FOREWORD
Lake sediment mineralogy and component analysis presents many new challenges to the student because it is a non-traditional problem for which neither books on marine minerals nor textbooks on thin section analysis offer much consolation. On the other hand, we can lean on the experience common to ocean sediments to develop a scheme and classification.
Lakes differ greatly from marine systems because of the many local factors that enter the picture and the closer links with the landscape geology and drainage basin. Lakes may vary greatly in chemistry determined by bedrock geology and hydrology, and may range over five orders of magnitute in total dissolved solids. Thus, a wide range of authigenic and evaporite minerals and components may be encountered that never enter the marine equation. Carbonates for example are typically inorganic (although bio-induced) precipitates in lakes, whereas almost all carbonate in the ocean is biogenic. Dolomite, siderite, and exotic Fe-Mn carbonates can occur in a number of lacustrine settings, but are rare in marine sediments. Amorphous silica precipitation may be quite common in alkaline lakes, although it is a unique oddity in marine environments. Biogenic and organic matter may be very abundant, and comprise any number of sources.
The following is a guide to optical determination of the major components that make up lake sediments. It is intended for working with smear slides (SM) or coarse fraction (CF) microscopy utilizing a petrographic microscope. The student is typically confronted with a spread of very fine grained particles and clumps, of which very few seem familiar in any way. It is intimidating. Be confident. With a few tips, study, and careful, systematic observations it soon becomes possible to generate most paleoenvironmental interpretations directly in the initial microscopic study. Surprisingly, even the eyeball estimates of percentages to 5% become rather consistent. The important step is to understand the likely genetic relationships among components and follow a systematic scheme of checks and elimination for sleuthing the clues to determinations and significance. Fear and loathing often arise today because only a very few geology students ever learn principles of optical mineralogy and crystallography anymore. Yet the microscope is the most powerful sediment tool, although much more dificult to use than a microprobe, x-ray diffraction, or isotope instrument.
Smear slide determinations derive from the practice of sediment petrography in the 1920s when heavy minerals were studied as grain mounts from sandstones to correlate oil well stratigraphy before micropaleontology. In the 1960s marine geologists discovered that even very fine-grained clays could be determined from simple smears on an object slide imbedded with Canada Balsam (RI=1.54). With the advent of Deep Sea Drilling (1968), shipboard smear slide analyses with visual percentage estimates have become the mainstay tool of marine core descriptions. Environmental concerns with the production of carcinogenic artificial resins such as CAEDEX (artifical Canada Balsam) added a major setback, but today a number of epoxy cements, such as NORLAND Optical Cement (RI=1.55-1.56) provide mounting media with stable strain-free optical properties.
Smear slides offer a cheap, powerful, rapid analytical tool to identify components and origins of lake sediment. They also offer a quick semi-quantitative measure of relative changes in components in a lithostratigraphic section.
There has never been a satisfactory manual for smear slide descriptions of lake sediments. This is a first attempt. I have been meaning to launch this effort since 1973 when during my first trip with DSDP I thought I was the only one on the Glomar Challenger who had never seen a smear slide. After meeting several famous colleagues also sneaking into the lab at 4 a.m. to review the shabby collection of marine examples, I eventually was relieved to discover that my prior schooling with titans of Teutonic mineralogy had eminently qualified me to become a smear-slide expert. We still hark back to the basic bibles such as Kerr, Optical Mineralogy (1959); or Tröger, Optische Bistimmung der Mineralien mit Tabellen(1967) but these only present classic fundamentals. So now with the advent of Website education I can pursue a manual of direct experience with lake sediments with the assurance that all mistakes are erased with a click, and others can complete the task once started.
Kerry Kelts, LRC Core Facility, January 1998
Thanks to inspiration and the role model of Rothwell, R.G., 1989, Minerals and Mineraloids in Marine Sediments: an optical identification guide. Elsevier, London, 279pp.
Note: This is a work in progress. Rather than wait until it is perfect we have opted to publish what we have thus far and add/subtract and edit as time permits. Please e-mail any comments, suggestions or corrections to schno005@tc.umn.edu.
INTRODUCTION
Smear Slides
Geologists are accustomed to petrographic study of rocks in thin sections where polishing the minerals to uniform (30mm) thickness allows a precise comparison of the birefringence as a major tool of identifications. Smear slides of lacustrine muds have a large range of particle thicknesses from the 2 micron clays to sand-size quartz, and thus present an unfamiliar spectrum of birefringence behavior. This complicates things. However, thickness can be estimated and tricks of refractive indices substitute as clues to mineralogy. Some minerals create confusion with special habits, such as micas that are commonly laying with the C-axis vertical, and thus show pseudo-isotropy in smear-slides rather than the rich pleochroism common in thin-sections. The range of authigenic minerals is equally unfamiliar to a standard petrographer.
For the study of LAKE SEDIMENTS we must make clear distinctions of the component origins within obvious genetic catagories. For this book we treat separately:
A) Authigenic, neo-formed, and diagenetic minerals and components
formed within the lake environment
B) Biogenic components such as shells, frustules, or parts
of the organisms dwelling in the catchment or aquatic system
C) Clastic and terrigenous components: the minerals transported
in from the landscape
D) Organic matter as a special case of biogenic components that
can be differentiated as terrigenous or aquatic sourced; pollen, charcoal,
coal, algal OM, bacterial sheaths, or other
E) Components treated separately because of their significance,
such as volcanic ash and tephra, or because they cannot be identified such
as many opaque grains and micro-nodules. Other examples might derive
from hydrothermal sources, or even cosmogenic particles.
Basically A-B-C can be plotted in a simple ternary diagram to
provide poplulation clustering needed to guide sediment classifications.
Keep in mind that the same mineralogy may occur in each of these.
For example, aragonite may be washed into a lake from ancient limestones,
marginal mudflats or travertine terraces, and also be precipitated in the
open waters from plankton productivity, as well as forming the major mineral
of a near-shore gastropod hash, or even precipate in organic rich sediments
as a diagenetic product of sulfate reduction.
The investigator needs to approach the description of smear slides with some kind of a standard check-list and scheme including percentage estimates that will allow an efficient noting of data. Different lakes will present different components so that it is most useful to modify check-lists to suit particular problems: example check list.
The first step is to make an educated guess on the TEXTURE of the sediment
(grain size distribution), mainly focusing on the non-biogenic fraction.
Be careful to make a scan of the whole slide because smear slides commonly
have coarser particles concentrated in a zone because of surface tension
created while drying on a hot plate.
Preparation
The preparation of good smear slides is an easy art. Because of varying grain size, a good smear is not just uniform but has patches where finer minerals are dispersed and larger ones grouped. Large sand grains create some problems. A small spatula or toothpick of sediment is dispersed usually in a few drops of calgon water, acetone, or alcohol on a microscope slide. You can also smear on the cover glass if you are very slick, careful, precise, and want all components on the same optical plane for viewing at higher magnification. The smear should be just barely transparent when dry, not too thick. Practice determines how much to swirl to get good particle separation, avoiding clumps of clay or organics.
The slide is labeled and dried on low heat (~60°C) with a hot plate.
A drop or two of Norland optical cement is dropped on the clean cover glass (or clean slide if a cover slip smear), and the cover glass placed on the warm, dry, smear. Avoid any possibility of contaminating the optical cement reservoir. Keep amounts to a minimum to just barely cover all the grains. You can adjust, spread, and press down with one or two pencil erasers.
The slide with cover glass and optical cement is placed under a strong ultraviolet-fluorescent lamp for 2-3 minutes to cure. The cement polymerizes quickly to a permanent mount under cold UV.
It is now ready for examination. Keep in mind that the slide may
have picked up some tricky contaminants such as microshreds of the toothpick
(high birefringent fragment), cigarette ash (looks like a zeolite with
holes), cloth threads (analine colors), or hair strands (shimmers).
General Microscopy
GRAIN ESTIMATES
Overview scans of smear slides at low power can provide estimates of grain size distribution. It is important to calibrate your eye to both the micrometer scale (usually a table posted at each microscope) and a sense of percentage diagrams. Note textural characteristics: well-sorted, poorly-sorted, bimodal, fragmented, whole, low matrix, etc.
PERCENTAGE ESTIMATES
Because these are particle slides, much of the space may be empty, and must be adjusted for (i.e. subtracted out) in the estimates. Ideally component percentages are given in terms of mass percents, rather than percent of slide covered (e.g. point-counts), although this requires an extra mental step. Multiplication of the area or volume estimate by the component's approximate specific gravity (e.g. 1 for organics, 2.7 for quartz or calcite, 5 for pyrite) allows for cross-checking with analytical weight abundances from carbon coulometry, loss-on-ignition, biogenic silica measurement, etc., and iterative refinement of the investigator's eye.
GRAIN SHAPES and CRYSTAL FORM
Granular, equigranular, prismatic, micritic, rod-shaped, acicular, rice,
sprays, needles, spheroidal, anhedral, polyhedral, euhedral, platelets,
cubes, rhombs, vesicular, ragged, fibrous, aggregate, cruiciform, bipyramidal,
lenticular, framboid, twinned, perlitic, pinacoidal, pseudomorph, tabular,
vermicular, angular, rounded, subangular, conchoidal, pitted, coated, clear,
polished, frosted, shard, vitreous . . .
Optical Characteristics for Microscope Determinations
CRYSTAL HABITS
If a mineral shows primary crystal faces (typically authigenic or diagenetic)
it may be possible to assign it to a characteristic crystal class:
cubic, orthorhombic (aragonite), trigonal (quartz, calcite), tetragonal
(rutile), or monoclinic (K-feldspars) and triclinic (plagioclase).
Primary faces are rare so that more commonly lines belong to cleavage or
twinning characteristics. The way a mineral breaks (cleavage) can
be very telling. Quartz for example is distinctive in that it has
no preferred cleavage or twinning orientation, whereas feldspars that look
similar to quartz almost always do.
TRANSPARENCY
Most microscopic minerals are colorless, but some have characteristic
color or are opaque due to carbon or Fe/Mn, e.g. pyrite. Iron oxides
glow faint red. Altered and weathered minerals (feldspars) and glasses
commonly are fuzzy, and brown or yellow or clear. Some heavy minerals,
pyroxenes, amphiboles, apatite, zircon, are characteristic greenish, bluish,
or yellow.
REFRACTIVE INDEX ( RI=1.4 to 1.9)
Note for many minerals the RI changes with crystallographic orientation.
Quartz and the mounting medium NORLAND provide standards of comparison
(usually 1.544 and 1.555). Minerals are easily classified as above
or below the medium using the BECKE Line. The rule is: From
the position of grain focus, as the tube is raised, the Becke Line migrates
into the higher RI component. For very fine particles, you can slide
in the quartz wedge until near the particle. Imagine the sun is shining
from the opposite side. If the particle has RI lower than the media,
it will appear as a pit, with the shadowed side toward the "sun."
RELIEF
Prominence of the mineral depends on the relative contrast of the RI
with the media. Small quartz grains for example are hard to even
see unless under crossed-nicols there are flakes of low bf.
BIRIFINGENCE
Rays traveling through crystals will create color because of splitting
as a function of crystal directions and refractive index differences.
The color charts are calibrated for 30 micron thin-sections, but still
function as relative guides. Typically carbonates stand out with
very high bf. Feldspars are lower than quartz, also low. Micas
generally show high dispersion, but in fact will appear very low to isotropic
because of their crystal habit to lie with 001 vertical. Heavy minerals
may have high bf and strong color, usually associated with iron.
Silica (diatoms) and glasses are of course isotropic.
EXTINCTION
Crystal structure will determine if minerals show characteristic extinction
under crossed-nicols polarization and usually at a specific angle or parallel
with the long axis. Isotropic minerals will remain dark. If
grains are anhedral or highly rounded, the extinction angles will simply
occur at 90° and not tell much except that the mineral is not isotropic.
PLEOCHROISM and DISPERSION
Some minerals such as micas or amphiboles have different color hues
depending on crystal angle under polarization. Dispersion is a property
of high bf and RI such that the mineral does not go extinct under crossed-nicols.
Typical for example of hematite.
ASSOCIATED MINERALS and INCLUSIONS
Many igneous and metamorphic minerals will have small characteristic
inclusions or accessory minerals (e.g. zircon in micas). In lakes,
these would generally indicate clastics except in the cases of evaporite
salts.
Components
| Dominant | 40-100% |
| Abundant | < 40% |
| Common | < 25% |
| Rare | < 5% |
| Trace | < 1% |
AUTHIGENIC COMPONENTS
Carbonate Minerals
Calcite
Calcite is the most common authigenic precipitate in low concentration
lakes. Water is commonly driven to calcite supersaturation by the
productivity of phytoplankton or picoplankton (Kelts and Hsu, 1978).
Marine calcite is predominantly biogenic, whereas lake chalk (calcite)
is almost entirely dominated by inorganic precipitation of mineral phases,
although bacterially- or plankton-mediated.
Authigenic calcite from pelagic rain may be quite uniform blocky polyhedra in a narrow size range (typically 8-30µm), or else a fine uniform micrite (< 5µm).
Authigenic calcite is anhedral, micritic aggregates, to subhedral-blocky polyhedral. With increasing magnesium, more prismatic forms appear. Variable, mod. high RI=1.486-1.658, lower than dolomite, weak twinkling, extreme bf 0.172, pastel shades of yellow to green, trigonal, perfect cleavage but rare in authigenic crystals, extinction parallel elongation and cleavage.
Mg-Calcite
Aragonite
Aragonite in lakes almost always a surface water precipitate.
Typically occurs as acicular neddles to rice-like, thin prisms, commonly
about 10µm. Transparent, colorless, mod RI=1.530-1.686, like
calcite directional, extreme bf in high order pastel yellow, gray, white,
orthorhombic. Aragonite generally represents a lake of higher concentration
and Mg/Ca molar ratios >5-12.
Biogenic shell forms - more prismatic, some curved, more commonly sand-sized. Often show growth striations. Lakes do not have the Pteropod aragonite oozes common in marine marginal basins, nor the coccolith-foram chalks of the pelagic realm.
Dolomite
Siderite
Manganese carbonates
Pisoliths, onchoids, oncholiths
Sulfide Minerals: pyrite, greigite, mackinawite, pyrrhotite
Reducing freshwater lake sediments commonly convert low sulfate concentrations
into a series of unstable monosulfides that are amorphous to opaque <
1µm. These will oxidize quickly upon exposure to air and thus
rarely occur in smear slides except as coatings. Greigite, Fe2S3,
the sulfide spinel, is magnetic, and will form as microconcretions.
Pyrite is relatively uncommon in freshwater sediments, but can be very
common with greater salinities, commonly occuring as round framboid aggregates,
or in cases as single small opaque cubes. These may show up better
under strong reflected illumination, eg with a fiber optic lamp.
Fe/Mn Nodules
Opaques form a continuum of Fe-Mn oxides and hydroxides from flakes,
micronodules, coatings, crusts, to concretions. Many micronodules
show tiny spheroid aggregates of prisms, and appear suggestive of bacterial
mediation. Sizes range greatly, but fine silt is common (4-10µm).
They are generally thought to form in a suboxic redox gradient where Fe
and Mn are mobile in reducing zones, then precipitate when hitting oxic
conditions. Commonly this is the sediment/water interface.
Limonite
Hydrated iron oxides commonly coat grains, or occur as irregular, microlitic
to amorphous flakes, or as small translucent to opaque yellow-brown grains.
Magnetite
Some very small (1-5µm) magnetite grains and chains occur as
euhedral octahedra, opaque, grown by magnetotactic bacteria in the sediment.
Vivianite
Authigenic, blue color, stellate habit. Crystals prismatic, often
in stellate groups, or renniform and globular.
Clay Minerals
Authigenic fibrous, magnesian clay minerals of sepiolite and palygorskite
may form in alkaline saline lakes.
Zeolites
Alkaline lakes may alter volcanic glass quickly to zeolites, or even
authigenic albite and K-feldspars.
Zeoloites, clinoptilolite and phillipsite, are prismatic euhedral to
ragged, usually transparent and colorless, with low RI=1.480-1.486, and
v. weak bf anisotropic, monoclinic,
Phillipsite usually better formed stubby prisms, some cruciform twinned.
Clinoptilolite favored by higher Si/Al.
Phosphates
Fish debris, coatings, vivianite (FePO4.10 H2O)
Silica
Opal C-T, chert
BIOGENIC COMPONENTS
Foraminifera
Lakes do not have forams, although some species may survive if introduced
into coastal lakes or paralic basins with near sea water salinities, for
example the late Pleistocene Carpenteria shelf. Similarly pteropod
aragonitic gastropods and calcite discoasters are unknown in lakes.
Coccoliths
Coccoliths do not occur in non-marine basins although distant cousins
in the green algae family may generate a calcareous test of calcite platelets
(eg. Phacotus). These are very rare in sediments.
Ostracods
Ostracods are the common calcite microfossil of lakes. They are
small (30-2000µm) ovate to kidney shaped, bivalve crustaceans, commonly
with ornamentation. Most are benthic. An animal may molt up
to 8 times, forming a shell within a few hours to days. Male and
female valves can be distinguished, as well as juvenile instars and adult
specimens. Ostracods are powerful environmental indicators with species
very sensitive to water chemistry, the shells containing trace metals,
Mg, Sr, Ba, etc, in ratios to the water chemistry, and the calcite to Mg-calcite
shells archiving stable isotopic compositions. Species level identifications
are difficult, but genus groups can be recognized:
Candona
Limnocytherids
In smear slides ostracod shells are commonly broken, and lie flat. The punctate surface is visible along with solid rims. The calcite appears surprisingly low birefringent because C-axes tend to be perpendicular to shell walls, which are also very thin (< 5µm). In fact, the fragments resemble cigarette ash flakes.
Molluscs, Gastropods
Charophytes
Stromatolites
Siliceous Microfossils
Radiolaria (siliceous zooplankton) and silicoflagellates (siliceous algae) are unknown in lakes.
Diatoms, Bacillariophyceae (Phylum Chrysophyta), siliceous algae, however, are very common as both benthic and pelagic forms. Lake diatoms tend to be more delicate than their marine cousins. The opaline test or frustule is secreted as two halves of a pill-box. They have common punctae in highly diverse forms, in ridges aligned. Most are 10-100µm, with a great variety of shapes.
Two Major Groups:
Centric diatoms (Centrales): spherical, circular, oblong, triangular,
or cylindrical with radial or concentric symmetry.
Pennate diatoms (Pennales): spindle, rod, wedge-shaped, bilateral
symmetry about a medial line.
Most planktic are centric, most benthic are pennate.
Sponge Spicules
Sponges occur in many lakes, not in abundance. Sponge spicules
are exclusively silica in lakes as minute skeletal elements to stiffen
sponge tissue. They are always a minor component, but stand out due
to their shape and relief. Spicules occur as needle-like or stellate
forms, often with a central canule. Names derive from number of rays:
monaxon, tetraxon, etc. They may range up to a few mm size.
Fish Debris
Fish contribute mainly phosphatic debris of bones, teeth (ichthyoliths),
vertebrae, and scales to lake sediment, although rare calcitic fish otoliths
are important sources of isotopic information. Generally they are
in low concentration, but in rare cases (e.g. L. Edward, E. Africa) may
form 5% of the sediment. Fish debris may provide important archives
of strontium isotopic information.
Pollen and Spores
Pollen counts are usually done on samples concentrated by destruction
of carbonate and silicate fractions. In normal smear slides from
lake sediment pollen and spore grains may be common enough to describe.
Spores may be trilete or monolete, and pollen grains saccate, subspheroidal,
oblate, pentagonal, rod-shaped, vesiculate, or irregular, etc.
CLASTIC COMPONENTS
Quartz
Quartz is almost always clastic in lake deposits unless in the form
of chalcedony in some cherts from alkaline lakes. Often the dominant
component of sand sizes. Aeolian grains may be frosted or coated
with hematite. Distinguished from similar-looking feldspars by lack
of cleavage, lack of twinning, conchoidal fracture.
Well-rounded to angular, conchoidal fracture, no cleavage, very rare euhedral. Inclusions or clear, wavy extinction from metamorphism, transparent. Low relief, n (1.533-1.544), weak bf (0.009), first order gray and white, trigonal , // extinct.
Feldspars
Ubiquitous clastic mineral, less common than quartz. Feldspars
are commonly monoclinic K-fels (orthoclase, microcline, sanidine) or triclinic
plagioclase varieties (Na-albite, oligoclase, andesine, labradorite, Ca-bytownite).
Grains are commonly subhedral, tabular to rounded, with characteristic good double clevage and twinning. Colorless to cloudy due to alterations, except sanidine. Variable RI = 1.51-1.54 K-fels, 1.53-1.59 plag; weak bf 0.005-0.011, extinction angle diagnostic of composition, depends on axis. Albite twin can determine composition with M.-Levy method.
Carbonate Minerals
Ancient limestones may be calcitic or dolomitic and deliver a variety
of carbonate component types as clastic components. These are generally
very irregular anhedral to subhedral grains of highly varied sorting, often
rounded or flaky chips. It may be difficult to recognize the difference
in a mixture with authigenic calcite.
Dolomite
Dolomite is a common clastic mineral in lakes in the silt to sand size
if the terrain has the right geology of dolomitic limestones. Authigenic
dolomite may occur around salt lakes but is almost always micritic, <
5 µm.
Clastic grains may show well-shaped cleavage rhombs. RI changes strongly with direction: 1.500-1.526 (below quartz) and 1.680-1.710 ( distinctly >quartz). Higher RI than calcite, twinkling common, extreme bf 0.180-0.190, high order yellow to white, trigonal, perfect rhombic cleavage, extinction symmetric to cleavage.
Micas
Micas are sheet silicates, almost always detrital from igneous and
metamorphics and may comprise a mixture of chlorite (greenish), biotite
(yellow to greenish), or muscovite (clear). Usually occur as ragged
flakes, larger than associated grains, commonly with some crystal edges.
Very fine muscovite and the clay mineral illite are nearly identical.
May be confused with fish debris.
o Low to moderate relief, varies, RI 1.55 (mus)-1.638 (bio),
perfect basal cleavage 001
o Monoclinic, some alteration possible, pleochroic, extinction // 001
Iron Oxides
Non-opaque iron oxides are also common as clastics, such as red hematite
or yellow hydrated goethite. Occur as irregular, translucent, flakes,
scales, and aggregates, commonly < 4µm, extremely high relief
(2.95), high bf, and dispersive. Ilmenite and detrital magnetite
are opaque, sometime shimmering bluish, usually blocky rounded.
Heavy Minerals
Heavy Minerals are commonly a ubiquitous accessory in clastics larger
than 30µm, and may be concentrated for study with heavy liquid separations.
Usually they are very rare or trace minerals of varied mineralogy and commonly
volcanic or plutonic in origin. Most are high relief, high density
minerals. Some of the most common are:
Amphiboles: green, prismatic, extinction * //, ragged cleavage
Apatite: euhedral, prismatic, egg, transparent, RI 1.630-1.655, weak bf 0.003, low order gray, hexagonal, // extinct
Epidote: prismatic subhedra, RI 1.720-1.763, mod. strong bf 0.014-0.045, monoclinic, perfect cleavage, pleochroic, // extinct, yellowish hue
Monazite: short prism, yellow transparent, v. high RI= 1.78-1.849, strong bf, dispersive, monoclinic, perfect cleavage, small extinction angle
Rutile: prismatic needles, transparent yellow to reddish, v. high RI= 2.603-2.903, extreme bf 0.286, tetragonal
Sphene: small diamond subhedra, colorless to brown-yellow, v. high RI= 1.887-2.054, v. strong bf 0.092-0.141, total reflection, strong dispersion
Zircon: equant prisms, transparent, to faint yellow, v. high RI=1.925, strong bf, tetragonal, inclusions, // extinct
Pyroxenes: chain structure ferro-magnesian silicate family, transparent pale to yellow to green, mod high RI 1.650-1.731, mod. bf, etching common, monoclinic and ortho, cleavage//, straight extinction ortho, 6°-50° clino-, similar to amphiboles in smear slides.
Opaques (including pyrite, magnetite, ilmenite, ores, sphalerite, galena, gold, platinum, graphite)
Clay Minerals
Most clay minerals in lakes are clastic-detrital, derived from the
weathering of catchment as a function of geology and climate. Cold
temperate crystalline terrains deliver more illite and chlorite, more weathering
adds smectites and mixed layer clays, whereas kaolinite is more common
in well-drained warmer latitudes.
Most are < 4µm, requiring high power objectives of 60-100x; low bf flakes. The refractive indices with the Becke line can distinguish chlorite and illite ( >1.54) from low RI smectite. Clay aggregates may show a dispersion or even polarization at first order color.
Evaporites
Gypsum and anhydrite may be encountered in lake sediments either as
clastic or authigenic components. In smear slides they are more commonly
reworked or redeposited so are treated here.
Crystals are euhedral to subhedral tabular of simple habit and platy
cleavage fragments. Irregular anhedra are less common. Cleavage
will produce rhombic fragments, that may show etching along edges.
Grains are colorless, transparent, although commonly clouded with occluded
clay or brine. Low RI= 1.520-1.529, weak bf 0.009 low order, similar
to quartz, but usually darker because thinner. Monoclinic, three
cleavages, extinction // to 010 cleavage, twins common swallowtail, polysynthetic
In hypersaline lakes, gypsum may precipate in the surface water and usually settles as thin prismatic mono-crystals, clear, colorless, and with bipyramids.
A serious problem is the gypsum, often fibrous, that grows during cold-storage of cores from salt lakes.
Anhydrite grains in lakes are commonly clastic, and distinguished
from gypsum mainly by a higher RI because of the less water in the crystal
lattice. 1.570-1.614, and higher bf 0.044 with bright color.
ORGANIC MATTER
Terrestrial Plant Fragments, Pollen, Peat
Plant material in smear slides may include pollen and fragments of
brown lignified wood fracments, translucent plant fibers, fibrous rootlet
fragments, and black carbonized debris or charcoal. Size may be very
variable within one slide. Darker fragments will be more likely reworked.
Plant debris may be mixed up with opaques, but commonly is more irregular,
or may even reveal cellularity.
Charcoal, Coal
Charcoal fragments may derive from grass fires, with distinctive cellular
shaped cuticles, or forest fires which supply either fine background soot,
or larger particles in the proximal regions. Large charcoal particles
may be indistinguishable from coal fragments weathered from bedrock geology
and transported as clastics into the lake. In reflected light, however,
coal fragments will usually shine more than charcoal grains, reflecting
their carbon maturity.
Aquatic Plant Macrofossils, Amorphous Aquatic Organic Matter, Algae,
Bacteria
Amorphous organic matter in smear slides us usually counted as an algal
or bacterial component. Under a fluorescence microscope organic matter
may reveal characteristic structure in the amorphous fraction that corresponds
to some types of green, lipid-rich algae such as Botryococcus.
SPECIAL COMPONENTS
Volcanic Glass
Lakes may be subject to air-fall tephras such as the Mazama Ash (6800
BP 14Cyrs) that function as important marker beds. They
may receive weathered and altered volcanic grains as a component of the
clastic mineral spectrum. Clastic grains will generally be more rounded
and altered, often with coatings. Hyaloclastites are also common
if eruptions are subaqueous.
Tephra
Glass shard ashes in lakes tend to be from more acidic explosive volcanism,
andesites-rhyolites-alkali. Basalt volcanoes however may easily
sputter significant amounts of glass into a lake system.
Shards are recognized as lunate, bubble-walled, or fragments of highly-vesicular
to fibrous glass. Typical air fall ashes may be well sorted in coarse
silt ranges, with some rare scattered feldspars or even pyroxenes.
Glasses are isotropic, clear, and have low RI = 1.46-1.62. RI is a direct measure of composition, becoming more basaltic at higher values. Basaltic glasses will also tend to be darker, more bubbly and less vesicular. RI=1.50 is 78% silica. Fresh glass is isotropic, but alteration may instill interference effects.
Alteration is common. Volcanic glass is inherently unstable and begins soon to devitrify, quicker for lower silica values. Hydration leads to cloudiness, and microcrystals may form. A version of yellowish-cloudy hydrated glass with pristine grain shape is called palagonite.
Extraterrestrial
High sedimentation rates in lakes make it very difficult to expect
to find concentrations of extraterrestrial components, microtektites, or
cosmic spherules. Nevertheless they are possible as spheroid to oval
glassy grains, commonly with pitting or fractures.