Practical Guide to Identification of the Basic Rock Forming Minerals:
Some of you may be wondering "Why are we looking at these bits of rock? What good will this do me?" The exercises we do in the first lab introduce you to concepts and practices you can apply to a million other things in life. Learning how to look at minerals will teach you how to classify, how to notice physical details (some of which might be very subtle), how to describe those details, and how to conduct an investigation of physical materials. This is an exercise about expanding the inquisitive part of your mind. We think minerals are the perfect tool. Besides, they're cool.
The first thing you need to have a clear understanding of is: What exactly is a mineral, what is a crystal, and what is a rock. Like this course, we'll start small and work up. At the very beginning of the formation of the planet Earth, there was a lot of molten material. Some scientists think the whole thing may have been melted at one time. The evidence for this is that the oldest rocks on the Earth and on the Moon (4.5 billion years or so based on isotope dating) are igneous. Igneous (ig nee us) rocks are made from cooled molten planetary material. As a "melt" cools, various collections of molecules become arranged in a stable framework, forming crystals. Different collections of atoms and molecules associate to create a specific crystal framework. There are common associations of elements that occur always in the same proportions because the atomic framework consists of the same group of elements built up over and over again. During cooling, there may be two or more different chemical associations building crystals simultaneously. These chemical associations have different crystal shapes and probably different colors. We observe them again and again in the rocks at the surface of the Earth. A naturally occurring chemical association that has a consistent crystal habit and other attributes is called a mineral (it will be bad for you if you copy this definition verbatim onto the lab exercise).
As you will see today, there are many different kinds of minerals, and usually several different kinds are found in any one rock. The kinds of minerals (and their size and habit), depend on the chemistry of the original material and pressure and temperature conditions present during cooling. Do not be discouraged that there are "many different kinds of minerals," because realistically there are only a few that are relevant. This makes the whole thing less intimidating and more fun. Rocks are simply "aggregates of minerals." In other words, rocks are made up of mineral crystals or of grains stuck together. You'll get more on this later - don't sweat it too much now.
There are 6 basic rock-forming minerals: Quartz, Feldspar, Mica, Pyroxene, Amphibole, Calcite. If you can identify these minerals and their relative abundances in the rock, you can make all sorts of useful and interesting inferences about the rock's origin and then, based on where you found it, of its subsequent history.
This guide has several other minerals as well as the big 6 because they are interesting and instructive:
| Pyrite | Garnet | Olivine |
| Galena | Magnetite | Chlorite |
| Halite | Kaolinite | Glaucophane |
| Gypsum | Fluorite | Serpentine |
The easiest mineral to identify (in my opinion). The relative abundance of quartz as
a mineral constituent is important in naming rocks, especially volcanics. If you miss this one, you're in big trouble.
| CRYSTAL FORM | Under conditions where free crystal faces can develop, forms short prisms with six sides terminated by three large faces and three very small faces (see figure). The sides of such prisms show a characteristic banding perpendicular to the long axis. In rocks, you'll never see good crystal outlines because quartz is usually the last mineral to crystallize. So learn what it looks like on the ideal samples in the lab. Also occurs as amorphous masses (flint, chert, chalcedony, agate, jasper). |
|---|---|
| COLOR | Clear, colorless to milky to translucent pink, yellow (citrine), red (carnelian), purple (amethyst) or gray (smoky). Noncrystalline masses can have any color you can imagine depending on whatever contaminants or minor constituents are present. |
| STREAK | White |
| LUSTER | Glassy to greasy |
| CLEAVAGE | None, has a characteristic conchoidal fracture |
| HARDNESS | 7 scratches glass, but not scratched by knife |
| SPECIFIC GRAVITY | 2.65 |
| CHEMICAL REAGENTS | Does not fizz in acid or dissolve in water |
The most abundant mineral in the Earth's crust. There are three end-members of this family of minerals, orthoclase, albite and anorthite. Their chemical relationships are shown on the ternary (triangle or three-way) diagram. Orthoclase can grade into albite by the gradual substitution of sodium for potassium in the crystal framework (this is called a solid solution). In the same way, albite can grade into anorthite by the substitution of calcium for sodium. There is no gradation between orthoclase and anorthite. Feldspars that fall on the Na-K side are called "alkalai feldspars" or "Potassium feldspars" or "Kspars" while those on the Na-Ca side are called "plagioclase feldspars." It is important to be able to tell the 2 groups apart because the relative abundances will help you name the rock in which it occurs.
This mineral has a composition that varies in the relative amounts of potassium (K) and sodium (Na). If it has more than 10% potassium, then it behaves as follows:
| CRYSTAL FORM | Forms a square-sided, stubby prismatic to tabular crystal (called Monoclinic). Often displays Carlsbad twinning (see figure). If the crystal has a mixture of Na and K, it often shows a "perthitic texture" which appears as short pale streaks within the mineral. Orthoclase takes its name from the Greek orthos, straight or right, and klasis, a break or fracture. |
|---|---|
| COLOR | Warm pink to white, very rarely pale green (in which case it's called Amazonite). When weathered or altered, can become chalky. |
| STREAK | White |
| LUSTER | Vitreous on fresh surfaces |
| CLEAVAGE | Breaks cleanly on two sides at right angles. |
| HARDNESS | 6, scratches glass, not scratched by knife |
| SPECIFIC GRAVITY | 2.54 to 2.62, this is average rock density at the surface of the Earth |
| CHEMICAL REAGENTS | None |
| DIAGNOSTIC CHARACTERISTICS | Nearly always exhibits perthitic texture, but not albite twinning. Perthitic texture looks like wispy intersecting streaks in the mineral. |
| CRYSTAL FORM | Forms a skewed version of the crystal described above. Plagioclase can also be stubby prismatic or tabular, but the sides will be slightly off square (called Triclinic crystal habit). Plagioclase takes its name from the Greek plagios, oblique, and klasis, a break or fracture. Plagioclase frequently displays Albite twinning that looks like tiny fractures or lines (striae) parallel to one side of the crystal (see figure). |
|---|---|
| COLOR | White to gray, sometimes iridescent gray (Labradorite). When weathered or altered, can become chalky. |
| STREAK | White |
| LUSTER | Vitreous on fresh surfaces |
| CLEAVAGE | Breaks cleanly on two sides at slightly oblique angles. |
| HARDNESS | 6, scratches glass, not scratched by knife |
| SPECIFIC GRAVITY | 2.54 to 2.62, this is average rock density at the surface of the Earth |
| CHEMICAL REAGENTS | None. |
| DIAGNOSTIC CHARACTERISTICS | Nearly always exhibits albite twinning, and never has perthitic texture. Albite twinning appears as tiny parallel striae on a cleavage surface. You can see it easily through a handlens as light reflects off the surface of the mineral. |
The main clues for distinguishing a feldspar from quartz are the cleavage and hardness; quartz doesn't cleave nicely and is harder. Calcite has a higher cleavage angle, is softer, and will fizz in acid. Other light-colored minerals will be much less abundant unless you're looking at a very strange rock so don't worry about them. You'll hardly ever be able to distinguish between the two feldspars if the rock you're looking at has crystals smaller than 5 mm or so because you just can't measure cleavage angles and it's really difficult to see the twinning. In that case you'll have to use other clues like color or association with other minerals as a guide. Once in a while a plagioclase will have a pinkish color, but usually, if it's pink, it's an alkalai feldspar (aka Kspar). Watch for the albite twinning (the tiny lines); if they are well-developed, then it's a plagioclase. However, if you don't see albite twinning, don't automatically conclude it's orthoclase, check the cleavage angles.
This group of minerals is characterized by a remarkably fine cleavage in one direction and by the thinness, toughness, and flexibility of the elastic sheets into which the cleavage permits them to be split, hence the classification as "phyllosilicates," (phyllo = sheet) For practical purposes, they can be divided into two groups: light-colored micas, or Muscovite; and dark-colored micas, or biotite.
| CRYSTAL FORM | Forms squat, flat, six-sided crystals that split readily along the base/top into thin sheets. |
|---|---|
| COLOR | Clear to smoky (Muscovite), to black, brownish, or greenish translucent (biotite) |
| STREAK | White, colorless |
| LUSTER | Splendent to pearly on cleavage faces |
| CLEAVAGE | .Perfect "basal" cleavage into thin, flexible sheets |
| HARDNESS | 2 to 3, easily scratched by a knife, but not by fingernail |
| SPECIFIC GRAVITY | 2.8 to 3.4 |
| CHEMICAL REAGENTS | Does not fizz in acid, does not dissolve in water |
Pyroxenes are an important constituent in igneous rocks, they are often the dark specks or are the reason for an overall dark color of a rock. They form in the presence of iron and magnesium and indicate a "basic" or mantle derived source. Most often confused with amphiboles which are similar in color and occurrence, pyroxenes are distinguished by their cleavage angles. A mnemonic we grad students use goes like this: the "x" in pyroxene indicates cross-wise (square) cleavage. Pyroxenes are readily altered to other minerals such as calcite or limonite.
| CRYSTAL FORM | Square-sided prisms, usually short and thick (see figure), though perfect crystals are only rarely found in rocks. |
|---|---|
| COLOR | Usually pale green to dull green to black, rarely white |
| STREAK | White to gray-green |
| LUSTER | Vitreous |
| CLEAVAGE | Two directions parallel to the prism sides, at nearly right angles (87° and 93°). The cleavage is good, not perfect - expect slightly irregular cleavage faces. |
| HARDNESS | 5 to 6, sometimes can just be scratched by a knife, but will not scratch glass |
| SPECIFIC GRAVITY | 3.2 to 3.6 (heavy due to the iron present) |
| CHEMICAL REAGENTS | Does not fizz in acid, does not dissolve in water |
Amphiboles are another important constituent of igneous rocks, often making up the dark colored fraction of minerals in an igneous rock. Hornblende is one common variety. Amphiboles alter readily to serpentine and/or chlorite (waxy greenish minerals common in the California Coast Ranges), limonite, carbonate and quartz. You don't generally find amphiboles and pyroxenes together. The presence of amphiboles indicates high pressure and the presence of fluorine and gaseous water (hence the hydroxyls). High pressure conditions suitable for the formation of amphiboles occur at great depth or under a tectonic load.
| CRYSTAL FORM | Forms prisms, often long and bladed with sides that meet at angles of 55° and 125°. In cross-section amphiboles have a diamond shape. Some varieties form very thin needlelike prisms and give a Belted appearance to the rock. A useful mnemonic is the "A" in Amphibole describes an angle of about 55°. Oh yea, and see that "h" in amphibole? It reminds you there's a hydroxyl at the end of the formula (OH). OK, ok, I'm stretching it, but it actually works. |
|---|---|
| COLOR | Highly variable, white to pale green to dark green, brown, and black. Most commonly in the darker colors. |
| STREAK | White to gray green or light brownish |
| LUSTER | Vitreous to pearly on cleavage surfaces |
| CLEAVAGE | Two directions parallel to the prism sides. Cleavage intersects at angles of 55° and 125°. Cleavage is perfect on the prism sides, but rough on the ends. |
| HARDNESS | S to 6, sometimes can be scratched by knife |
| SPECIFIC GRAVITY | 3.0 to 3.5 |
| CHEMICAL REAGENTS | Does not fizz in acid, does not dissolve in water |
Calcite is a carbonate and so behaves is a most characteristic manner, it dissolves in acid. This is a very handy feature, because most self-respecting geologists carry around a small bottle of dilute HC1 (hydrochloric acid) that they can dribble onto an unknown white mineral, and if it fizzes, then "ta da," it's calcite.
| CRYSTAL FORM | Rhombohedrons (see figure) along free surfaces when formed from flowing waters (crystals lining geodes), also microcrystalline to massive (dripstone, limestone, travertine), and coarsely crystalline (marble, a metamorphic version of limestone). |
|---|---|
| COLOR | Normally white to clear, but occasionally pink or yellow, rarely salmon-colored, purple, gray, or green (in the presence of impurities) |
| STREAK | White to gray |
| LUSTER | Vitreous |
| CLEAVAGE | Perfect cleavage in three directions, making perfect rhombohedrons with interfacial angles of 78° and 102° |
| HARDNESS | 3 |
| SPECIFIC GRAVITY | 2.7 |
| CHEMICAL REAGENTS | Fizzes in cold, dilute acid. Does not dissolve in water. |
One of the most widely distributed minerals. This oxide of iron is found in many different types of rocks, and is the coloring agent for reddish sandstones and red soils.
| CRYSTAL FORM | Rarely forms distinct crystals, generally occurs as either "specular iron ore," "micaceous hematite" (specularite), or "common red hematite." |
|---|---|
| COLOR | Specular and micaceous varieties are silvery metallic, while common red hematite is dull red-brown |
| STREAK | Red brown to brown which distinguishes it from magnetite (black) and limonite (ochre yellow) |
| LUSTER | Splendent to metallic for specular or micaceous varieties, earthy for common red hematite |
| CLEAVAGE | None |
| HARDNESS | 5.5 to 6.5, the earthy form is softer than the metallic form |
| SPECIFIC GRAVITY | 5.26 |
Commonly known as "Fools' gold," this sulfide commonly occurs in metamorphic rocks, notably shales that have been heated and along with economic ores such as copper and gold. Sometimes it replaces fossils (this is especially cool). Stupid prospectors did not know to test the hardness of this yellow metal which distinguishes it from softer gold.
| CRYSTAL FORM | Isometric, frequently as cubes, or pyritohedrons or octahedrons (see figure). When it replaces fossils it is microcrystalline. |
|---|---|
| COLOR | Brassy yellow metallic |
| STREAK | Brownish black |
| LUSTER | Metallic to splendent |
| CLEAVAGE | indistinct, fracture: conchoidal |
| HARDNESS | 6 to 6.5 |
| SPECIFIC GRAVITY | 5 |
| CHEMICAL REAGENTS | Does not fizz in acid, does not dissolve in water |
This lead sulfide is not all that common to find, but we happen to have a lot of it for this lab exercise and due to our lack of pyrite, have substituted it for certain questions. Galena is an important lead ore found in many different situations: Sedimentary rocks, hydrothermal veins, and with deposits associated with intrusive bodies (pegmatites).
| CRYSTAL FORM | Isometric, forms cubic and occasionally octahedral crystals |
|---|---|
| COLOR | Lead-gray metallic (diagnostic feature) |
| STREAK | Lead-gray |
| LUSTER | Metallic opaque |
| CLEAVAGE | perfect in three directions, forming cubes (diagnostic feature) |
| HARDNESS | 2.5 |
| SPECIFIC GRAVITY | 7.6 |
| CHEMICAL REAGENTS | Does not fizz in acid, does not dissolve in water |
Halite, also known a rock salt, or just plain old "salt," is one of the not-so-abundant, but interesting minerals in this lab. You will know halite when you lick it (except not with the sample specimens in this lab). What many folks don't realize is that salt occurs in vast thick layers beneath the surface of the Earth is some places (Gulf Coast of USA, Iran, and more).
| CRYSTAL FORM | Isometric; occurs as cubes, or as "hopper" crystals where the edges grow out faster than the faces (see figure). |
|---|---|
| COLOR | Colorless to white |
| STREAK | White |
| LUSTER | Greasy to silky |
| CLEAVAGE | Perfect in three directions, forming perfect cubes with each bash of the hammer |
| HARDNESS | 2.5 |
| SPECIFIC GRAVITY | BOW |
| CHEMICAL REAGENTS | Dissolves in water (diagnostic feature); has saline taste |
Gypsum forms at low pressure and temperature in the presence of water. It often occurs with halite (rock salt) or sedimentary rocks. You can find gypsum crystals growing in soils around the Bay area.
| CRYSTAL FORM | Flat, tabular crystals with no right angles. Gypsum is commonly twinned in "arrowhead" or "swallowtail" shapes (see figure). Within rocks, it can be foliated (small flakes) and show curved surfaces. |
|---|---|
| COLOR | Colorless or white in transparent or translucent crystals; massive varieties can be red or brown or even black due to impurities |
| STREAK | White |
| LUSTER | Vitreous to pearly |
| CLEAVAGE | Perfect cleavage along the flat side of the crystal, allowing thin sheets to be peeled off (almost like with mica). Two other cleavage planes exist, but not as perfect as the side cleavage, they form a rhomb with angles of 66° and 114°. |
| HARDNESS | 2, easily scratched by a fingernail (diagnostic feature) |
| SPECIFIC GRAVITY | 2.3 |
| CHEMICAL REAGENTS | Does not fizz in acid, does not dissolve in water |
Usually found in metamorphic rocks, but sometimes in igneous rocks. This is one of the merely cool minerals that's easy to identify because of the hardness and the crystal shape.
| CRYSTAL FORM | Isometric, which means that it makes really nifty dodecahedrons or trapezohedrons (see figure). Though sometimes it can occur as masses with no distinguishable crystal forms. |
|---|---|
| COLOR | Variable, commonly dark red or reddish brown, but sometimes green or brown or pink or yellow |
| STREAK | White or a pale shade of the crystal's color |
| LUSTER | Vitreous to resinous |
| CLEAVAGE | none |
| HARDNESS | 6.5 to 7.5 will scratch glass |
| SPECIFIC GRAVITY | 3.6 to 4.3 |
| CHEMICAL REAGENTS | Does not fizz in acid, does not dissolve in water |
| CRYSTAL FORM | Crystals octahedral or dodecahedral (complications on an isometric form), though usually massive or granular. |
|---|---|
| COLOR | black |
| STREAK | black |
| LUSTER | dull metallic |
| CLEAVAGE | none, massive stuff has a hackly or uneven fracture |
| SPECIFIC GRAVITY | way heavy |
| DIAGNOSTIC STUFF | magnetic |
A decomposition product of feldspars.
| HABIT | Usually as earthy aggregates with mineral grains too small to see. |
|---|---|
| COLOR | white, often stained brown or gray |
| STREAK | white (duh) |
| LUSTER | dull, earthy |
| CLEAVAGE | can't see the cleavage, grains too small |
| SPECIFIC GRAVITY | way light |
| DIAGNOSTIC STUFF: like chalk, but doesn't fizz in acid |
Not something you're likely to stumble on in the hills, but we have some really nice specimens in the lab.... (hint)
| HABIT | Crystals isometric, usually in cubes, less often as octahedrons |
|---|---|
| COLOR | Colorless to wine-yellow, green, green-blue, violet-blue, also white, gray, skyblue, purple, bluish black or brown. Color often two-toned |
| STREAK | clear, white |
| LUSTER | vitreous |
| HARDNESS | 4 |
| CLEAVAGE | Cleaves perfectly in § directions, 45 degrees off cubes axes into an octahedral shape |
| SPECIFIC GRAVITY | medium-heavy |
| DIAGNOSTIC STUFF | has the same crystal shape as halite, but doesn't dissolve in water and is harder |
A mineral mostly found in quick-cooled rocks (lava-rocks like basalt). Again, important because we have nice specimens that you'll be seeing.
| HABIT | Crystals equant. In granular masses and as rounded grains in dark-colored igneous rocks |
|---|---|
| COLOR | Olive-green to brown-black |
| STREAK | white or gray |
| LUSTER | vitreous |
| HARDNESS | 6.5 |
| CLEAVAGE | indistinct |
| SPECIFIC GRAVITY | medium-heavy |
| DIAGNOSTIC STUFF | has the same crystal shape as halite, but doesn't dissolve in water and is harder |
An important metamorphic mineral.
| HABIT | Crystals flat, pseudohexagonal, in fine-grained or earthy masses or scaly aggregates |
|---|---|
| COLOR | Characteristically green |
| STREAK | white to pale green |
| LUSTER | vitreous to earthy |
| HARDNESS | 2.5 |
| CLEAVAGE | small, brittle, perfect flakes (different from biotite which has flexible flakes) |
| SPECIFIC GRAVITY | medium |
Another important metamorphic mineral; a primary constituent of blue schists.
| HABIT | Prismatic to acicular, can be fibrous | |
|---|---|---|
| COLOR | Pale blue, lavender blue, dark blue to black. Darker with more iron. | |
| STREAK | white to blue-gray | |
| LUSTER | vitreous, silky in fibrous varieties | |
| HARDNESS | 6 | |
| CLEAVAGE | Perfect in 1 direction | |
| SPECIFIC GRAVITY | medium-heavy | |
| DIAGNOSTIC STUFF | Color and crystal habit! |
The state mineral of California. Contains other minerals along with pure serpentine: lizardite, chrysotile, and antigorite. Commonly found in the California Coast Ranges and Sierra Foothills. Forms as the alteration product of basalt (ocean floor volcanic material). When found in very large, sheet-like occurrences, serpentine represents slivers of uplifted ocean crust.
| HABIT | Generally as structureless masses, sometimes in fibrous masses. | |
|---|---|---|
| COLOR | Usually green, also yellow, brown, reddish brown to dark gray, almost black. Often mottled or speckled. | |
| STREAK | white | |
| LUSTER | waxy or greasy in massive forms, silky when fibrous | |
| HARDNESS | variable, 4 to 6 | |
| CLEAVAGE | no cleavage, breaks along smooth, curved planes | |
| SPECIFIC GRAVITY | average | |
| DIAGNOSTIC STUFF | This is just one you'll have to look at a lot and get used to what it looks like, nothing else is similar, and it's hard to describe, but you will see it on exams! |
Some of the above descriptive material was derived from the following sources: Louis V. Pirsson, revised by Adolph Knopf, 1955, Rocks and Rock Minerals, published by John Wiley & Sons, London. , Berry, L.G., and B. Mason, 1959, Mineralogy: Concepts, descriptions, determinations, published by W.H. Freeman and Company, San Francisco.
First, does the mineral even have a crystal habit? Some minerals do not always form in conditions that allow the formation of perfect crystal faces:
Anhedral (-hedral = faces; an = without)
Subhedral Some minerals form a few nice crystal faces, while the rest is anhedral
If the crystal is completely bounded by its own "rational faces," and its growth during crystallization was not restrained or interfered with by adjacent grains, it will have nicely formed faces on all sides:
Euhedral
(eu = good)
Prismatic (Acicular for long, thin crystals)
(Columnar for shorter, thicker crystals)
Prisms can have 3, 4 or 6 primary sides (with additional auxiliary sides).
Equant (equal dimensions around crystal) varieties: cubic and dipyramid
Bladed
Tabular
Sides can intersect at right angles (orthorhombic), or at oblique angles (monoclinic or triclinic)
Lamellar or "Platy"
Typical of micas. Crystals form in stacks of sheets.
Some crystals grow like Siamese twins creating characteristic shapes that help to identify it. Common forms of simple twins are Contact and Interpenetrative. A more complex type of twinning is called Polysynthetic which simply means "lots of 'em."
Example of a Contact twin in cassiterite (SnO2).
Example of Carlsbad twinning (a type of contact twin) in orthoclase (KAlSi308).
Example of Interpenetrative twin in fluorite (CaF2)
Example of interpenetrative twin in staurolite (FeAl4si2olo(oH)2
Example of polysynthetic twinning in albite (also known as albite twinning). If you see this type of twinning in your mineral, you can be certain it is not a potassium feldspar (K-spar), but a
plagioclase.
As crystals grow, the free surfaces assume an outer shape dictated by the molecular arrangement. Perfect calcite crystals (which you'll see in lab) form prisms with 6 (or a multiple of 6) sides and a complex arrangement of faces coming together to a point at the end. Sometimes you'll be lucky and find perfectly formed crystals in a rock you're identifying. But mostly the crystals will be intergrown with other minerals and you won't be able to see the perfect free-face shape of the crystal. Well, what do you do then?! No problem, look at how the mineral breaks. Just like the outside shape is controlled by the molecular arrangement, internal planes of weakness are formed by the molecular arrangement. The way the mineral breaks (cleanly or imperfectly), and the angles between broken planes are characteristic for each mineral and can be used to identify an unknown. When a mineral has a definite plane or planes of weakness along which it breaks, it is called CLEAVAGE. (And no, a Wonderbra is not necessary.)
Sometimes a mineral has no weak plane, all directions are strong. So when it breaks, it just busts anywhere. That's called FRACTURE. Cleavage planes are flat surfaces which reflect light sort of like many tiny mirrors at slightly different elevations. Fracture surfaces are very uneven and won't reflect light the way a cleavage plane does.
Look at the examples laid out in the lab for different kinds of cleavage (perfect, good, poor) and fracture (conchoidal, hackly, uneven).
