It
is a rare case when a mineral can be identified on color alone, but
can be very important for a first pass. This is because color can
vary so much for some silicates – pyroxenes in particular.
(How to decide what type of pyroxene you are looking at? If you can manage, check the type of extinction, and if you're lucky, compare birefringence. For clinopyroxene – inclined extinction and higher birefringence, orthopyroxene – parallel extinction and lower birefrigence). Also sediments can become stained so that they appear to
be a nonstandard color (example: iron stained pits in quartz and feldspars). It is also possible for minerals to become oxidized or discolored. Perhaps the most interesting case is that of pleochroic halos on biotite - damage to the crystal structure by alpha particle bombardment from embedded zircons. Vivianite’s blue color in plane polarized light is very diagnostic in lacustrine sediments, but is an oxidized feature in iron-rich sediments. It appears white in unoxidized sediments, but very rapidly turns bright blue when exposed to oxygen.
Shape
Euhedral,
anhedral, angular, rounded, and flaky. These are all useful
observations. It is important to remember that you are not working
with a petrographic thin section; shape can vary a lot (in 3D, even!)
since there is no control on thinness or thickness of sample. A lot
of principles of sedimentary petrology apply well to lacustrine smear
slides. You are more likely to see euhedral shapes of minerals grown
in the sediments (calcite rhombs are the most common euhedral mineral you are likely to see) or near a
volcanic setting. In most cases, minerals are worn down in transit
to the bottom of a lake via chemical and physical weathering
processes. Minerals high on the Mohs scale can be more resistant to
physical weathering,
so shape will at times be more well preserved (example: dodecahedral garnet, zircon, apatite).
Extinction
If you
have an isotropic mineral, it will be pretty easy to sort it out
(garnet or spinel). Do not be fooled by volcanic glass, which lacks a
crystal lattice structure, and thus will not extinguish when you
rotate the stage in cross polarized light. Extinction can also be
skewed as the light passes through an irregularly shaped object, so
it may not be as sharp as in thin section. In tectonically active
settings, you may see strained, otherwise known as undulatory, quartz
(the extinction sweeps across the grain, as the crystal lattice has
been bent – move the stage up and down in cross polarized light as
well to confirm this) or twinned calcite (this is how calcite
accommodates strain – it shortens itself under pressure). However,
compaction in the sediments is strong enough to develop some thin
twins in calcite. Thicker, patchy, or kinked twins would indicate
deformation before settling to the bottom of the lake bed. Extinction
is tough for micas as well – biotite’s extinction is
pseudoisotropic in a smear slide, as you tend to be looking down the c-axis of the
mineral and the thickness of a plate of mica is so small. To my eye,
the extinction is a faint dark green that is easier to see if you
turn the brightness way up before you rotate the stage (remember to
turn it back down before you flip back to plane polarized light so
that you don’t blind yourself!). It can be tough to determine
whether you have flakes of chlorite or biotite (both are colored,
transparent micas) in your sample. If the extinction looks bumpy, you are surely looking at biotite (this is called bird’s
eye extinction).
Twinning Patterns
You
will encounter twinning most commonly in calcite and in feldspar. As
it turns out, scientists have discovered a myriad of twinning
varieties in minerals, but a few are diagnostic for feldspars, so
much so that you can identify a compositional end member. For albite,
there are polysynthetic twins – these appear as thin parallel lines
that alternate in extinction.
Orthoclase rarely has more than one twinning plane in a single
crystal [IMAGE]. You can identify this under a microscope when half
of a grain goes extinct at a time. Microcline is known for its
combination of twinning types. These two twinning styles combine to
show a cross-hatched pattern of extinction.
Do not confuse exsolution lamellae with twinning patterns. Exsolution
lamellae are often found in microcline and orthoclase (this texture
is called perthitic), as well as pyroxene at times. These features
can be seen in cross polarized light as distinct regions of stringy,
pinched out, or subparallel to anastomosing meshes of different
chemical compositions within a mineral.
Zoning
Tourmaline,
garnet, plagioclase – can be visible in plane or cross polarized
light, depending on the mineral that you are looking at. Zoning looks
like a concentric pattern of alternating colors (somewhat like tree
rings). Tourmaline zoning often grades from bright aqua to olive
green in plane polarized light. Zoning in plagioclase is
difficult to see in plane polarized light, but will be very obviousin cross polarized light.
Birefringence
This
is by far the trickiest characteristic to utilize for mineral
identification in smear slides, so it is best to concentrate on the
previous characteristics. For example, quartz will be pale yellow to
white when the thickness of the grain is around thirty microns, and will be higher order when thicker (always work
with a scale bar present). Calcite almost always has extreme
birefringence (high order pastels). Chlorite has a variable chemical
composition that can sometimes lead to anomalous birefringence colors
under crossed polarized light – ranging from a curious brown to
bright purple.
Relief
The
Becke line test (following the plane of focus up and down over a
grain) seems to work well in smear slides as well as in thin
sections. This quick test is also useful for tracking cleavage planes
up and down a grain.
Lithic Fragments
The
terrestrially derived grains that you see were worn from a rock at
some point. A rock is an agglomeration of minerals – even if they
are all the same mineral or a long list of components. If you are
looking at a smear slide of large (sand sized, approximately) grained
sediments, it is possible that you may find a lithic fragment. A
lithic fragment is a piece of a rock, not just a lone mineral. It may
be a chunk of limestone, chert, quartzite, phyllite, siltstone,
shale, or basalt. These can be identified by the presence of grain boundaries within a single grain. One could easily misidentify a fecal pellet as a lithic fragment –
these pellets often occur as clots of material, but there is no heavy
line defining its edge and generally there is a fair amount of
organic material as well as minerals inside of a pellet.
This will help you determine that you are indeed looking at a lithic
fragment or not.
Abundance
A
little research at times: what does the presence of this mineral
imply about the depositional environment? If you
think you have identified an obscure mineral, perhaps one that would
only exist in a very specific setting, take a moment to make sure it makes some sense before making any drastic interpretations. Example: You think you’ve
found a few sparse, long volcanic glass shards in your slide. Is
there a volcano nearby? Double check that you aren’t looking at a
sponge spicule.
No comments:
Post a Comment