BORON. Minerals containing Boron or any of its compounds as an essential component part are comparatively few in number, and ouly found in a few spots; it may be therefore considered as one of the least predominating of the elements. It is the basis of Sassoline, or native boracic acid ; Borax, or borate of soda ; Boracite, or borate of magnesia ; Datholite, or borate and silicate of lime ; and Botryolite.
It also enters as boracic acid into the composition of Axinite and Tourmaline, but only in small quantity, most analyses giving between two and three per cent, of the acid in the former mineral, and between four and five per cent. in the latter.
The presence of Boron in any mineral may be readily detected with the blow-pipe, owing to the beautiful green tint communicated to the flame by the boracic acid. The facility with which the tint is obtained depends on the element with which the boracic acid is combined ; in every instance however it may be detected by the following process :—Let a flux, composed of 44 parts of bisulphate of potash and one of finely-powdered fluor-spar, be well mixed with about an equal quantity of the assay, which must then be formed into a paste by the addition of a little moisture. A small quantity of this being taken up on the extremity of a platinum wire must first be dried, and then exposed to a high temperature until it is fused, being held within but near the extremity of the blue flame. When the mass is fused it appears for a few moments enveloped in a pure green flame, which soon disappears, and cannot be again produced. The theory of the changes is this :—The fluorine of the flux being set free by the excess of sulphuric acid unites with the boron of the assay, forming the fiuoboracic acid, which at the moment of its volatilisation communicates the green tint to the flame. This process is, however, only necessary for the detection of the boracic acid in axinite and tourmaline, as the flame 'is permanently coloured by sassoline, boracite, datholite, and botryolite ; and the same effect is produced by moistening the glass of boron with sulphuric acid and again fusing it.
The native boracic acid is found as a deposit in several of the lagunes of Tuscany, and also in considerable abundance in the hot springs near Sasso in the same country, whence it has been called Smoothie. It occurs in the form of thin scaly particles, or crystalline grains either loose or aggregated in the form of a crust. These crystalline grains are hydrated boracic acid, the constitution of which may be expressed by the formula— Boron 1 ; Oxygen 6; Water 6, as given by Berzelius, 100 parts of sassoline being composed of boracic acid 56.37, water 43'63: their specific gravity is P48. The lustre is pearly, and the colour is grayish or yellowish-white : they are slightly translucent.
It loses its water of crystallisation and fuses at a very low tempera ture, forming a glassy globule, which is a non-conductor of electricity, and becomes resinously electric on friction. It has also been found more recently by Dr. Holland to be a deposit of the solfatara within the crater of Volcano, one of the Lipari Isles, being an exhalation of the funiaroles, around the edges of which it forms thin filaments or cakes on the surface of the sulphur.
Borax, or borate of soda, is principally employed in the arts as a flux in several metallnrgical processes, and is very advantageously used in the process of soldering metals. To the chemist it is an invaluable re-agent in experimenting with the blow-pipe.
Borax is soluble in twelve times its weight of cold and twice its weight of boiling water, from which it may be readily obtained in very perfect crystals the oblique prismatic system. The more usual form of these is represented in the accom panying figure, where the faces r are the vertical prism, the angles of which are, accord ing to the measurements of Phillip, 86° 30' and 93° 30', the acuter edge of which is truncated by M, the obtnser by T, while P is the inclined terminal plane, and makes with M an angle of 106° 30'; 0 are the faces of a hemi-octohedron. The following are the measurements given by Phillips on ..... 86° 30' P on r . . . . . 101° 30' M on r . . . . . 133° 20' P on M . . . . . . . 106° 30' P on 0 . . . . . . . 139° 15' 0 on 0 . . . . . . . 122° 34' It is very common to find the edges between 0 and r truncated. The specific gravity varies from P5 to P7 ; the hardness from 2 to 2'5. When coloured it is of a light yellowish-green : the fracture is conchoidal and of a resinous lustre.
The chemical composition as given by Berzelius is Boracic Acid 36'52 Soda 16'37 Water . . . . . . . . 47'11 Boracite is in many respects one of the most interesting bodies of the inorganic kingdom. It was first described by Lasius in 1787 under the name of cubic quartz, and was found in the gypsum rocks near Luneburg in Brunswick, where it occurs in small crystals, which areyerfectly developed on every side and imbedded in the gypsum.
The crystals usually present a combination of the cube, dodecahedron, and the two hemi-octohedrons, in which combinations sometimes the one sometimes the other form predominates. The locality was for some time the only spot where boracite was found, until the crystals were discovered in a gypsum rock called Segeberg in Holstein, at the foot of which is situated a small village of the same name. The crystals of the Boracite of this spot possess the same characters as those of Lihieburg, and add considerable interest to the very peculiar rock in which they are found, which is itself a very remarkable object from its abrupt elevation over the sandy plain of Holstein. It is described in the Geognostisehen Aufsatzen ' of Steffens, who considers it to be of the same formation as the gypsum of the Paris basin.
Boracite has been analysed by Stromeyer, who found it to be com posed of boracic acid 67, magnesia 33. The specific gravity is 2'9; it is transparent, but also frequently opaque ; the hardness is to 7; it is brittle, and has a conchoidal fracture. The lustre is vitreous, inclining to adamantine. The colour is usually a yellowish or greenish gray.
It fuses easily before the blowpipe, at first with much foam, and then forma a glass globule, which crystallises on cooling, so that the surface is covered with fine acicular points. When just so much soda is added as will form with it a clear glass, it will then crystallise as perfectly as the phosphate of lead.