Optic Axial Angle Research Articles

Optical Crystallographic Properties of Organic Compounds. I. Barbituric Acid Derivatives

The following optical and crystallographic data for several barbituric acid derivatives have been determined: the principal indices of refraction, alpha, beta, and gamma; birefringence; the optic sign; type and extent of dispersion; elongation; habit; crystal system; extinction; and the relationship, of the ellipsoidal axes to the crystallographic axes, when possible. The size of the optic axial angle, 2 V , has been calculated from the indices of refraction. The optical orientation commonly observed is noted when constant. The optical crystallographic properties of the several barbituric acid derivatives are useful physical constants for purposes of identification.

生藥成分の結晶學的研究 第1報

Crystallographic studies were made of pure santonin crystals to prepare its crystal system, facial indices and stereograph. The index of refraction, optic axial angle, sign of elongation and dispersion of optic axes were determined. These figures were compared with those of crystals obtained by microsublimation from Artemisia cina and A. monogyna Waldst. et Kit. in order to determine the presence of santonin. The same measurements were made of artemisin and temisin which are often present in crude drugs and have similar chemical structures to santonin, in order to facilitate observation.

Mahadevite—A new species of mica

The physical properties and chemical composition of a species of mica from the Warangal district in Hyderabad have been studied. Thick sheets are bronze-coloured and opaque, while thin sheets transmit light of a greenish yellow tint, the pleochroism being weak but distinct. The optic axial angle is 13°48′. Th plane of the optic axes is perpendicular to the plane of symmetry. The composition is SiO2 38·98%, Al2O3 29·94%, Fe2O3 and FeO 4·12%, Mgo 13·2%, K2O 9·6% water 3·8%. On the basis of Pauling’s model of the mica structure, the numbers of atoms of various elements in six, and twelve co-ordination positions are calculated. Approximately equal number of Al and Mg atoms are found in six co-ordination. The mica appears to be of a type intermediate between muscovite and phlogopite.

Optical properties of olivines from Ubekendt Island, west Greenland

The olivines, which are the subject of this note, were obtained from a series of basaltic lavas and dikes on Ubekendt island, which lies off the west coast of Greenland in latitude 71 ° N., longitude 54 ° W.The optical properties of the olivines were investigated in order to determine any range in composition; such variations might have been expected in the very thick lava series. From this aspect, however, the results are disappointing, the composition range being small. However, it is thought that the results may have a certain interest in showing the relative accuracy obtainable in the measurement of refractive indices and optic axial angles. The usual determinative methods have been slightly varied. It is the purpose of this note to describe these variations and to compare the relative speed and scope of the method used with those obtained by other methods.

Lamellar Structure of Potash-Soda Felspars

IN an extremely interesting paper, Ito1 has recently described the results of an X-ray examination of a Korean moonstone which he assumes to be similar to a specimen (described by E. Spencer2) containing approximately equal amounts of potash felspar and soda felspar, showing a blue schiller, and resembling sanidine in its glass-clear appearance and in having an abnormally low optic axial angle. Ito concludes that this crystal is an intimate intergrowth of a (monoclinic) potash felspar having the sanidine structure, and a corresponding high-temperature monoclinic form of soda felspar.

The variable composition of cordierite in the Dartmoor granites

The recognition of pronounced varietalism in both the garnet and the basic mica of the Dartmoor granites prompted the authors to inquire whether the cordierite associated with these species was similarly variable in composition.The inquiry was directed in the first instance to possible variation in the optic axial angle 2V, which is determinable with accuracy on the unaltered parts of cordierite in thin rock sections. Determinations made on the sector-twinned cordierites in the garnetiferous granite of Sweltor a gave the following peculiar results, which Mr. Rama Rao verified repeatedly, using a Fedorov microscope in perfect adjustment.

A critical review of the data for a revision of the enstatite-hypersthene series

At the suggestion of Professor H. H. Read the optic axial angle and the refractive index y were measured in four hypersthenes from Aberdeenshire norites. As the values obtained suggested that the minerals were iron-rich, it was decided to have chemical analyses made. These and the optical data are set out in table I below. At the same time the literature has been searched for analyses and optical data of enstatites and hypersthenes, in order to supplement those used by A. N. Winchell in his diagram of variation of the properties of the series. The references given by Winchell have been examined, and it is found that some of the data is not reliable.

Letters to Editors

The following preliminary optical examinations have been made. Chrysene crystallised in monoclinic plates showing the forms (001), (110) and (100). There is a marked tendency to twin on the c face. (010) is the optic axial plane with making 10° to the c axis in the obtuse angle. The birefringence is very high and negative with a fairly large optic axial angle.

Crystal Structure of Vitamin B1 and of Adenine Hydrochloride

CRYSTALS of vitamin B1 (hydrochloride) from various sources have been examined. All have been found to be substantially identical in form and X-ray patterns. A very slight difference in optical properties may, however, point to the existence of a variable amount of impurity in solid solution. The angle between the c axis and the principal direction γ is 19° 40′ ± 20′, 19° 20′ ± 20′, 19° 20 ± 20 and 19° 0′ ± 20′ in four specimens prepared respectively by Windaus and Tschesche, Jansen and Donath, and Peters (two specimens separated at different pH). The crystals were monoclinic, tabular on b and slightly elongated along c, showing faces (010), (110), (Oil), and sometimes (1 KT) and (2 KT). The cell dimensions are: a = 12.62 A.; b = 20.53 A.; c = 6.96 A.; = 66° 5. The space group is probably Assuming four asymmetric molecules per cell and the measured density 1.43, the molecular weight is 358, in good agreement with 355 calculated from the formula of Windaus1, C12H18O2N4S, 2HCl. The optical properties are: optic axial plane b, is 19° 20 from c in acute angle; optic axial angle large, undeter mined, sign probably negative, birefringence high.

Report on the optical axial angle and relation of its plane to the plane of symmetry in mica

Report on the optical axial angle and relation of its plane to the plane of symmetry in mica

The determination of the piezo-electric moduli of ammonium seignette salt

In a former paper the author compared the elastic properties of the two Seignette salts. It was shown that these properties were changed considerably on replacing the atom of potassium by the ammonium group. The ammonium salt, when subjected to tensile and torsional forces, was less elastic than the potassium salt, so that its deformation values were considerably increased. Other known differences were referred to, including the sizes of the unit cells, the change in optic-axial angles, with change in composition of the salts in “solid solution,” and the dextro- and lævo-rotation of polarised light for the two dextro salts in the directions of the optic axes. The potassium salt has pronounced piezo-electric properties. When a prism is pressed in certain directions with a pressure of one dyne per square centimetre, an electric moment of the abnormally large value of 1000. 10 -8 E. S. U. per unit volume is developed. It was therefore decided to determine the piezo-electric moduli of the ammonium salt and see the effect of the chemical change in the molecule and the associated piezo-electric change.

On the determination of the optic axial angle and crystal-forms from observations by the Becke method in thin sections

Some years ago the writer read a paper on the determination of the optic axial angle of a crystal by the Becke method. This proposed method was to record the trace of the optic axial plane when horizontal, the position of one axis, and the extinction-direction. From these data and the established constants of the microscope, the position of the second optic axis may be found, as the extinction bisects the angle between the centre of the pole of the section and the two optic axes. Hence, the optic axial angle may be determined.Considerable errors arise in these determinations owing to defects in the optical system of the microscope, and also to the inability to determine the exact position when the trace of the optic axial plane is parallel to the E-W. lines in the microscope.

A Monticellite-Nepheline-Basalt from Tasmania: a correction to mineral data

F. P. Paul described in 1906 (T.M.P.M., vol. 25, p. 309) a nepheline-eudialite-basalt from Shannon Tier, Tasmania, containing an unknown mineral referred by him to Ca2SiO4. Subsequently Bowen (Amer. J. Sci., 1922, p. 30) pointed out the close correspondence between the optical properties of Paul's mineral and of the artificial βCa2SiO4 prepared by Day, Shepherd and Wright. Still later (Geol. Mag., 1927, p. 43) the writer of this note ventured to apply the name shannonite to the mineral of this singular occurrence. Since that time through the kindness of Mr. P. B. Nye, Government Geologist of Tasmania, a specimen of the rock described by Paul from Shannon Tier has been obtained and examined. That the specimen examined is identical with the rock described by Paul is clearly indicated by his detailed description and the photomicrograph which accompanies his paper. Unfortunately my own determinations are not in agreement with those of Paul, and the purpose of the present note isto state the nature of these discrepancies. The rock is a felspar-free nepheline-basalt of which the chief constituents are idiomorphic nepheline, a titaniferous augite, olivine (chrysolite), the unknown mineral of Paul, biotite, apatite, magnetite, perofskite, and minor amounts of interstitial analcime. The described eudialite has not been discovered, and from the description attached to the photomicrograph this mineral appears to have been confused with nepheline. I find further that the properties of the unknown mineral agree, not with those of any of the artificial calcium orthosilicates, but with those of monticellite. The two olivines are readily distinguished in thin section, the chrysolite occurring as chadacrysts and the monticellite as oikocrysts, the latter appearing as shapeless grains often surrounding chrysolite and enclosing numerous idiomorphic crystals of nepheline and magnetite, and thus belonging to a late stage in the crystallization history of the rock. The chrysolite moreover is readily distinguished—apart from these textural differences—by its higher birefringence and larger optic axial angle (near 90°).

Societies and Academies

LONDON. Mineralogical Society, June 15.—S. I. Tomkeieff: On some chloritic minerals associated with the basaltic Carboniferous rocks of Derbyshire. Certain leptochlorites occurring as vesicular infillings in the Carboniferous lavas (‘toadstones’) of the North: Derbyshire area are described. The chemical analysis of a finely spherulitic chlorite from Calton Hill places it definitely in the delessite-diabantite series. Some i other chlorites from Miller's Dale are less crystalline oand show a peculiar development of bacteria-like aggregates, similar to those observed in the chlorophæite of Dalmahoy Hill, near Edinburgh. All these chlorites can be compared with the chloritic palagonite occurring in the mesostasis of the non-vesicular basalt of the same lava flow, and it is suggested that both varieties of chlorite are primary, and were formed during the final stages of the solidification of the magma (autopneumatolitic).—F. L. Stillwell: On the nature of berthierite. A chemical analysis of berthierite from Nullamanna, near Inverell, New South Wales, gave the formula 3FeS.4Sb2S3. Microscopical examination of polished and etched sections of the material shows an inter-oo growth of about 18 per cent. of stibnite. Deducting this from the results of the chemical analysis, the formula of berthierite becomes FeS.Sb2S3.—L. J. Spencer: A sperrylite crystal from the Transvaal. Crystals of sperrylite (the rare platinum arsenide,,PtAs2) up to half-an inch across have recently been found in the Potgietersrust platinum fields, Transvaal. The crystal examined measures 5.0-5.5 mm. across and weighs 1.294 gm. It is a brilliant cubo-octa-hedron developed on all sides and with the corners and edges much rounded. The rounded areas give a profusion of scattered reflected images, few of which lie in the principal zones on the crystal. The only forms identified with certainty are (100), (111), (110), (210), (211).—H. E. Buckley: The anomalous optical properties of some new series of isomorphous double tartrates. In addition to the mixed crystals {m NaK, n Na(NH4)} C4H4O6. 4H2O, previously examined, optical anomalies of the same kind have now been determined for the series {m KNa, n K(NH4)} C4H4O6. 4H2O and {m (NH4)Na, n (NH4)K} C4H4O6.4H2O. All these crystals are orthorhombic and isomorphous, but when freshly prepared they show crossed dispersion characteristic of monoclinic crystals (borax type). On standing, the crystals show a slow change in the size of the optic axial angles and in the position of the optic axial planes for different colours; and finally, after a lapse of some time, they settle down with all the optic axial planes lying in the plane (100) or (010) or in both, giving in the latter case crossed axial-plane dispersion of the orthorhombic (brookite) type. This change is hastened by rise in temperature and retarded by pressure. Sections from the outer and inner portions of the crystals showed a difference, evidently due to the time taken for growth; but in the final state they are identical, suggesting that the crystals are homogeneous. Other isomorphous series of mixed crystals containing only two of the three bases, namely {m Na2, n Na(NH4)}, {m (NH4)2, n (NH4)Na}, {m Na2, n NaK}, and {m K2, n KNa}, showed, in the range of the visible spectrum, the optic axial planes all in one plane, but with wide differences in optic axial angle.—G. T. Prior: On the South African meteorites, Vaalbult, Witklip, and Queen's Mercy. The Vaalbult meteoric iron is a very deeply and broadly pitted mass weighing about 26 lb. which was found on a farm in the Prieska Division, Cape Province. It is a coarse octahedrite having a percentage of nickel of about 7. The Witklip meteoric stone fell on May 26, 1918, at about 9.40 A.M., after the appearance of a luminous meteor and loud explosions, on the farm Witklip in the Carolina District, Transvaal. Fragments weighing together only about 22 gm. appear to have been preserved. It is a grey chondrite closely resembling the Cronstad meteoric stone. Of the Queen's Mercy meteorite a large stone, a foot and a half long, fell, on April 30, 1925, at Queen's Mercy, about twenty miles from Matatiele, and was broken into small pieces by the natives. A second complete smaller stone, weighing about 950 gm., which fell about fifteen miles from Matatiele, was obtained from Chief Jeremiah Moshesh and is now in the Natal Museum at Pietermaritzburg. The meteorite is a veined grey bronzite-chondrite containing about 15.5 per cent. of nickel-iron, in which the ratio of iron to nickel is about 10.5, and about 5.5 per cent. troilite, and having a ratio of magnesia to ferrous oxide in the pyroxene of about 5.

A contribution to the study of the optical properties of mixed crystals

The optical properties of isomorphous mixtures produced in the laboratory have been studied by various crystallographers since the time of Senarmont, who found that the optic axial angle of mixtures of the potassium and ammonium-seignette salts was intermediate between those of the simple salts and varied gradually with the composition of the mixtures. Such artificially prepared mixtures offer better material for investigation than do the mixtures occurring as minerals, since their composition is simpler and can be varied at will. The first quantitative results were obtained by Dufet, who measured the mean refractive index (β) of mixtures of the orthorhombic sulphates of magnesium and nickel and enunciated the law: "The differences between the refractive indices of a mixture of two isomorphous salts and those of the component salts are inversely proportional to the numbers of molecules of the two salts in the mixture.” This is equivalent to the statement that in (orthorhombic) mixed crystals the refractive index of a mixture is a linear function of its composition as expressed in molecular percentage. Thus, if N n' n" are the refractive indices of the mixed crystal and of the two components, respectively, and m' m" are the molecular percentages of the two components N = m'n' + m"n" /100.

Note on cordierite in a cordierite-gneiss from Madura district, Madras, India

The specimen of cordierite-gneiss under notice was collected by Mr. T. N. Muthuswami (Assistant Professor of Geology, Presidency College, Madras) in a quarry at Ganguvarpatti in the Madura district. On examining tliin sections of the rock, he found that many of the cordierite crystals were optically positive, and, thinking that the mineral might be worth further investigation, he sent a specimen of the rock to me. The investigation was carried out ill the Geological Department of the Imperial College of Science and Technology, and my thanks are due to Professor W. W. Watts and Dr. J. W. Evans for their kindness in affording me facilities for this work and also to Dr. H. H. Thomas, of H.M. Geological Survey, for his kindness in placing Fedorov's universal stage at my disposal for the measurement of the optic axial angle.

A New Method for Measuring the Optic Axial Angle of Minerals

A method is presented which to a large extent eliminates the errors of observation of the optic axial angle by the Michel-Levy method. Instead of standardizing circles of reference, observations on a mineral of known 2E are made directly through the interference figure of the unknown. The method is accurate to within a degree if the known mineral and the lines on the micrometer ocular are placed exactly in the 45° position.

The dispersion phenomena and the influence of temperature on the optic axial angle of Sanidine from the Eifel

In the course of my study of the dispersion phenomena of the felspar group in the Mineralogical Laboratory of the University of Cambridge, Professor Lewis advised me to study the remarkable properties of sanidine from the Eifel, which were first noticed by Des Cloizeaux, and he very kindly placed the mineral sections in the Museum at my disposal. In order to be certain of the material used, application was made to Professor A. Lacroix of Paris, who with the courtesy and generosity characteristic of his countrymen was good enough to send some fragments of the very crystals used by Des Cloizeanx in his classical researches.

The dispersion phenomena of some monoclinic Felspars

In the present paper the dispersion of the refractive indices of monoelinie felspars from four different localities is given, likewise that of the optic axial angle and of the optic axes in some of them.The method of investigation and instruments for these studies have been already mentioned in my previous paper. As the refractive indices and dispersion of the glass hemisphere of the total-reflectometer have not so far been given, I shall in the first place state the way in which they have been determined and their values and the degree of accuracy attainable, for the dispersion of the glass hemisphere differs to au appreciable degree from that of other instruments of the same kind.

Societies and Academies

LONDON.Geological Society, December i.-Dr. A. Smith Woodward, president, in the chair.-Dr. J. W. Evans: Petrological methods. The different methods of obtaining the directions-image ("interference figures") of a small mineral in a rock-slice, unaffected by the light from neighbouring minerals, were discussed. The author prefers the use of a diaphragm in the focus of the eyepiece, in conjunction with a Becke lens; he also described the inferences that might be drawn from the form, position, and movement on the rotation of the stage of the isogyres (dark bars or bushes) in the directions-images, both of chance sections and of those cut parallel to planes of optical symmetry or at right-angles to optical axes. He showed how the character or sign of the crystal and its approximate optic axial angle might be determined.