Alumn Stone Enlarge
Exotic Mineralogy
Argilla silico-sulphata

Alumn Stone

  • Syn.
    • Alaunstein, of the Germans.
    • Alumenilite. La Meth. II, 113.
    • Siliferous subsulphate of Argill. Sowerby’s List of Minerals, 3.

This mineral has long been known by the German writers on mineralogy, nevertheless it has bees neglected by many French authors, and appears to have been almost entirely unknown in England until some specimens were imported from Vienna in 1819. The crystallized form is a late discovery; the varieties from Tolfa near Civita Vecchia are destitute of any approach to crystallization, they are massive with a smooth and splintery or rough and rugged fracture, and want that glimmering which the surfaces of minute crystals dispersed through the masses give to the varieties from Hungary; these masses are also cellular, and the cells whose form is very irregular and ragged are lined with glistening crystals, most commonly of such small dimensions, and so closely set, that their forms cannot be distinguished; in some specimens however the crystals are larger (perhaps the 20th. of an inch wide) and arranged in little globules, and some are sufficiently insulaled to shew that they are six-sided plates with alternately inclined edges, their surfaces are very brilliant and flat. The rarest form of this mineral is in well formed rhombs whose edges are the 10th of an inch long, grouped closely upon the surfaces of hollows in the same substance.

The sides of the rhombs are generally slightly curved like those of Brown spar, and their apices truncated, leading to the minute plates above mentioned (see the outlines); the plain produced by the truncation is very flat and brilliant, the others arc shilling ; the fracture is uneven and rather dull in every direction except that perpendicular to the axis of the rhomb, in which it is foliated and brilliant. The crystals are soft enough to yield to the knife easily with a grating sound, but the cellular mass contains a greyish substance that is so hard as to injure the edge of the knife and probably contains more silex than the crystals imbedded in it, which ought to be considered as pure Alum-stone. The crystals, are transparent on the edges, the other parts opaque.

The spec. grav. varies from 2,587, to 2,633; it is difficultly fusible.

The following arc the analyses given by Vauquelin and Klaproth.

Vauquelin Klaproth from Tolfa. Ditto from Hungary
Argilla 43.92 19.0 17.5  
Silex 24.00 56.5 62.25
Sulphuric acid 25.00 16.5 12.50
Potash 3.08 4.0 1.0  
Water 4.00 3.0 5.0  
100.00 99.0 98.25

It is evident that this mineral contains all the constituent parts of Alum which has long been extracted from it, by first roasting so as to destroy the adhesion of the particles, then washing from the Silex and superabundant Argill the soluble portions and evaporating the water to obtain the alum in crystals.

Fig. 1, represents the largest crystals I have met with; they are from Beregshaez.

Figs 2 and 3 are from the cellular variety found also at Beregshaez in Upper Hungary.

Fig. 4, the most compact variety from Tolfa, it is accompanied by Quartz.

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