The crystals are feebly doubly refracting, and in polarized light exhibit a banded structure parallel to the cube faces.
Crystals have the form of small, sharply defined cubes of an oliveor grass-green colour, and occur together in considerable numbers on the matrix of the specimens.
It forms dark red crystals isomorphous with ferrous sulphate, and readily soluble in water.
It forms monoclinic crystals which are very soluble in water.
The crystals possess a perfect cleavage parallel to the plane of symmetry and are usually bladed, in habit; they are soft (H= 2), flexible and sectile.
Boron dissolves in molten aluminium, and on cooling, transparent, almost colourless crystals are obtained, possessing a lustre, hardness and refractivity near that of the diamond.
It is decomposed by water, and with a solution of yellow phosphorus in carbon bisulphide it gives a red powder of composition PBI 2, which sublimes in vacuo at 210° C. to red crystals, and when heated in a current of hydrogen loses its iodine and leaves a residue of boron phosphide PB.
It forms slightly coloured small crystals possessing a strong disagreeable smell, and is rapidly decomposed by water with the formation of boric acid and sulphuretted hydrogen.
Recent analyses prove the presence of a small but variable amount of potassium (K 2 O, 2.68-4.13°/x) in the Cornish crystals, though in those from Hungary there is only a trace; this constituent appears to take the place of basic hydrogen in the above formula.
Natural crystals are sometimes honey-yellow to brown in colour, but this appears to be due to alteration.
Pharmacosiderite is a mineral of secondary origin, the crystals occurring attached to gozzany quartz in the upper part of veins of copper ore.
Crystals are prismatic, acicular or scaly in habit; they have a perfect cleavage parallel to the brachypinacoid (M in the figure).
Sammetblende or przibramite is a variety, from Przibram in Bohemia, consisting of delicate acicular or capillary crystals arranged in radiating groups with a velvety surface and yellow colour.
Acicular crystals, resembling rutile in appearance, ` sometimes penetrate crystals of pale-coloured amethyst, for instance, at Wolf's Island in Lake Onega in Russia: this form of the mineral has long been known as onegite, and the crystals enclosing it are cut for ornamental purposes under the name of "Cupid's darts" (fleches d'amour).
Each cell contains a zinc plate, immersed in a solution of zinc sulphate, and also a porous chamber containing crystals of copper sulphate and a copper plate.
Cadmium sulphate, CdSO 4, is known in several hydrated forms; being deposited, on spontaneous evaporation of a concentrated aqueous solution, in the form of large monosymmetric crystals of composition 3CdSO 4.8H 2 O, whilst a boiling saturated solution, to which concentrated sulphuric acid has been added, deposits crystals of composition CdSO 4 4H 2 0.
Of the suspended substances, grains of caoutchouc, drops of resin and oil, proteid crystals and starch grains may be mentioned.
They are composed of a homogeneous proteid substance, and often contain albuminoid or proteid crystals of the same kind as those which form the pyrenoid.
In the red variety of Cucurbita pepo these crystals may consist of rods, thin plates, flat ribbons or spirals.
These are, according to Meyer, acicular crystals, which he calls tricizites.
Albumen crystals are also to be found in the cytoplasm, in leucoplasts and rarely in the nucleus.
Such crystals occur either in cavities in mineral-veins and in granitic rocks, or as a lining in agate geodes.
Many of the hollow agates of Brazil and Uruguay contain a crop of amethyst-crystals in the interior.
As crystals, and as stalactitic, encrusting, reniform, massive, earthy and occasionally pulverulent forms as "sulphur meal."
Crystals of sulphur are transparent or translucent and highly refractive with strong birefringence; they have a resinous or slightly adamantine lustre, and present the characteristic sulphur-yellow colour.
Free sulphur may also result from the decomposition of pyrites, as in pyritic shales and lignites, or from the alteration of galena: thus crystals of sulphur occur, with anglesite, in cavities in galena at Monteponi near Iglesias in Sardinia; whilst the pyrites of Rio Tinto in Spain sometimes yield sulphur on weathering.
Cesena and Perticara are well-known localities in this district, the latter yielding crystals coated with asphalt.
Fine crystals occur at Conil near Cadiz; whilst in the province of Teruel in Aragon, sulphur in a compact form replaces fresh-water shells and plant-remains, suggesting its origin from sulphur-springs.
Commercial sulphur forms yellow crystals which melt at 113° and boil at 444'53° C. under ordinary pressure (H.
The solution obtained may be evaporated in vacuo until it attains a density of 1.46 when, if partially saturated with potassium hydroxide and filtered, it yields crystals of potassium pentathionate, K 2 S 5 0 6.3H 2 0.
Crystals of azurite belong to the monoclinic system; they have a vitreous lustre and are translucent.
- On the theory that crystal form and structure are the result of the equilibrium between the atoms and molecules composing the crystals, it is probable, a priori, that the same substance may possess different equilibrium configurations of sufficient stability, under favourable conditions, to form different crystal structures.
In the article Crystallography the nature and behaviour of twinned crystals receives full treatment; here it is sufficient to say that when the planes and axes of twinning are planes and axes of symmetry, a twin would exhibit higher symmetry (but remain in the same crystal system) than the primary crystal; and, also, if a crystal approximates in its axial constants to 'a higher system, mimetic twinning would increase the approximation, and the crystal would be pseudo-symmetric.
A sublimate may be formed of: sulphur - reddish-brown drops, cooling to a yellow to brown solid, from sulphides or mixtures; iodine - violet vapour, black sublimate, from iodides, iodic acid, or mixtures; mercury and its compounds - metallic mercury forms minute globules, mercuric sulphide is black and becomes red on rubbing, mercuric chloride fuses before subliming, mercurous chloride does not fuse, mercuric iodide gives a yellow sublimate; arsenic and its compounds - metallic arsenic gives a grey mirror, arsenious oxide forms white shining crystals, arsenic sulphides give reddish-yellow sublimates which turn yellow on cooling; antimony oxide fuses and gives a yellow acicular sublimate; lead chloride forms a white sublimate after long and intense heating.
It forms silky crystals which melt at 6° C., and boil at about 144° C. with decomposition.
It forms very hard metallic-looking crystals, burns in oxygen and is not attacked by acids.
All these are strikingly alike in appearance and general characters, differing essentially only in chemical composition, and it would seem better to reserve the name cerargyrite for the whole group, using the names chlorargyrite (AgC1), embolite (Ag(Cl, Bl)), bromargyrite (AgBr) and iodembolite (Ag(C1, Br, I)) for the different isomorphous members of the group. They are cubic in crystallization, with the cube and the octahedron as prominent forms, but crystals are small and usually indistinct; there is no cleavage.
Crystals of a different form are deposited from a strong boiling solution of the acid.
Crystals of barytes are orthorhombic and isomorphous with the strontium and lead sulphates (celestite and anglesite); they are usually very perfectly developed and present great variety of form.
The crystals of prismatic habit represented in figs.
Crystals of barytes may be transparent and colourless, or white and opaque, or of a yellow, brown, bluish or greenish colour.
Well developed crystals are extremely common, but the mineral occurs also in a granular, earthy, or stalactitic condition.