JOURNAL MINERALOGICAL SOCIETY OF AMERICA 125

DISPERSION OF MINERALS

A. N. Wrncr.arl-, Uniaersily o! Wisconsin.

The new methods of studying powdered minerals lead naturally,not only to a knowledge of their indices of refraction in ordinarylight, but also and simultaneously to a knowledge of the variationin their indices with variation in the wave-length (or color) oflight; that is, the new methods lead to a knowledge of the dis-persion of minerals. Of course, the dispersion of a mineral is asdefinite a physical property as its refringence or birefringence and,therefore, may be used to identify it. At present, data regardingthe dispersion of minerals are remarkably scanty and in somecases not very accurate. All available data on dispersion (ofreasonable accuracy) are assembled in the following .table. Inorder to make these data useful for determinative purposes, theminerals are classified first in groups of increasing dispersion, andthen arranged in each group in the order of increasing refringence.

The dispersion given is always the difierence between the indexin l ight of 4861 A wave-length (:h" Frauenhofer l ine F or theB line of hydrogen) and the index in light of 6563 A wave-length(:the Frauenhofer l ine C or the a l ine of hydrogen). In the tablesthis dispersion is meant by the expression F-C.

fn anisotropic crystals the dispersion of the substance is, ingeneral, not the same for light vibrating in directions which arecrystallographically unlike. Therefore the dispersion for I[o isnot the same as for No, and the dispersion for ly'- is not equal tothat for ly'n nor to that for /y'o. However, these differences areusually small, and the only dispersion given in the table is thatfor N of isotropic substances, ly'o of uniaxial substances and -ly'^o{ biaxial substances (with rare exceptions, in cases where data arenot available for N-).

In the literature the dispersion is rarely given directly and theindices measured for different wave-lengths do not always includethe wave-lengths of the F and C lines, which are the standardsused in computing the dispersion. Therefore, it is necessary toobtain this dispersion by graphic (or mathematical) solution inmany cases; when this involves considerable extrapolation thevalue of the dispersion is followed by the * sign. In order topermit checking bf this extrapolation and also to permit thepreparation of dispersion curves for all minerals, the original data

126 TE E AM ERICAN MIN ERALOGIST

are given for yellow (the sodium D line unless otherwise noted),blue (or green-the exact wave-length given whenever known)and red (the wave-length given, if known). ff a chemical analysisof the mineral sample whose dispersion was measured is availatrle,the name of the mineral is followed by an asterisk. Finally, inorder to permit verification of the data, the name of the observerand the place of publication are given in all cases. If a laterpublication of the data is probably more accessible than the first,that is commonly given preference.

The dispersion of minerals increases, in general, with theirrefringence; nevertheless, minerals of the same refringence do notnecessarily have the same dispersion. An interesting and possiblyimportant illustration of this fact is furnished by the data regard-ing sillimanite and mullite. These two minerals are so much alikethat Eitell and Rinne2 consider that they are not essentiallydifferent. They are both orthorhombic and have crystal formswhich are very similar. The common prism faces are nearly atright angles to each other and both minerals have perfect cleavageparallel to the brachypinacoid. In fact, Wyckofis was at firstunable to distinguish between them by means of X-ray patterns,though he later founda "very slight but probably real difierences."In optical properties, also, they are almost identical. Accordingto Bowen, Greig and Zies,5 they both have the optic plane parallelwith the perfect cleavage and in both the slow ray vibrates parallelwith the vertical axis, giving positive elongation. They have thesame optic sign, nearly the same refringence, nearly the samebirefringence and nearly the same optic angle; in fact, whenmullite contains four to five percent of (TiOz*FerOr), it is in-distinguishable from sillimanite even by means of accuratemeasurements of these optic properties. According to Bowen,Grieg and Zies, such mullite can be distinguished from sillimaniteby chemical analysis or by the fact that it is colored and pleochroicwhile sillimanite is colorless. Since difierences between mulliteand sillimanite are so difficult to find, it seems important to notethat the dispersion of mullite (.026) is more than double that of

L Zei.t. Kryst., LXIV, 1927, p. 535.2 Zei,t. Kryst., LXI, 1925, p. 113.3 f our. Am. Cer. Soc.,Yll,1924, p.253.a Ann. Jour. Sci., CCXI, 1926, p.459.6 JouruWash. Acad. Sci., XlV, 1924, p. 183.

JOURNAL MINERALOGICAL SOCIETY OF AMERICA 127

sillimanite (.01 to .012) according to data now available. Thisdifierence, like the color, is doubtless due in part to the smalltenor of (TiOrf FerOa) in the mullite. It seems improbable to thewriter that it is due wholly or even chiefly to the titanic acid andferric iron, and, if mullite containing none of these substances hasa dispersion notably higher than that of sillimanite, it would bedificult to explain that condition on the basis of submicroscopicinclusions of corundum in sillimanite, since the dispersion of corun-dum is no greater than that of sillimanite. Of course, it is alsodiff icult to understand why admixed corundum (with N:1.76)should lower the indices of sillimanite from about 1,67 to about1 .65 .

It is clear that minerals differ in their dispersion, and in somecases this difference will be very useful in distinguishing betweenthem. The scanty data now available are summarized in thefollowing table. Such data will doubtless become much moreabundant as the new dispersion methods of measuring indicesof refraction come into wide usage.

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