CLUES TO RECOGNIZING MINERALS IN SMEAR SLIDES
     Calcite is highly birefringent so that even tiny fragments appear blue, red, and/or yellow under crossed polars; often they are outlined in black. Most calcite flakes are probably tiny fragments of foraminifer tests: look for associated identifiable foram tests and fragments to confirm this. If fragments are too small to identify as foraminifers (or nannoplankton), lump them as"undifferentiated calcite" in percentage estimates.
Pavement calcite appears under crossed polars as a flat bluish grains with irregular black lines (the interlocking pavement blocks). Fragments were probably originally foraminiferss, but now altered to mineral calcite.

Aragonite needles, look like minute stubby rice grains to more elongate shreds. Like calcite, they are highly birefringentand even small fragments show color under crossed polars.  (Aragonite is most often seen in the shells of pteropods, see Biogenic components.)

          Aragonite needles (Image downloaded from Doug Schnurenberger's web 
         site at http://lrc.geo.umn.edu/smears/.)
    Dolomite is rare, but sometimes occurs as tiny equidimensional rhombs or flakes with high relief and very high birefringence, higher even than that of calcite.
     
               Dolomite Rhombs      Gypsum crystals in smear slides look like gypsum in hand sample: they have the typical elongated rhombic form (or some twinned variation thereof). Gypsum is colorless in plane polarized light and is distinguished from quartz by its crystal habit. Gypsum fragments can appear as scuzzy yellow under crossed polars. Halite crystals, if they survive the smear-slide preparation, are isotropic and appear as clear squares (cubic) under plane-polarized light and are black under crossed polars.

    Quartz is very common, especially in sediments derived from terrigenous sources. It has low birefringence and low relief. Grains are usually rounded or subrounded with relatively high sphericity. Small fragments are grey, but typical silt- and sand-sized grains show concentric rings of blue, red, yellow. A sand-sized quartz grain looks like a colorful topographic map of a low hill. Very small fragments of quartz are often difficult to distinguish from feldspar.
     


    Quartz in plane-polar and cross-polarized light. (Image from Reidel 
    and Tway's LithoWise .)
    Feldspars shows low relief and low birefringence; twinning is common. It occurs commonly with quartz and although small angular fragments are difficult to distinguish from that mineral, larger feldspar grains can be recognized by their more platy form (feldspar looks like a topo map of a plateau rather than of a hill), and slightly lower birefringence (some what more browns and yellows and oranges as opposed to reds and blues. Feldspars often show twinning under the crossed polars; quartz is never twinned.
     
     Micorcline feldspar displays a unique wavy-lined "tartan" twinning pattern
           that cannot be confused with anything else. (Image from Riedel and Tway's,
           LithoWise.)
    In fine-grained clastic sediment where the distinction between quartz and feldspar is difficult, lump as quartz/feldspar or simply as "siliciclastics" for the purpose of percentage determinations.   Ferromagnesian Minerals (Amphiboles, Pyroxenes, Olivines) occur as discrete grains in pelagic and terrigenous deposits. Grains are often larger than the "background clay" in pelagic sediments and generally show high relief and high birefringence. Some are gray or greenish gray in plane-polarized light; some are pleochroic (change color lightly when stage is rotated in plane-polarized light). A distinctive "hacksaw" habit is also common. Although we do not often recognize the specific minerals for reconnaissance smear-slide determinations, some clues are
      Olivine - green in plane-polarized light and showing high order colors under crossed polars; concoidal fracture.

    Amphiboles - (esp.hornblende) - high relief, moderately elongate, prismatic habit, green, greenish gray in plane-polarized light, pleochroic, moderate birefringent colors, but may be masked by mineral color. Green hornblende is relatively common in near-shore sediments
     

    Green hornblende in plane polarized and cross-polarized light
         (Image from D. Schnurrenberger's web site at
                                         http://lrc.geo.umn.edu/smears/.) 

    Pyroxenes - Most common ferrumagnesian mineral in marine sediments. High relief, moderately high birefringence, prismatic. Can be distinquished (sometimes) from amphiboles as the pyroxenes are not usually colored or pleochroic in plane-polarized light. Also individual crystals tend to be more stubby than amphibole crystals.

          Heavy Minerals (non-opaque) comprise a variety of minerals of over 2.89 specific gravity. Many (not all) are recognized by very high relief, high birefringence, rounded subhedral grains. They occur mostly in near-shore sediments in association with quartz and feldspar. (High birefringent, high relief mineral grains in deep-sea sediments are more likely to be ferromagnesian minerals, though both heavy and ferromagnesian minerals occur in near-shore sediments.) Some heavy minerals which are recognizable in smear slides are:

        Apatite - colorless in plane-polarized light. Moderately high relief, weak    birefringence (small fragments are gray, larger fragments are blue and red, yellow.) Apatite is distinguished quartz by its much higher relief. It also occurs frequently as elongated prismatic crystals.

          Zircon - rounded grains with very high relief (perifery looks like a Marks-A-Lot line), very high birefringence, colorless in plane-polarized light, inclusions.

        Rutile - very high relief, very high birefringence, yellow or brown in plane- polarized light, diagonal striations.

         Garnet - very high relief, rounded octahedreal crystal and isotropic. It is the only relatively common heavy mineral which is isotroic (black under crossed polars) May be pink or reddish brown under plane-polarized light and show distinctive v-shaped etched surface.

         Tourmaline - elongate prismatic form, moderate relief, moderate birefringence, often colored in plane-polarized light and highly pleochroic, parallel extinction. Distinguished from hornblende by parallel extinction and greater pleochroism.

         Fe Oxides - hematite, limonite is reddish brown in plane-polarized light, and may be in the form of small transparrent grains or the red color dissiminated throughout the clay fraction. Highly birefringent, but often this is masked by strong brown mineral color. Iron oxides appear earthy and reddish brown in reflected light. Magnetite and ilmenite are opaque. If gross sediment color is red, it probably contains iron oxides; it doesn’t take much iron oxide in the sediment to give it a red color.

          Fe/Mn Micronodules - opaque, spherical to subspherical nodules.     Pyrite - opaque, commonly seen in microfossil tests, especially diatom tests, as spherical (framboidal) clumps. Also seen as opaque cubes and octahedra or just opaque spheres and grains scattered throughout the sediment. Appears metallic brassy yellow under reflected light. Sometimes pyrite looks reddish (oxidized) in plane-polarized light.

   Zeolites - Phillipsite is the most common zeolite in younger marine sediments. It occurs as small transparent (plane-polarized light) prismatic grains with distinctive v-shaped terminations or as distinctive cruciform twins or clusters. Look for zeolites in red and brown clays (sediments deposited below the CCD) and below beds rich in volcanic ash: zeolites are thought to be an alteration produce of ash.

Because Clinoptilolite is more common in older sediments, from below 700m, we rarely see it in the SIO cores. But if a zeolite seen in the form of small, ragged shreds instead of the distinctive phillipsite crystals, suspect clinoptilolite. Usually, at the level of smear-slide descriptions, simply lumping the minerals as zeolites is sufficient.


Zeolite individual crystals and interpenetrated cruciform clusters. Volcanic shards (glass) are angular, clear isotropic (black under crossed-polars), fragments, often with spherical vessicles (gas bubbles) and parallel "pipe vessicles." Brown shards are usually mafic glass: colorless shards are usually felsic glass. Use the Becke line also to distinguish between mafic and felsic shards. If the Becke line moves inward as objective is raised, then ash is mafic. If it moves outward, the ash is felsic. Keep in mind when determining percentages that If volcanic ash is abundant in the smear slide, much of the angular, isotropic material in the clay fraction may also be volcanic ash. Mafic shards from glassy rinds of pillow basalts don’t have the vesicular structures.      Volcanic glass. (Image from Rothwell, Minerals and Mineraloids.)

Palagonite - flaky, scuzzy yellowish orange in plane-polarized light. Amorphous, scuzzy under crossed polars. Found in association with volcanic shards. Remnant vesicles may be seen in palagonitizing shards.

Micas occur as large thin flakes ... looks like mica in hand specimen. In smear slides mica flakes are often larger than the other grains owing to their slower settling velocity. Although an anisotropic mineral, because mica flakes settle on the broad flat side and present the isotropic optic axis to the viewer, the appear isotropic (dark, no birefringence) under crossed polars.) Generally, under plane-polarized light chlorite is green, biotite is brown or brownish green, and muscovite is colorless.
 

Mica Flake (Image from D. Schnurrenberger's web site.) Glauconite - grains are a scuzzy olive green in plane-polarized light and isotropic (cryptocrystalline) under crossed polars. Glauconite seen off the Mendocino Fracture zone, however occurs in large (.1 - .5mm) gray-green pellets, often with reddish brown (limonite) sutures. Some pellets appear to be casts of fossil tests or fecal pellets(?).They appear as large opaque masses in the petrographic scope. Look for them under lower in the reflected-light ‘scope. Clay (Component) - Clay, as a component, comprises a suite of specific minerals, but in practice all the very fine-grained background material which can’t be otherwise identified is usually lumped under "clay."
 
  Clay (Image from D. Schnurrenberger's web site at http://lrc.geo.umn.edu/smears/.) Clay (Texture) - grains less than .004mm. Sometimes even these small grains can be identified as diatoms, volcanic shards or FeOxides so be sure to lump these components back into the clay size when making sand/silt/clay determinations. Clay (component) and clay (texture) percentages may vary for the same slide.

Non-Disaggregated Material - best to recognize as a separate category, as the clumped material can comprise a
variety of components.

(Last revised 3/2002 by pworstell@ucsd.edu)