thin sections tutorial

7/25/2019 Thin Sections Tutorial

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THIN SECTIONS TUTORIAL

The study of rock minerals and textures is called petrography. Thin section

petrography is the study of microscopic features using a "polarizing" or"petrographic" microscope. Following are two dierent images of apetrographic microscope with parts labeled. There are also web sites thatgo into this in more detail if you would like to learn more.

What are thinsections? Thinsections are madefrom small slabs of arock sample glued to aglass slide ! inch by# inches$% and thenground to a speci&edthickness of '.'(mm(' microns$. )t this

thickness mostminerals becomemore or lesstransparent and canthen be studied by amicroscope usingtransmitted light. Thin


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sections are timeconsuming and costlyto prepare.

Thin sections are *iewed using a petrographic microscope under twodierent lighting conditions+plain polarized light andcrossed polarizers.

,lane polarized light is lightthat is constrained to a singleplane. The light wa*e is asimple sine wa*e that has the*ibration direction lying in theplane of polarization.

When *iewing under planepolarized light% a singlepolarizer lower polar$ is used.-nserting the upper polarizer is

referred to as crossed polarizers or% crossed nicols$% the name gi*enbecause the two polarizing lenses are set at right angles to each other.

inerals can be classi&ed as anisotropic or as isotropic% dependingon their light properties. -sotropic minerals show the same *elocityof light in all directions% while anisotropic minerals show the*elocity of light *arying in dierent orientations.

For a good look at what someone sees with a polarizing microscope *isitthis site for their *irtual tour

/ock+forming materials that are isotropic include glass and minerals suchas Fe+Ti oxides magnetite and ilmenite$. Volcanic glasswill appear assome shade of color tan to brown$ when *iewed under plane polarizedlight% but will be black when *iewed under crossed polarizers. Fe-Tioxidesare black under either condition.

)nisotropic minerals show dierent colors when *iewed under crossedpolarizers. The colors seen are called interference colors% which can beused to help identify the minerals. Quartappears as white when *iewedunder plane polarized light% and will be white or some shade of gray when*iewed under crossed polarizers. 0nfortunately% !lagioclaseis also whitewhen *iewed under plane polarized light% and also has similar interference


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colors. Clino!"roxenewill be tan to green when *iewed under planepolarized light% and will be a more *ibrant color such as blue% pink% or greenwhen *iewed under crossed polarizers.

For more information on a polarizing microscope and descriptions of shapeand clea*age% interference colours% relief% extinction angle% color andpleochrosim% twinning% opacity% and *ibration directions *isit this site. Theyha*e excellent descriptions of what we are looking for in a thin sectionsusing the polarizing microscope1

The following photos are thin sections of minerals and rock textures youmay encounter in the study of 2assia ountains ash+3ow tus. Thinsections of minerals and *olcanic microtextures are from the -40 5eology6epartment% as well as from se*eral -nternet sources% with many from the5eology 6epartment of the 0ni*ersity of 7. 2arolina.

#inerals

Augite+ 7ote the !igeonitetwinlamellae thin parallel linear$ in thisgrain. ,igeonite is a 2a+poor clino!"roxene. The *iew isunder crossed polarizers. The sample is

from 5eorgia.$

)nother exampleof clino!"roxene. This one is in

basalt. The *iew is in planepolarized light.


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This is the same *iewof clino!"roxenein basalt% butunder crossed polarizers.

$lagioclase+ this slide showcasesone of plagioclase8s *ery commonfeatures1 its polysynthetic twinning.

9iew is under crossed polarizers.

Quart+ this slide shows:uartz in a range of crystal

orientations% all showingtypical grayish interferencecolors. 9iew is under crossedpolarizers.


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Volcanic %icrotextures

R&"olite+ This crystal+rich rhyolitecontains phenocrysts of :uartz% ;+feldspar sanidine$% plagioclase% andbiotite in a &ne+grained groundmass.7ote the interesting shape of thepartially resorbed?skeletal?$ 'uartz grain in the centerof the photograph. (iotite brown$ isin the upper leftcorner% !lagioclaseis in the lowerleft. 9iew is under crossed polarizers.

Scoriais another name for ahighly *esicular almost "frothy"$basalt. The )lac*+ o,al eaturesinthis photomicrograph are ,esicles.7ote the acicular+ .&ite!lagioclaselaths throughout andthe white oli*ine crystal at the lowerright. 9iew is under crossedpolarizers. 4ample is from


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some polymorph of 4i=#% and in thiscross+polarized shot% appear asround ob>ects with dark crosses.7ote the large crystal% which formsthe nucleus of one of the spherulitesat center+left. 9iew is under crossedpolarizers. 4ample is from Wyoming.

-n this tu% the irregularl" s&a!edglass s&ardsare still relati*elyundeformed. )lso note the phenocrystsof 'uartclear$ and )iotitedarkred$ in this slide. 9iew is under planepolarized light. 4ample is from 7e*ada.

,oorly welded r&"olitic tu/+ inthis sample% the glass shards arestarting to get deformed. 7ote thephenocrysts of 'uartclear$and )iotitedark red$ in this rock.9iew is under plane polarizedlight. 4ample is from 7e*ada.


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Welded rhyolitic tu+ in this sample%the glass s&ardsare fused togetherin a swirly mass% and the large

!u%ice rag%entat center right is3attened. -ncontrast% 'uartphenocrysts arerelati*ely undeformed. 9iew is underplane polarized light. 4ample is from7e*ada.

This photo is of a welded tu withnumerous glass s&ardsandbroken !lagioclasegrains. The *iewis under plane polarized light.

This photo is showing extinctglass %atrixelongated blackshape$% lit&ic clastlower left$% andbroken !lagioclasegrains. The*iew is under crossed polarizers.

=bsidian *itrophyre + ) ,itro!&"reisanother name for a phenocryst+bearing obsidian. The phenocrysts in

the photomicrograph aremostly !lagioclase. The groundmassis ,olcanic glass. The *iew is undercrossed polarizers. 4ample is fromontana.$


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9olcanic glass that has completelyde*itri&ed will often ha*ea elt"texture% as shown in thisphotograph. The minerals that youcan see are plagioclase white$%clinopyroxene shades of green$% andFe+Ti oxides black$. 9iew is underplane polarized light.

9olcanic glass

showing !erlitictexture concentriccur*ed fractures$. ) bleb of glasswith clinopyroxene and plagioclase islocated below and to the right of thelarge plagioclase phenocryst.

-

This slide shows a texturecalledgranophyric. 0rano!&"rictextureis an intergrowth of:uartz and alkali feldspar. -nthis case% the granophyrictexture radiates out from largeplagioclase grains lower left+gray% lower right+grayblack$.9iew is under crossed

polarizers.

thin sections tutorial

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