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Lab Exercise 1 - Mineral Identification

Purpose: In this exercise we will examine common rock-forming minerals and attempt to identify some unknown minerals by testing them. We will study some of their physical properties and learn simple tests that can be applied to distinguish minerals. Some of these test have value when testing gem substances in their rough form, but few of these test should be applied to cut gemstones.

This laboratory exercise is associated with the concepts introduced in Chapter 1.

WHAT WE WILL DO IN THIS LAB: We will do simple tests, but even these can be done with considerable precision if practiced. Since minerals are the building blocks of most rocks, what we learn today, will be applied to recognizing rocks and gem materials in future laboratories.
  • 1) identification of unknown minerals fragments in trays based on physical properties (1 hr.)
  • 2) identify crystal shapes on ideal specimens (10 minutes; done with your group)
  • 3) relate shape to the crystalline structure of minerals (10 minutes; done with your group)
  • 4) observe crystal structures of the selected crystal systems using models and natural crystals (10 minutes)

  • METHODOLOGY and PROCEDURES: A systematic (logically ordered) testing of unknowns and reference to mineral identification tables will allow us to make educated guesses as to the identity of the unknowns in project 1 (identification of mineral fragments). For these exemplar minerals, extreme quantitative (numerical) precision will not be necessary, but careful recording of results and observations are essential.

    The student should become familiar with the tests described below (see Review Questions and terminology at the end of this web page). The order in which the tests are done should be based on the data tables used (see next section). Not all tests are necessary or need be applied. The goal of the tests is to get enough characteristic results to slowly separate a large pile of minerals into smaller and smaller piles, and ultimately individual identified samples. Of course, this may take several steps.

    Click on images for a larger view throughout this website.
    Common rock-forming minerals
    Figure 1. Examples of common rock-forming minerals.
     
    The instructor will give you a list of numbers corresponding to the labeled minerals in our trays of mineral fragments you will test from your trays. Most of these are broken pieces not crystals.

    Record your observations: Use the Laboratory 1 Worksheet sample worksheet to record mineral sample numbers. Write the sample numbers in the in the answer spaces provided.

    Observing Characteristics of Minerals

    The following exercises (Steps A-G) are intended to encourage observation skills relating to descriptive properties of minerals. Selected minerals specimens are intended to introduce students to the unique characteristics if mineral related to the chemical composition and associated physical properties. Note that in the natural world over 4,000 different minerals have been identified, of which only several dozen are considered "common" on Earth. The samples provided for this exercise are common to moderately rare. All the minerals associated with this lab have economic significance, important mostly as used in manufacturing of household, industrial, and commercial products, including gemstones.

    Properties to be evaluated include mineral luster, color, hardness, cleavage and fracture, streak, crystal form, and other auxiliary properties.
    Note: Most samples needed for this laboratory exercise are available from local rock shops, gem & mineral clubs, or commercial sources.

    1) Luster

    Luster is the effect created by light reflecting from the mineral’s surface. The most important types are: 1) metallic luster and 2) non-metallic luster. Metallic luster is a property of some minerals that have shiny, metal-like appearance, although the mineral may not be a true "metal" in chemical form (like gold or copper). Example of minerals with metallic luster include pyrite and galena. Another type, 3) dull and earthy luster is often grouped with metallic luster (which you can do today). Though these two groupings (1 &2) are enough for common minerals, more specific types of luster terminology important to gemstones include: adamantine (like a diamond), chatoyant (such as cat’s eye), schiller (iridescent), greasy, pearly, silky, resinous (like amber or polyurethane), vitreous (looking like glass, a good example is clear quartz), and waxy. Gemstone luster will be carefully examined in a later laboratory, but demonstration examples will be displayed in room today.

    Lab Exercise Step A: Separating minerals by luster

    You have been given a box of minerals. Separate them into the groups based on basic luster characteristics of metallic, earthy or dull, and nonmetallic.
     
    Metallic
    Earthy or Dull
    Nonmetallic
    Sample numbers:      

    Step B: Demonstrations of Special Lusters:
    Special characteristics related to luster are demonstrated with special mineral samples provided for this exercise (if available). Familiarize yourselves with the following luster characteristics:

    • Adamantine means "having the hardness or luster of a diamond." Clear diamond is a highly "radiant" in bright light. Other minerals with high radiance include cubic zirconium, and "Herkimer diamond" (a unique variety of very clear quartz crystal).
    • Chatoyancy is the character of having a fibrous texture as seen in tiger’s eye. Tiger’s eye has fibers embedded in quartz and has a strong chatoyancy, but other minerals such as tourmaline and cat’s eye (chrysoberyl), or chrysotile also show this.
    • Schiller is luster property best seen in labradorite feldspar that varies in color as the mineral is moved and looks like the wings of some iridescent butterflies. Labradorite makes an attractive building material and semiprecious stone. Schiller is also seen in some gems such as moonstone.
    • Pearly luster as seen in the mineral selenite (gypsum) ulexite (sometimes called the TV stone).
    • Greasy luster as in some chalcedony, a type of microcrystalline (also called cryptocrystalline) quartz.
    • Vitreous luster as seen in broken glass.

    • Resinous luster as seen in amber (a fossilized tree resin; not a mineral)

    Lab Exercise Step B: Evaluating special luster characteristics. Put the correct numbers and your assumed names for the mineral samples based on their special luster characteristics.

     
    Chatoyancy
    Schiller
    Pearly
    Greasy
    Vitreous
    Resinous
    Waxy
    Sample numbers:              

    2) Color

    Color is how a mineral looks in reflected light. If the mineral is transparent this too can be noted. See what color most closely matches those used in the descriptions of minerals, but also note that there may be considerable variation. For instance, quartz occurs in many colors such as colorless, milky, rose, smoky, purple (amethyst), and yellow (citrine) and also varies from transparent to translucent (not allowing much light to pass through).

    Geologically, iron-rich, dark colored minerals are separated from iron-poor, light colored minerals. This distinction creates two separate subdivisions and speeds identification. It is also logical because most rocks are classified by the percentages of these lighter and darker minerals. Dark colored minerals are sometimes called ferromagnesian minerals (rich in iron and magnesium).

    Lab Exercise Step C: Separating minerals by color
    Place mineral’s numbers into the boxes below. Dark colored minerals include the following colors: Black, Brown, Green, Red, dark gray. Light colored minerals include the following, White, Light Brown, Yellow, Pink, Light gray. Also list the colors of minerals you consider to have metallic luster. Use the following or create your own descriptor for the metallic minerals, Brassy, blued (gun metal), tarnished (like silver), Rusty, Brick red, specular (reflecting light like many tiny mirrors).

    Step C
    Dark Minerals
    Light Minerals
    Metallic Minerals
    Samples numbers:    







    3) Hardness

    Hardness is defined as resistance to scratching. Commonly geologists and gemologists use a scale created by Friedrich Mohs in 1812. Mohs Hardness Scale is a relative scale and is based on 10 common minerals arranged from softest 1 to hardest 10. If a mineral of known hardness can scratch another of unknown hardness, the unknown is softer than the test mineral. You can bracket a mineral between two known minerals and then state its hardness. A mineral very close in hardness will just barely scratch a test mineral that it is close to. Half hardness definitely scratch the one below and clearly don’t scratch the one above.

    We will not scratch minerals, but will try to scratch test materials of known hardness with the minerals. Choose a sharp, clean, and representative edge or point of the mineral specimen to scrape against the test object. Caution! Always put the glass plate for testing on the table! Never, ever hold it in your hand while scratching a mineral on it!
    Mohs’ Scale of Hardness of Minerals
    1. Talc (softer than skin; feels slippery),
    2. Gypsum (fingernail is about 2.5 and easily scratches gypsum, but not calcite = 3)
    3. Calcite (a little harder than the average fingernail; a copper penny is 3.5)
    4. Fluorite (forms triangles and cubes)
    5. Apatite (your teeth are this hard, glass is about 5.5)
    6. Orthoclase Feldspar (a pink or white feldspar will just scratch glass; a steel file is about 6.5)
    7. Quartz (6 ½ porcelain plate will be scratched by quartz; quartz is found almost everywhere!)
    8. Topaz (varies in color, some is a precious gem)
    9. Corundum (includes ruby and sapphire)
    10. Diamond (which scratches everything, even other diamonds).
    Note that many minerals can be bracketed between two test minerals. In this lab exercise, if you think it is between them in hardness, you say ½. For instance Olivine can scratch orthoclase but not quartz. In this lab, we don’t need to get ½ hardnesses.
    Lab Exercise Step D: Separating minerals by hardness

    Using the minerals given by your instructor, list the minerals by hardness. Use the 3 categories below. Later when your identify the minerals, refer to these results:
    1) Soft. These minerals can be scratched by your fingernail.
    2) Intermediate. These minerals can not be scratched by a fingernail, but can be scratched using a steel nail (soft steel is about 5.5 on Mohs’ Scale; though this is the same hardness as glass, if the mineral is close to this hardness, it is much easier to see a scratch on glass).
    3) Hard. These minerals can scratch a glass plate. It important that you put the glass plate on the table and push the mineral away from you and others when you do this test to avoid injury.

    These are only relative hardness and if absolute hardness of diamond is compared to that of corundum it can be up to 150 times harder (diamond’s hardness is directional).

    Using hardness, luster, and color characteristics, classify the lab mineral samples and add their sample numbers to the table.
     
    Metallic or Earthy
    Nonmetallic and Dark
    Nonmetallic and Light
    Soft
    (fingernail)
         
    Intermediate
    (steel nail)
         
    Hard
    (scratches glass)
         
    Caution! Remember not to test fine mineral or gem samples for hardness unless there is a safe inconspicuous place to scratch. Destructive test are always a bad idea. Never do a hardness test on a cut gem in class. You can just ask your instructor how hard the cut gem is.

    4) Cleavage and Fracture

    Some minerals break with smooth sides when hit by a strong, direct blow. Other minerals will fracture unevenly. A mineral that cleaves with smooth sides will reflect light from the cleavage just as a wristwatch glass does (Have you seen someone shine sunlight around the room with a watch glass? Must have been a boring class). Shiny, reflective sides suggest cleavage, while dull sides of a mineral are usually due to fracture (some metallic minerals may have tarnish that reduces both luster and shine off the surface). Fracture surfaces are uneven and won’t reflect light like a watch glass (see Figure 3).

    Minerals displaying good and poor cleavage
    Figure 3. The flat or stepped surfaces of cleavage reflect light all in the same direction, like a mirror. A fractured uneven surface reflects light in all directions. Cleavage shows a strong flash of light when the specimen moves.
    Cleavage and Fracture In Minerals

    A mineral may have both fracture and cleavage (see Figure 4). Minerals become flat-sided geometric solids if three (3) or more good cleavages exist. A cube for instance is formed by 3 cleavages at right (90°) angles to each other (Example: halite). Less than 3 cleavages means at least one fracture surface. Some minerals (Example: quartz) have no cleavage at all!

    Cleavage is due to planes of weakness in the mineral’s crystalline structure. It is result of bonding and ordering of atoms. It is important, for example in diamond cutting and also can create flaws. A cut gemstone may show a cleavage crack internally. Sometimes a cleavage crack reflects a rainbow of colors.

    Cleavage planes of different minerals including quartz, muscovite, feldspar, halite, and calcite
    Figure 4. Examples of cleavage and fracture in selected minerals: quartz (lacks cleavage); muscovite (single-plane of cleavage, fracture in other directions), feldspar (2 directions of cleavage, fracture in other directions); halite (cleavage in 3 directions at 90°); and calcite (cleavage in 3 directions and angles greater and less than 90°).
    Lab Exercise Step E. Look at the angles at which the cleavages meet (assume you are looking at mainly cleavage or fracture surfaces in your box samples), are they right angles (like a box), acute angles (less than 90°), or obtuse angles (greater than 90°). Though it is difficult to be precise, you may estimate the actual angle created when two cleavages meet. First list if you see cleavage at all (cleavage = flat and reflective surfaces). A mineral having only one cleavage will make a sheet-like flat mineral, more than one cleavage will make a 3-D or boxy mineral piece.
    Metallic or Earthy
    Nonmetallic Dark
    Nonmetallic Light
    No Cleavage

       
    Cleavage Present

       

    More than one
    cleavage present
    (estimate angles)





       

    Streak

    The color of a powdered mineral is called the mineral’s streak. Just as when a piece of chalk is drawn across a black board, it leave a streak. Streak is when a mineral is drawn across a porcelain plate. Any mineral that leaves no powder on the plate is harder that the porcelain (6.5 on Mohs’ scale, same as a hardened steel file).

    Many minerals have a different colored powdered than they do in their crystal or massive form. The color may be entirely different, or it may be a different shade. Look at pyrite, fool’s gold, for instance.

    Lab Exercise Step F: Report the color of the powdered mineral on the porcelain plate as streak, as it may differ from the color appearance of the larger specimen. Do not waste the streak plate. Start by doing a few examples suggested by your instructor.

    Do minerals with metallic luster and dark colored nonmetallic minerals. Light colored minerals’ streak are usually not needed for identification.
     
    Step F, Streak test for minerals. Report number and color. Start with suggestions from your instructor.
    You can test other minerals later if you are having trouble identifying them.
     
    Metallic or Earthy
    Nonmetallic Dark
    Nonmetallic Light
    (not important)
    Color of streak
    (put colorless
    if streak powder
    has no color)
         

    Crystal Shape

    The shape of a crystal can be a characteristic property. Most minerals crystallize with a particular shape referred to as the mineral’s crystal habit. The crystal’s habit is a result of the internal ordered arrangement of atoms in the mineral’s structure.

    The habit or shape is formed by flat-sided surfaces referred to as crystal faces. Crystal faces are the result of growth of the mineral. A crystal with all flat surfaces bounding it is called euhedral (unless it is a mineral with perfect cleavage). Euhedral crystals are rare and are formed when crystals have room to grow without interruption.

    Most crystals in a rock hit other growing crystals and do not have flat faces. These irregular crystals are called anhedral. If you look at the rock granite, the visible crystals all run into each other, thus every mineral is anhedral (see side table granite piece). Crystal shape can be characteristic for a mineral. For instance, beryl often crystallizes in an elongate 6 sided prism(hexagonal prism) with flat ends. One of the most common euhedral minerals seen is quartz. Quartz often forms hexagonal prisms with a pyramid at the tip (termination). If a crystal has flat faces at both ends it is said to be doubly terminated.
    Figure 5. Examples of mineral crystal forms. mineral crystal shapes
    Lab Exercise Step F. Use the illustrations above to identify the unknowns on the side table labeled “crystal shape. ”Warning! Do not Scratch these minerals! Use color and luster plus shape to determine your answers. Note that the minerals are listed in your tables at the back of the chapter and you can look up luster and color there.
     
    Octahedral
    shape
    Cubic shape
    Complex
    shape
    Twinned
    Prism
    Sample number          
    Your guess at a name          

    Understanding crystal shape is very important for correctly orienting gems for cutting and polishing. Along with cleavage and fracture the proper orientation of the crystal is important for bringing out the best qualities of a gem.

    Auxiliary Test

    These tests should only be done if you suspect the mineral might have this property based on looking at the Tables of Identification at the back of this chapter. So do not test every mineral with these tests, only use them later. Still become familiar with them for possible later use.

    Reaction to Acid:
    This test is used on minerals that are suspected of being in the carbonate family. A drop of dilute hydrochloric acid is placed on the mineral. If the mineral bubbles, it is a carbonate and the acid has dissolved it and released carbon dioxide (CO2). Clean the acid off by drying it with a paper towel immediately after testing. It can stain clothes! If you leave an acid soaked mineral in your box, everyone at your table loses a point on the next quiz!!!

    Lab Exercise Step G - As needed, check the minerals you suspect may be carbonates with acid.
    Which sample or samples
    fizz when exposed to acid?
     
    Specific Gravity (also called "heft"): Basically specific gravity (SG) is a comparison of the weight of a mineral to the weight of an equal volume of water. The resulting number is very much like density (grams/cm3 [a division problem]); however, since you divide grams by grams in specific gravity, the units (grams) cancel out and SG is dimension less (no weight or volume given). SG of minerals are tabulated and usually can be calculated to 1 or 2 decimal places in the lab; for instance quartz has an SG of 2.65 while the metallic mineral galena has an SG of 7.6.

    However, we will study SG in a separate lab, but today you can observe heft. Heft is the weightiness of an object in your hand. For instance, two oranges of the same approximate size can be tested for heft. The one that feels heavier is juicer. Two minerals of the same size can be hefted and the heavier mineral has a higher SG. Try this for say for two (2) sets of light colored, two dark colored, and several metallic minerals. Put the general results in the table below, remember you are approximating and sort of treating all the minerals as if you are testing oranges for juiciness. So heavy = most heft and light = least heft.

    Again, density and specific gravity are basically the same thing and we will use them in later labs. Heft itself may be valuable when comparing say a quartz to a topaz. See the side table for a piece of each. Which is greater heft, Quartz or Topaz?
    Lab Exercise Step H: Determining heft (approximate density)
    Report number and approximate heft of the mineral. Try and find minerals of similar size and report heft as light, medium, heavy.
    Only do a few tests.
    2 of each mineral color
    or luster maximum
    Metallic or Earthy
    Nonmetallic Dark
    Nonmetallic light


       


       

    Magnetic Attraction

    Few minerals are attracted to a magnet. Magnetite, an iron mineral, is attracted. Ferrous (iron minerals) are more attracted than nonferrous substances. Some gems, such as synthetic diamonds are attracted to magnets because they have iron inclusions. Iron meteorites are also magnetic.

    Lobestone is a magnetic form of magnetite that will hold an iron staple. Find it in your box!


    Meteorite with a magnet stuck to it.
    Figure 6: A iron-nickel meteorite with a magnet stuck to it (example is a Diablo Canyon meteorite from Arizona).

    Tenacity

    Tenacity is more than just strength, it is essentially overall durability and behavior related to toughness. Minerals can have tenacity such as being:
  • Brittle (breaks or shatters under force)
  • Elasticity (snapping back)
  • Malleable (ability to be hammered into sheets)
  • Ductile (ability to be drawn into a wire), etc.

  • Interestingly, tenacity is not entirely related to hardness, for example topaz is harder than quartz, but breaks more easily because it has cleavage and quartz has no cleavage. Thus most people would suggest that quartz has better tenacity. Gold’s tenacity is great for jewelry.

    Taste

    Salt has a taste as do other some other non-gem minerals. However, don’t taste minerals! You could get sick. Although most are OK, some have toxic metals, arsenic, or other concerns. It is advised to wash your hands after handling mineral samples.

    Smell

    A few minerals smell when broken or powdered. Usually sulfur, but also some clays. Petroleum smell is common is some porous rock and mineral samples derived from oil-producing regions.

    Transparency

    Lean the meanings of transparent, opaque, and translucent. Transparent is clear (like glass) and objects can be seen through it; opaque means light will not pass through it; translucent means that light will move into it, but not project an image (like frosted glass or white plastic).

    Luminescence

    So minerals give off light when stimulated by: ultraviolet light, heat, pressure, electricity, X-rays, etc. We will study luminescence a little later in the course.


    Warning! You may need to retest some of these minerals if you do not find a match in the tables. The reason is that poor technique and impurities in the specimens can lead to inaccurate results. Sometimes you should borrow a neighbor’s sample just in case!
     

    LIST OF MATERIALS NEEDED FOR THIS LAB EXERCISE

    Note: Most test objects and mineral samples needed for this laboratory exercise are available from local rock shops and gem & mineral club shows, or commercial sources. Be sure to protect demonstration samples and encourage everyone to conserve samples for future use.
    1) Test objects: glass plates, porcelain plates, steel nails, calcite pieces, hydrochloric acid (7%), plastic magnifiers, magnets (or staples)

    2) Demonstration minerals: quartz crystal, calcite crystal, corundum crystal, tourmaline crystal

    3) Models of crystal shapes: of salt and diamond, wooden models of crystal shapes.

    4) Examples of minerals for evaluating luster.
  • metallic: pyrite, galena, magnetite, specular hematite.
  • earthy or dull: hematite and limonite.
  • nonmetallic: quartz, gypsum, feldspar.

  • 5) Special lusters:
  • adamantine: diamond or cubic zirconium
  • chatoyancy: tiger’s eye
  • schiller: labradorite
  • greasy: opal or chalcedony
  • pearly: ulexite or gypsum (satin spar)
  • earthy: oolitic hematite (red) and limonite
  • vitreous: quartz
  • resinous: amber

  • 6) List of minerals needed for hardness tests: talc, gypsum (selenite), calcite, fluorite, apatite, feldspar, quartz, topaz, corundum, diamond (industrial diamond, or carborundum as a substitute)

    7) Specific gravity (heft): quartz and topaz

    8) Magnetism: magnetite or iron metal

    Review Questions and Terminology

    1) List the important words in defining a mineral.

    2) List the important words in defining a gem.

    3) What is gem rough?

    4) List important physical properties that we test for in the lab.

    5) List physical properties tests that are destructive to the mineral/gem samples.

    6) How hard is your fingernail, a soft steel nail, a piece of glass, a porcelain plate?

    7) On a cut (faceted) gemstone, where is the best place to do a hardness test?

    8) Find out, from an outside source, what tarnish is? What may cause it to occur?

    9) A flat piece or surface on a mineral is more likely,_______________, but could also be __________________.

    10) Can you explain your answer to question 9 (above)? Why is one more likely?

    11) Can a mineral have a cube form only by growing as a cube in the first place?

    12) Why would anyone name a trash/garbage bag “Hefty?”

    13) Quartz is one of the most common minerals present, you run into it on a beach, in house
    dust, etc. What property should a gem have in greater/excess than quartz?

    Click below to proceed to:
    http://geologycafe.com/gems/labs/lab1.html
    9/17/2014