1.1 Introduction
This is a book about identifying minerals in thin section. It is not a text on optical mineralogy. If you wish to read about the fundamentals of optical mineralogy, consult chapter 5 of Mineralogy.
Mineralogists have described more than 6,000 minerals; most are rare. The following pages contain descriptions of about 75 minerals, including all the major rock-forming minerals you are likely to see in thin sections of common igneous and metamorphic rocks. We have included some sedimentary minerals too, but sedimentary petrography is not a focus of this book.
Many of the minerals described here are abundant; others are widespread but exist in small amounts. Still others have been included because they have a special significance or special properties. However, if you are studying rocks of unusual composition or history, you may encounter minerals that are not listed.
We have tried to show and describe typical or characteristic optical features of all minerals. But every mineral, even common ones, can have unusual features in some rocks. You may find something in your rock that doesn’t look quite like our images because we never saw that before!
Video Recordings: We have included videos for many minerals. The petrologist who recorded the videos tells you what he sees that lets him identify that particular mineral. And he also points out potential ambiguities.
Photomicrographs: We also have thin-section photos for most minerals. A single mineral may have a range of appearances in thin section, so we have included photos that show different appearances or characteristics. For all photographs, we list the width (field of view or FOV) in millimeters.
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About the Photographs Photomicrographs generally appear first in a plane polarized (PP) view. When you mouse-over the image, it will switch to a crossed polars (XP) view. You can switch back and forth quickly using your mouse. For devices that lack a mouse, tap on the image to switch to XP view. Tap outside the image to return to PP view. Links below figure captions allow you to see larger versions of the photos. |
Most of the photos are ours, taken from our research and teaching collections. We provide citations for photos we obtained from web sources.
Mineral Descriptions: The focus of this book is on what you need to know to be able to identify different minerals in thin section. So we describe common occurrences and distinguishing features first. We also list similar minerals and the properties useful for distinguishing them.
Optical Properties: We have tabulated the optical properties for each mineral. These properties generally include crystal system, refractive indices, birefringence, relief, interference figure, and more.
For most minerals, we have included drawings, like the ones seen here, that show relationships between crystallographic and optic axes. The drawings also show principal cleavage directions as fine lines. Mineral crystals, however, have many different morphologies; the drawings we include are just examples.
In the drawings, a, b, and c (in green) are labels for the crystallographic axes. In contrast, X, Y, and Z (in red) are three mutually perpendicular light vibration directions. By convention:
••light vibrating parallel to X has the smallest refractive index (α)
••light vibrating parallel to Y has intermediate refractive index (β)
••light vibrating parallel to Z has the greatest refractive index (γ)
Cubic minerals are isotropic; tetragonal and hexagonal minerals are uniaxial. Orthorhombic, monoclinic, and triclinic minerals are biaxial. Isotropic minerals have no optic axis, uniaxial minerals have one, and biaxial minerals have two.
In these drawings, optic axes are shown by blue circles with partial ellipses. Although unlabeled on the drawings, the acute angle between optic axes in biaxial minerals is 2V.
In biaxial (+) (“biaxial positive”) minerals, β is closer to α than to γ, and 2V is centered on the Z direction. In biaxial (-) (“biaxial negative”) minerals, β is closer to γ than to α , and 2V is centered on the X direction. The optic plane (a plane that includes both optic axes) is shown for biaxial minerals by a dashed line, unless it would make the sketches too cluttered.
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The last chapter (link) of this book contains an automated unknown mineral search routine. The search starts assuming your mineral can be one of the 75, or so, that are described in this book. You click to select the properties that you have observed, and minerals are eliminated from the list. In principle, with enough properties and clicking, you will get down to one or just a few possibilities. There are, however, ambiguities and complications that can make this process not so straightforward. Perhaps the best thing about the Search Routine is that you can explore. You can see which minerals stay on the list or disappear as you change properties. So, you can figure out which properties are key for identifying particular minerals. You can also just wander around. Curious about which minerals are isotropic? Click and you will find that there are five. (Yes there are other obscure ones, but they are not in our database.) Or maybe you like minerals that appear red in thin section? Click and you will find there are also five. Uniaxial minerals? There are 12. |
1.2 Work plan for ID’ing minerals
Identifying minerals could be as simple as scanning through the images and videos until you find something that looks like what you see in your thin section. But that’s a little like trying to find a book on quantum physics without knowing how books are catalogued. You might start at one end of the library, see if the first book is on quantum physics. No? OK, pick up the next one. Still no? OK, pick up the next one. And so on. It would work…eventually. But it’s not very efficient.
Wouldn’t it be better if you knew how books were organized? Even if you didn’t know exactly where quantum physics was catalogued, you’d be a lot better off if you knew where the physics section was, or even the science section. You wouldn’t be scanning ten shelves of cookbooks, forty shelves of romance novels, the complete works of Charles Dickens or Karl Marx, etc., to get there.
Well, so how are minerals organized? In textbooks like this one, we organize them according to mineral structure – framework silicates, sheet silicates, etc. That’s good organization from a theoretical standpoint, and it has its uses (for example, many micas share similar optical properties, as do amphiboles, etc.). But that’s still a little abstract for a beginning student. How do you know if a colorless low-relief mineral is a tectosilicate or a chain silicate? After all, you can’t see how the silica tetrahedra are arranged.
Instead, the way petrologists commonly approach mineralogy is to look at the rock first – what type of rock is it? Is it an igneous rock? Volcanic? Plutonic? What’s its overall bulk composition? Mafic? Felsic? These kinds of considerations temper our expectations and narrow down the range of minerals we expect.
For example, if we’re looking at a basalt, we wouldn’t expect to see quartz or K-feldspar among all the possible low-relief, colorless minerals. It’s not impossible, but the more likely minerals are plagioclase and possibly zeolites (if the basalt has infilled vesicles). Conversely, if we’re looking at a granite, we don’t expect to find olivine or pyroxenes.
Rocks have many surprises – that’s one of the things that makes petrography so fun! Some very rare basalts do contain quartz crystals (usually grains they picked up from wall rocks), and that’s important for understanding their history. But most rocks have similar assemblages of minerals, and it’s worth learning what’s likely. True, we can’t list all possibilities, but we have tried to provide a useful list of the most common minerals in the most common rock types, along with their most diagnostic optical properties.
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Subtle Colors A note on color in plane-polarized light: Yes, we understand – colors can be subtle. If you want to see colors more distinctly, switch to conoscopic illumination, or otherwise turn up the light intensity. Chloritoid and glaucophane will look really blue; garnet can actually look pink; pyroxenes and actinolite will look green. However, it’s hard to see relief or cleavages with conoscopic illumination, and focusing is problematic. So, apply conoscopic illumination when you want to verify color (or look at interference figures), but prefer normal illumination to assess most optical properties. |
1.2.1 Common and diagnostic minerals
Rather than repeat the optical properties for the same minerals in different kinds of rocks, we’ve constructed the following table of common and diagnostic minerals as a reference (also available here as a pdf):
| Very Common Minerals | |||
| Mineral (r.i.) |
Relief | Interference Color | Distinctive Characteristics |
| Quartz (1.55) | Low | 1st order white | Undulose (wavy) or domainal extinction (different blocks go extinct sequentially, like falling dominoes) |
| Plagioclase (~1.55) | Low | 1st order white | Albite and pericline twinning; often optically zoned |
| K-feldspar (1.53) | Low | 1st order white | “Tartan” twinning (microcline); Carlsbad twins (sanidine) |
| Muscovite (1.6) | Medium | typically 3rd order |
Bird’s Eye Extinction (BEE) but colorless in PPL; single excellent cleavage |
| Biotite (1.62) | Medium | 2nd-3rd order |
BEE, pleochroic brown or brownish green in PPL; may host pleochroic halos; single excellent cleavage |
| Hornblende (1.65) | Medium | 2nd order | Pleochroic green, brown, greenish brown or greenish blue; inclined extinction; pleochroic halos; good cleavages intersecting at near 60-120o |
| Clinopyroxene (1.7) | High | 1st–2nd order | Greenish in PPL (may be pale); inclined extinction; fair to good cleavages intersecting at near 90o |
| Orthopyroxene (1.7) | Medium | 1st order yellow |
Subtle pink/green pleochroism; parallel extinction; fair to good cleavages intersecting at near 90o |
| Olivine (1.67) | High | 2nd-3rd order | Curved fractures; does not occur with quartz |
| Garnet (1.8) | High | (none) | Lacks cleavage; often contains quartz and oxides |
| Calcite (1.5-1.65) | Variable | Pearly white |
Planar twin domains may appear colored in PPL |
| Common or Diagnostic Minerals | |||
| Mineral | Relief | Interference Color | Other characteristics |
| Epidote (1.75) | High | Anomalous blue; or up to 2nd order | Occasional twins; very saturated interference colors, e.g., lemon yellow, bright magenta |
| Kyanite (1.72) | High | upper 1st order | Good cleavage and one parting; ductile folding common; looks like colorless staurolite, but with good cleavage; often shattered or plucked (has holes) |
| Sillimanite (1.66) | Medium | 1st -2nd order | Good cleavage; rhombs or rectangles in cross section with diagonal cleavage; also can be fibrous in fibrolite mats |
| Andalusite (1.64) | Medium | 1st order yellow |
Chiastolite crosses (sometimes); faintly pink pleochroism sometimes in cores. |
| Chlorite (1.6) | Low | Green-brown/blue; typically 1st order |
Pleochroic green; pleochroic halos |
| Staurolite (1.75) | High | 1st order orange |
Pleochroic yellow; looks much like kyanite but lacks cleavage |
| Chloritoid (1.75) | High | 1st order yellow but typically looks blue | Pleochroic blue/green; may have hourglass structure; similar to staurolite but different color |
| Cordierite (1.55) | Low | 1st order white |
Looks like quartz, but can have yellow pleochroic halos around included minerals; sillimanite often present |
| Glaucophane (1.65) | Medium | 2nd order | Pleochroic blue/purple; amphibole cleavage |
| Actinolite (1.65) | Medium | 2nd order | Very pale green to colorless; amphibole cleavage |
| Accessory Minerals (small or tiny, rarely abundant) | |||
| Mineral | Relief | Interference Color | Other characteristics |
| Apatite (1.65) | Medium | 1st order gray | Usually small; looks a lot like garnet but lower relief, and not quite isotropic |
| Tourmaline (~1.65) | Medium | 2nd order | Usually small; diverse strong colors; reverse pleochroism |
| Spinel (1.8) | High | (none) | Dark bottle green or reddish brown |
| Zircon/Monazite (1.9) | Very high | 3rd-4th order |
Usually tiny; causes pleochroic halos in biotite, chlorite, cordierite, and hornblende |
| Ilmenite | N/A | (opaque) | Usually small and elongate; rarely blood-red or brownish purple at thinned edges or in flakes |
| Magnetite, chromite sulfides | N/A | (opaque) | Usually small; blockier than ilmenite; chromite often occurs as inclusions in olivine |
| Titanite (1.95) | Very high | 4th order pastel |
Usually small; brown/tan in PPL; similar to calcite but much higher relief |
| Rutile (2.75) | Very high | 4th order pastel | Usually small to tiny; blobby or needley; Orange/brown/amber-colored in PPL |
1.2.2 Common rock types and their minerals
Here is a simple table of common rock types and their mineralogy (also downloadable here as a .pdf):
| Common rocks and their mineralogy | |
| Igneous rocks | Minerals |
| Mafic (basalt) | Olivine, plagioclase, clinopyroxene, orthopyroxene, magnetite/chromite; zeolites or calcite as alteration or infilling of vesicles |
| Mafic (gabbro) | Olivine, plagioclase, clinopyroxene, orthopyroxene, magnetite and/or ilmenite |
| Intermediate (andesite) | Plagioclase; hornblende; clinopyroxene or orthopyroxene; magnetite and/or ilmenite |
| Intermediate (diorite) | Plagioclase; hornblende; more rarely clinopyroxene or orthopyroxene; biotite; magnetite and/or ilmenite; apatite |
| Felsic (rhyolite) | Quartz, K-feldspar (sanidine), plagioclase; often biotite; zircon |
| Felsic (granite) | Quartz, K-feldspar (orthoclase or microcline), plagioclase; often hornblende and/or biotite; either titanite or ilmenite and/or magnetite; apatite, zircon, tourmaline, muscovite (sometimes), epidote (sometimes), garnet (rarely) |
| Sedimentary rocks | Minerals |
| Clastic rocks | Quartz, plagioclase, K-feldspar as clasts; quartz, calcite, or hematite as cements; rock fragments (diverse types); chlorite, clays, muscovite (rarely), zircon, tourmaline, apatite |
| Carbonates | Calcite and/or dolomite, quartz, clays |
| Chert | Quartz (super fine-grained) |
| Metamorphic rocks | Minerals |
| Low-grade pelitic schist | Quartz, plagioclase, muscovite, chlorite, ilmenite or magnetite, zircon, apatite, tourmaline |
| Medium-grade pelitic schist | Quartz, plagioclase, muscovite, biotite, garnet, staurolite, kyanite (higher pressure), andalusite (lower pressure), cordierite (lower pressure), zircon, apatite, tourmaline, monazite, ilmenite or magnetite or rutile (rarely), chlorite (often retrograde) |
| High-grade pelitic schist | Quartz, plagioclase, K-feldspar (orthoclase or microcline), biotite, garnet, sillimanite, cordierite, spinel (rarely), zircon, apatite, tourmaline, monazite, chlorite (retrograde) |
| Greenschist | Quartz (often), plagioclase (albite), chlorite, epidote, actinolite (sometimes), ilmenite or magnetite or titanite |
| Amphibolite | Plagioclase, hornblende, biotite, epidote (often), quartz (often), garnet, chlorite (sometimes), calcite (sometimes), clinopyroxene (rarely), ilmenite or magnetite or titanite or (rarely) rutile |
| Granulite | Plagioclase, clinopyroxene, orthopyroxene, garnet, hornblende, biotite, ilmenite or magnetite |
| Blueschist | Plagioclase (albite), quartz, glaucophane, epidote OR lawsonite, muscovite (phengite), rutile and/or titanite, chlorite, garnet (sometimes) |
| Eclogite | Garnet, clinopyroxene (sodic), glaucophane (rarely), quartz, rutile, titanite (retrograde), muscovite (phengite), kyanite (rare), retrograde amphiboles (common – actinolite, glaucophane, or a mixed composition) |
| Low-grade calc-silicate | Calcite, dolomite, quartz, plagioclase, K-feldspar, biotite (pale), talc, tremolite, clinopyroxene, epidote, garnet (sometimes), vesuvianite (sometimes) |
| High-grade calc-silicate | Calcite, dolomite, quartz, plagioclase, K-feldspar, biotite, epidote, olivine (not with quartz), wollastonite, scapolite, brucite (after periclase), garnet (sometimes) |