XLIII.—On Circular Crystals

Abstract

In 1836, Mr Fox Talbot communicated to the Royal Society a paper “On the Optical Phenomena of certain Crystals” which he obtained by dissolving a crystal of Borax in a drop of somewhat diluted Phosphoric acid. When the acid and the salt are in proper proportions, “the field of view of the microscope is seen covered with minute circular spots, each of which is like a tuft of silk radiating from a centre, and is composed of a close assemblage of delicate acicular crystals forming a star.” Among these crystals are seen interspersed “a number of circular transparent bodies, which are tufts or stars of acicular crystals, in such close assemblage as to be in optical contact with each other, and to produce the appearance of a single individual.”

Bernauer′s Bands

“…Rhythmic precipitation in spherulites, as distinct from Liesegang banding, was first observed by Brewster [10] and discussed at length by Hedges.…”

“…Rhythmic precipitation in spherulites, as distinct from Liesegang banding, was first observed by Brewster [10] and discussed at length by Hedges. [11] Bernauer's push to ascribe all textures to one mechanism is mystifying because rhythmic deposition could be established in some cases with an optical microscope.…”

An optical test to unveil twisting of birefringent crystals in spherulites

“…In other words, the twisted crystal will show a uniform and lower birefringence along its length. Moreover, because along each helix within a twisted crystal, the direction of the optical axis is reverted every half-pitch, and a crystal is made of sets of co-axial helices for the whole length, the optical test settled by Brewster [7] to establish the birefringence sign of spherulites allows us also to reveal twisting. Indeed, by using a retardation plate, a hypothetical low birefringent spherulite constituted of thin, twisted radial crystals should show four luminous sectors of the same colour or alternating bands with the same colours, although exchanged, in all sectors.…”

“…The optical behaviour of an elongated crystal of sub-micrometre size would be very difficult, if not impossible, to reveal by means of an optical polarizing microscope because of the limited resolving power and magnification level of the equipment. Fortunately, almost all substances solidified by cooling from melt form spherulites [7,20–23], which, under particular conditions, can reach dimensions of hundreds of micrometres or even more. Furthermore, in such a close assemblage, radial crystals are in contact to each other so as to produce the appearance of a single individual [7,22].…”

“…Fortunately, almost all substances solidified by cooling from melt form spherulites [7,20–23], which, under particular conditions, can reach dimensions of hundreds of micrometres or even more. Furthermore, in such a close assemblage, radial crystals are in contact to each other so as to produce the appearance of a single individual [7,22]. The circular arrangement of elongated crystals reflects the optical behaviour of each crystal and, because of the large dimensions of spherulites, allows investigations otherwise hardly feasible on separate crystals.…”