Iceland spar and its early applications in microscopes and

other optical equipment Leó Kristjánsson

Institute of Earth SciencesUniversity of Iceland

Plenum lecture given at the Scandem2009 conference on microscopy, Reykjavík June 2009 (with minor modifications)

Visitors to Iceland perhaps ask themselves...

...did this remote and sparsely populated island ever contribute anything significant to the outside world ?

Yes, at least in one case which in fact has much to do with microscopes, crystals, materials science, and optics

It is Iceland spar, “...der isländische Doppelspat, dieses unvergleichliche Mineral...der Grundstein unserer physikalischen Kenntniss der Krystalle...” (G. Tschermak, 1881)

Our story begins in 1668, when the King sent a stone-mason to East Iceland to recover crystals found at a site close to a local trading post

Utstekkur farm, Reydarfjördur. Photo: Mats Wibe Lund

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The crystals were studied by the scientist Rasmus Bartholin, who published a booklet

on his observations in 1669

The dot A under this transparent crystal is seen as two dots

For one image C, the light ray obeys Snell’s law of refraction

The “extraordinary” image B rotates with the crystal

Optics, crystals, etc., 1670-1710

• This phenomenon of double refraction was noted in some other minerals, but is difficult to observe in most of them

• A chapter in Chr. Huygens’ book Traité de la Lumière, 1690, dealt with this phenomenon. He showed that the properties of light changed on traversing Iceland spar

• Huygens assumed light to be a wave. A point source of light within a crystal like Iceland spar would give rise to two waves, one circular and the other ellipsoidal

• Isaac Newton, in contrast, expected light to be a stream of particles in his “Opticks” 1704-06.

Most scientists accepted Newton’s

view to 1800. Only a few papers on double refraction appeared in 1710-80, this one

in 1761

The subject was revived

by the “father of crystallography”, the abbot R.J. Haüy, in

1780-1805 after experiments with

Iceland spar.

Light was demonstrated to be a wave in Th. Young´s

slit experiments, 1802

Iceland spar and optics, early 19th century

• E.L. Malus discovered in 1808 that reflection from smooth non-metallic surfaces changed a light beam in much the same way as its passage through Iceland spar. He called it “polarization” of light

• D. Brewster found experimentally for many materials in 1811-15, that this polarization by reflection was complete at an angle of incidence i = arc tan(n)

• F. Arago saw in 1811 brilliant colors in sheets of mica viewed through Iceland spar.

• This “chromatic polarization” which was shown by Th. Young in 1814 to be due to

interference of polarized waves, became the foundation of petrographic

microscopy D. Brewster

Brewster realized that some crystals have one axis of symmetry for light propagation, others

have two such axes

The interference figures appearing in crystal plates in convergent polarized light have been valuable in the

identification of crystals

Uniaxial crystal Biaxial crystal

A. Fresnel’s experiments on

diffraction further confirmed the

wave hypothesis. In the 1820s he

then developed equations for the

reflection and refraction of light

waves at boundaries, as well as for

the propagation of light in crystalline materials

Fresnel realized that in order to

explain double refraction, light had

to be a transverse wave

His theories were tested several

times, with the aid of Iceland spar

A. Fresnel

What was so special about Iceland spar, which is only

a variety of the common mineral calcite (CaCO3)?• Calcite has unusually high birefringence

• The Iceland spar crystals were often:– Large and homogeneous– Very pure (ca. 99.95%) – Very transparent, and free from cracks, twinning

etc.– Easily cleaved, leaving very clean smooth surfaces

How did the crystals find their way to scientists?

- with local merchants and traders - on fishing vessels from Europe - with occasional travellers

Mining of the crystals for export began in 1850, and continued intermittently to 1925. The major effort was in 1863-72

The crystals were deposited by hydrothermal alteration of basalt lavas within a large (eroded) volcanic center

of about 11 M.y. age

The mine, now under protection, is ca. 30 m wide and 10 m deep

Small crystals can still be seen there

Around 1830, scientists realized that the polarization state of light provided a very sensitive means for probing the

nature of matterUsing an Iceland spar rhomb for such observations was sometimes inconvenient, due to image overlap

So in 1828, W. Nicol invented an Iceland spar prism where one image was discarded. Thousands of Nicol prisms have been made. Nicol prisms

arealso valuable infinding the direction of vibration inpolarized light.

In the 1830s, the first microscopes equipped with calcite rhombs or Nicol prisms were described, e.g.

by G.B. Amici

Apparatus with glass- mirror polarizers was also employed for studying crystal plates

Amici 1832/1844 Nörrenberg 1842

Spar

Microscopes for the study of crystals developed gradually from the 1830s The type on the right was in use around1860, for instance by A. Des Cloizeaux

Powell 1841

Nicol prism

In 1863 F. Zirkel combined H.C. Sorby’s technique of making thin sections of rocks, with a microscope

containing two Nicol prisms

These lithographs are from an 1876 paper by Zirkel

Biologists also began to appreciate by 1860, that polarized light brought out previously invisible structures in tissues

One example is E. Brücke’s microscope picture of striations in muscle fibers, 1858

A main reason for the use of polarized light in microscopy

is that differences in the birefringence of minerals appear as

different colors

This was the discovery made by

Arago in 1811

Gabbro. Top: ordinary light, bottom: crossed nicols

F. Zirkel

Polarizing microscopes, made commercially for the study of thin sections in parallel or

convergent light, appeared in the 1870’s

Advantages of using polarized light in microscopes for

mineralogical/petrographical studies, ca. 1880 -1900

• It offered a fast way of identifying and analysing minerals

• Many minerals previously thought to be rare, were in fact common

• Associations between minerals were useful in prospecting

• Alteration, zoning, twinning, strain effects etc. became evident

• With heated stages, phase transitions could be studied up to 1500°C

• The technique aided in synthesis of minerals and rocks

Specialized instruments for the study of individual

mineral grains were designed, as well as teaching equipment

Axial-angle apparatus, ca. 1880

Projector, 1875

In the mid-1880s, stocks of Iceland spar from 1863-72 were exhausted, and

extraction of optical-quality crystals at the East Iceland site also became

difficult

This caused a “spar famine” among many users for decades, and led to attempts at

finding other mines or substitutes

From around 1900, minor quantities of (mostly small) usable calcite crystals were

obtained from various other locations e.g. in Crimea (Russia), U.S.A. and Germany

Sites in South Africa became the chief

supplier after 1920

In spite of the “spar famine”, the development of petrographic microscopes

continued...Swift/Dick 1889

R. Fuessca. 1900

C. Reichert 1909

A. Nachet1900-10

Bausch & Lomb ca. 1910

E. Leitzca. 1905

.....and continued

Zeiss/Winkel 1918, universal stageLeitz 1919, with reflected-light and other options

Photo: D. Kile

What about the results ?

“...und nur der mikroskopischen Untersuchung dankt [die Gesteinskunde] die ungeheuren Fortschritte, welche sie im letzten Drittel des 19. Jahrhunderts zu verzeichnen hatte.” (E. Weinschenk, 1901)Other uses of polarizing microscopes to the 1920s included

- Industrial materials: Portland cement, slags, abrasives, paint fillers, refractory compounds for furnaces, glass, ceramics,....

- Micro-chemical analysis

- Liquid crystals (for which polarized light was essential), from 1888

- Biological materials: bones, fibres, muscles, shells, gallstones,...

- Ore minerals, using reflection of polarized light (after 1922)

Another important application of Nicol prisms from Iceland spar was in

polarimeters, which measure optical activity

It occurs in some crystals and in various organic liquids and solutions.

The direction of polarizationof light rotates on passage through these substances.

Polarimeters were availablecommercially before 1850.

A typical polarimeter with two Nicol prisms, 1874. An adjustable quartz wedge compensates for rotation caused by a fluid such as a sugar

solution in a long glass tube (not shown)

Polarimeters were mostly employed in organic chemistry

- A prime discovery was made by L. Pasteur in 1848: optical activity is due to a mirror-image asymmetry of molecules. This led later to the realization that the valency bonds of carbon are three-dimensional

- Structure of sugars and other carbohydrates, e.g. by E. Fischer

- Research on amino acids, plant oils, alkaloids, nucleic acids, etc.

- Reaction kinetics and equilibria, e.g. in hydrolysis and fermentation

Other applications of polarimeters included

- Sugar industries: growing, processing, sugar trade and taxation, uses,...

- Industries where sugar compounds were made from starch

- Production (from plants or synthetically) and quality control of

essential oils and other chemicals such as vanillin and camphor

- Physical chemistry, e.g. A. Werner’s work on coordination compounds

- Compounds with optical activity due to atoms other than carbon

- Medicine, primarily clinical and research uses connected with diabetes

One more class of instruments containing Iceland spar, was

photometers for measuring light intensity

With two Nicol prisms in tandem, you can attenuate a light beam by a known ratio

König-Martens spectrophotometer

Polarization photometers had many functions

- Physics: interactions of light with matter, e.g. reflection and fluorescence

- Chemistry/biochemistry: chemical analysis by absorption or emission

- Astronomy: measurement of the brightness of tens of thousands of stars

- Technology: gloss, dyes, photography, illumination, testing of lamps,....

- H. Wanner’s optical pyrometer, a modified polarization photometer,

was useful in much high-temperature research and technical applications

(furnaces) early in the 20th century.

Some areas of progress in physics and materials science before 1930, where Iceland spar was

used in experimental situations - Discovery of M. Faraday’s magneto-optical effect (1845)

- The electro- and magneto-optical effects of J. Kerr (mid-1870s)

- Tests of J.C. Maxwell’s electromagnetic theory; also tests for “aether drift” before and after appearance of A. Einstein’s theory of relativity (1905)

- Optical properties of metals and of thin films (ellipsometry)

- Participation in research on the Zeeman effect (1896), photoelectric effect, etc.

- Iceland spar was one of the first substances whose anisotropic behavior in elastic, magnetic, and thermal properties was investigated

...ctd.

Areas of progress........continued

- Photoelastic studies with polarized light, on e.g. machine- and building-parts subjected to stress

- Photoelastic tests for thermal stresses in glass, were especially important in the manufacture of lenses and prisms for microscopes

.....ctd.

Sophisticated polariscopefor testing whether lenseshave been properly annealed(H. Schulz, 1912)

Areas of progress........continued

- Photoelastic studies with polarized light, on e.g. machine- and building-parts subjected to stress

- Photoelastic tests for thermal stresses in glass, were especially important in the manufacture of lenses and prisms for microscopes

- Extensive research for many decades on the scattering of light by particles

- Iceland spar was employed in studies on ultraviolet radiation

- Chemists used Iceland spar as a reagent e.g. in atomic mass determinations

- Research on colors, color perception, and other aspects of vision

.....ctd.

Iceland spar crystals (from Iceland and elsewhere) were from 1915 used as a wavelength standard in X-ray spectroscopy, including many fundamental studies

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Areas of progress........continued

A.H. Compton’s X-ray spectrograph;C is a diffracting crystal from calcite

Concluding remarks

In this brief presentation, I hope to have shown how Iceland in the past contributed something to progress in science and technology.

The fortuitous existence in East Iceland of a single “hole in the ground, miles from nowhere”, as is sometimes said, advanced some fields of research in natural sciences, such as yours, by tens of years

Iceland spar, in fact, took part in many Nobel-prize discoveries, ofwhich those by Fischer, Werner and Zeeman have been mentioned

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I wish you a successful meeting and a pleasant stay in Iceland.