Atlasofrock-formingmineralsinthinsections.pdf

1、Atlas ofrock-forming mineralsin thin sectionsW.S.MacKenzie and C.GuilfordLongman Scientific&Technical1980This full-colour handbook illustrates the appearance of common rock-forming minerals as seen in thin section under the polarizing microscope.It is not intended to replace a mineralogy textbook-ra

2、ther it is designed to be used as a laboratory manual alongside the standad texts,by all students of earth sciences from sixth form to honours degree level.The book contains over 200 photomicrographs accompanied by short descriptions and summaries of the optical properties of the various minerals.Th

3、e photographs are taken in either plane-polarized light or under crossed polars,and are careefully chosen to show the features by wich the minerals can most easily recognized.W.S.MacKenzie,PhD,FRSE,FGS,is Professor of Petrology at the University of Manchester.C.Guilford,FGS,is the Superintendent of

4、the Department of Geology at the University of ManchesterCONTENTSPrefaceivMuscovite54IntroductionvBiotite55Birefringence chartviStilpnomelane57Olivine1Pyrophyllite58Monticellite3Talc59Chondrodite4Chlorite60Zircon6Serpentine62Sphene7Prehnite63Garnet8Microcline64Vesuvianite9Perthite&Microperthite65Sil

5、limanite10Sanidine66Mullite12Anorthoclase67Andalusite13Plagioclase68Andalusite&Sillimanite14Quartz70intergrowth15Myrmekite72Kyanite16Granophyric texture73Topaz17Tridymite74Staurolite18Cristobalite75Chloritoid19Nepheline76Sapphirine20Sanidine&Nepheline78Eudialyte21Leucite79Zoisite22Nosean80Epidote23C

6、ancrinite81Piemontite24Scapolite82Allanite25Analcite83Lwsonite26Corundum84Pumpellyite27Rutile85Melilite28Perovskite86Cordierite30Spinel87Tourmaline32Brucite88Axinite34Calcite89Orthopyroxene35Dolomite90Augite36Apatite92Titanaugite37Fluorite93Clinopyroxene&Orthopyroxene Deerite94intergrowth38Howeieite

7、95Aegirine-augite39Zussmanite96Jadeite40Yoderite97Wollastonite41Index98Pectolite42Anthophyllite-Gedrite43Cummingtonite-Grunerite44Tremolite-Ferroactinolite45Hornblende46Kaersutite48Glaucophane49Arfvedsonite50Aenigmatite51Astrophyllite52Lamprophyllite53INDEXActinolite45Muscovite54Aegirine-augite39Myr

8、mekite72Aenigmatite51Nepheline76-78Allanite25Nosean80Analcite83Olivine1-2Andalusite13-15Orthite25Anorthoclase67Orthopyroxene35Anthophyllite43Pectolite42Apatite92Perovskite86Arfvedsonite50Perthite65Astrophyllite52Piemontite24Augite36-37Plagioclase68-69Axinite34Prehnite63Biotite55-57Pumpellyite27Bruci

9、te88Pyrophyllite58Calcite89Quartz70-71Cancrinite81Rutile85Chlorite60-61Sanidine66-78Chloritoid19Sapphirine20Chondrodite4-5Scapolite82Cordierite30-31Serpentine62Corundum84Sillimanite10-11,15Cristobalite75Sphene7Cummingtonite44Spinel87Deerite94Staurolite18Dolomite90-91Stilpnomelane57Epidote23Talc59Eud

10、ialyte21Topaz17Fayalite1-2Tourmaline32-33Ferroactinolite45Tremolite45Fluorite93Tridymite74Forsterite1-2 Vesuvianite9Garnet8Wollastonite41Gedrite43Yoderite97Glaucophane49Zircon6Granophyric texture73Zoisite22Grunerite44Zussmanite96Hornblende46-47Howieite95Idocrase9Jadeite40Kaersutite48Kyanite16Lamprop

11、hyllite53Lawsonite26Leucite79Melilite28-29Microcline64Monticellite3Mullite12PREFACEThe purpose of this book is to illustrate the appearance of many of the common rock-forming minerals in thin section under the microscope.It is not our intention that it should be used as a substitute for a mineralogy

12、 textbook but rather as a laboratory handbook for use in practical classes together with one of the standard textbooks on mineralogy.The idea of producing a series of photographs of minerals in thin section came from two sources.The son of one of the authors,I.R.MacKenzie,then in his second year as

13、a student of geology,suggested that these would be a useful aid in recognizing minerals under the microscope.On questioning undergraduates in second-year Geology classes in Manchester University,why they preferred certain textbooks to others,the answer was invariably that they found those books whic

14、h contained illustrations accompanying the text particularly useful,especially when they could recognize under the microscope features which could be seen in the photographs.Some of the textbooks which,in our opinion,contain the best photomicrographs or drawings of minerals are rather old and are no

15、t readily available to the student of today.Rosenbuschs Mikroskopische Petrographie der Mineralien und Gesteine,published in 1905,has some excellent photomicrographs printed in black and white,while Tealls British Petrography,published in 1888,has beautiful drawings which appear to have been hand-co

16、loured before reproduction by printing.H.G.Smiths Minerals and the Microscope,first printed in 1914,has been found useful by generations of students of elementary mineralogy because of the high quality of the illustrations.It seemed to us that if we could reproduce faithfully,by colour photography,t

17、he appearance of minerals under the microscope both in plane-polarized light and under crossed polars,the usefulness of photomicrographs as a teaching aid would be increased enormously.The majority of the photographs were made from thin sections of rocks in the teaching collections of the Geology De

18、partment in Manchester University and we are grateful of our colleagues in Manchester for providing us with thin sections.ivINTRODUCTIONThe minerals represented here are arranged in the same order in which they appear in Deer,Howie and Zussmans Introduction to Rock Forming Minerals(relevant page num

19、bers given at the end of each entry in square brackets),except for a few minerals which are not described by these authors,viz.deerite,zussmanite,yoderite and lamprophyllite.The decision as to which minerals to include has been based mainly on two considerations,firstly,how frequently they occur and

20、 secondly whether a photograph can be a useful aid in identification.In the headings for each mineral we have listed the chemical formula(simplified in some cases),crystal system,optic sign,the values of the refractive index for biaxial minerals and the and ray refractive indices for uniaxial minera

21、ls together with the birefringence.These figures have been quoted from Deer,Howie and Zussmans book with their permision.The rock type and locality of the specimens are quoted,where these are known,and the magnifications used in taking the photographs are given.Each pohotograph is accompanied by a b

22、rief description of the field of view illustrated but,in general,only properties which can be seen in the photographs are discussed.Thus we have omitted reference to optic axial angle,sign of elongation and dispersion.In most cases at least two photographs have been made of each mineral,one in plane

23、-parallel light with the polarizer in two orthogonal positions.In the case of isotropic minerals we have tended to omit the view taken under crossed polars.With few exceptions the polarizer has been set parallel to the edges of the photograph but we have not made much use of this fact since discussi

24、on of extinction angles is omitted except in the case of the plagioclase feldspars,because this would necesitate reproducing a number of photographs taken under crossed polars.In order to show pleochroism,we have used rotation of the polarizer rather than rotation of the stage of the microscope for

25、two reasons.Firstly,this makes it easier to compare the photographs and observe the change in colour shown by any one crystal and secondly it has been done to encourage the use of this method for detecting weak pleochroism.Although we have adopted the procedure of retaining the thin section in the s

26、ame orientation for all three photographs,this has one disadvantage.If there are only a few crystal in the field of view,or the crystals have a strong preferred orientation in the rock section used,we have been unable to show the maximum change in absorption colour on rotation of the polarizer throu

27、gh 90 since the extreme absorption colours are shown by a crystal when its vibration directions are parallel to and perpendicular to the polarizer.In these positions the crystal would be at extinction when viewed under crossed polars and ideally we wish to show the characteristic interference colurs

28、 near to their maximum intensity.We have not specified in which of the two orthogoanal positions the polarizer is set in the photographs taken in plane-polarized light.As mentioned above we have quoted the numerical value of the birefringence for each mineral,but in the description of the photograph

29、 we have generally referred to the order of the interference colour.To enable the reader to translate birefringence to a particular colour we have included a photograph of a quartz wedge with a birefringence scale along its length.This should not be used as a Michel-Lvy chart since the thickness of

30、the section is not taken into account,it being assumed that the section is of standard thickness,viz.0.03 mm.Thus the mineral names are reproduced against the highest-order colour which they show in a thin section of standard thickness rather than opposite radial lines which show the variation in co

31、lour with thickness and birefringence of the mineral as in a Michel-Lvy chart.The faithful reproduction of the interference colours of minerals in thin section or in a quartz wedge as seen under crossed polars,depends to a large extent on the type of film used and also on the printing process.Some o

32、f the Michel-Lvy charts that have been published depart slightly from the true colours and one fault which is fairly common concerns the middle of the second-order colours where a broad band of bright green is sometimes shown between blue and yellow.Observation of a quartz wedge under crossed polars

33、 reveals that the second-order colour betweenblue and yellow is a rather pale green in contrast to the fairly deep green in the third order.Only in minerals which are colourless and have negligible dispersion,is it possible to distinguish these two greens and even only after considerable experience.

34、In some of the photographs of minerals of moderate birefringence the edge of the crystals can be seen to be wedge-shaped and thus the order of the interference colour can be determined fairly readily.Some of the common minerals which are usually considered difficult to identify(e.g.cordierite)are re

35、presented by more than one rock section if we considered that additional photographs would give a better idea of the variations in appearance which may be expected in different rocks or if it was impossible,in one field of view,to illustrate the diferent porperties which we wished to show.In a few c

36、ases the photographs taken in plane-polarized light show pale pink and green colours due to stray polarization produced in the photographic equipment:when such colours are present we have noted this in the description of the photograph.OLIVINEMg2SiO4-Fe2SiO4Symetry-Orthorhombic(+)(-)RI -1.651-1.869B

37、irefringence-0.035-0.052 The olivines form a complete solid solution between the magnesian end-member,forsterite,and the iron end-member fayalite.These photographs show two olivine phenocrysts in a fine-grained groundmass of plagioclase feldspar,pyroxene and iron ore.The upper photograph,taken in pl

38、ane-polarized(PP)light,shows the typical shape of olivine crystals;the irregular cracks and slight alteration along the cracks are characteristic of this mineral:there are signs of cleavage along the length of one of the crystals.In the lower photograph,taken under crossed polars(XP),one of the crys

39、tals is cut very nearly perpendicular to an optic axis and so shows a very low interference colour;it is an anomalous brown caused by dispersion of the optic axes.The other crystal shows a second-order blue on the rim whereas the main part of the crystal shows a slightly lower colour.The higher birr

40、efringence on the rim of the crystal is an indication of a higher iron content.The reverse effect,viz.lowering of the birefringence colour due to the wedge shape of the crystal boundary,can be seen on the bottom edge of one of the olivine crystals and also on a clinopyroxene phenocryst part of which

41、 just appears at the bottom of the field.1Specimen from ankaramite,Mauna Kea,Hawaii,43X1OLIVINEMg2SiO4-Fe2SiO4Symetry-Orthorhombic(+)(-)RI -1.651-1.869Birefringence-0.035-0.052 The olivines form a complete solid solution between the magnesian end-member,forsterite,and the iron end-member fayalite.Th

42、ese upper photograph,taken in PP light,shows olivine(brownish-green colour,occupying most of the field)intergrown with a calcic plagioclase.The high relief of the olivine against the feldspar is noticeable.A pale colour in olivine seen in PP light is common but it does not show pleochroism-the more

43、Fe-rich members of the series show a yellowish-brown colour.The cracks in the crystals are quite characteristic as is the slight alteration of the mineral along the cracks.In the lower photograph,taken under XP,the interference colours are mostly second order;the highest colour showing in this view

44、is the yellow in the small crystal just above the centre of the field-these colours indicate a Mg-rich olivine since birefringence colours well into the third order are only seen in olivines with high iron contents.1 Specimen from gabro picrite,Border Group,Skaergaard intrusion,East Greenland,23X2MO

45、NTICELLITECaMgSiO4 Symetry-Orthorhombic(-)RI -1.646-1.664Birefringence-0.012-0.020 In the upper photograph,taken in PP light,the dominant mineral is monticellite with subordinate calcite.In PP light the calcite can be recognized by its good cleavage and twin lamellae.The high relief of the monticell

46、ite against the mounting material can be seen at a small hole near the top edge of the slide.In the lower photograph,taken under XP,the interference colours are seen to be low first order;the highest colour seen here is the orange-yellow colour.It should be remembered that in rocks without quartz or

47、 feldspar present it is sometimes difficult to judge the correct thickness of a section and this section may be slightly thin.10 Specimen from monticellite-spinel-phlogopite rock.Barnavave,Carlingford,Eire.32X3CHONDRODITEMg(OH,F)2.2MgSiO4 Symetry-Monoclinic(+)RI -1.602-1.627Birefringence-0.028-0.034

48、 Although members of the humite group,of which chondrodite is one,are frequently yellowish in colour,in this case the chondrodite is nearly colourless in thin section.In the upper photograph,taken in PP light,the high relief is distinctive,as also is the calcite(brownish colour)and two crystals of m

49、uscovite.In the lower photograph,taken under XP,the muscovite-crystals show a bluish-yellow interference colour,while the calcite is grey or dark grey.Multiple twinning is shown in two of the crystals of chondrodite and this is a characteristic of the monoclinic members of the humite group.It may be

50、 that this section in slightly thin because the highest interference colour in this field is the first-order red shown in the crystal in the top right part of the field,and from the birefringence we should expect to see colours up to second-order red(see photographs on p.5).11 Specimen from marble,N