatively flat surfaces -- especially look for flat surfaces that parallel one another. You will want to look with your hand lens to look for pieces of cleavage planes that repeat themselves. You do not need to break apart your mineral to determine its cleavage properties (especially if you wish to resell your rock kit)!
You may not find any cleavage planes but instead you may notice that a mineral is full of curved fractures (these are called "conchoical fractures" and they are curved like the inside of clam shells, for example, or broken glass). "Fracturing" happens when a mineral doesn't break along any particular direction -- it just breaks randomly like glass.
Look at your remaining minerals and try to answer these questions for each one:
Write these answers on your sheet or index cards for each sample. See if you can identify the specimens that do NOT exhibit cleavage (that is, have conchoidal fracture or no obvious breakage patterns at all). Find out which of your promising suspects fall in the "Uneven fracture" and "Poor cleavage" categories in Appendix D. Match your minerals up with the most likely suspects.
Checking on your confidence level:
Last Questions: Look back on the comments above describing "cleavage planes". If you think your last minerals have relatively flat surfaces which might be cleavage planes, here are a few more questions to ask:
With these observations written down, compare these remaining minerals with those that fall in the "Cleavage" categories in Appendix D. Again compare your minerals to the "usual suspects" and complete your identification work.
Checking on your confidence level:
You have completed the Week 3 Lab -- Part 1. Please use the submission form to turn in your answers. THEN......
Answer Those 30 Multiple-Choice Questions in the Week 3 Lab Part 2: With your index cards on which you have recorded your observations, determinations, and identifications of your minerals, proceed to answer your lab questions with relative ease (hopefully) and submit this last part of your lab homework. Use the form in your In-Box labeled as "Lab Homework Week 3 -- Part 2."