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Volume 2, 1869
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Art. XXVII.—On the production of certain Crystalline Phosphates and Arseniates.

[Read before the Wellington Philosophical Society, February 9, 1869.]

A Great many minerals occur in a natural state, which, in their chemical constitution, their crystalline form, or both combined, have not yet been artificially produced.

It seems very desirable to know, both upon chemical and geological grounds, the conditions necessary for their production, and especially in those forms which they assume in nature.

The metallic phosphates and arseniates—a group of salts which, almost without exception, are only known in the laboratory as gelatinous or pul-verulent precipitates,—stand conspicuous among those native minerals, which we have hitherto been unable to obtain by artificial means in their crystalline forms.

I have recently attempted the crystallization of some of these compounds, with a certain degree of success; and further, in the course of my experiments, I have succeeded in crystallizing some phosphates, which, hitherto, have not assumed such a crystalline form, either naturally or artificially.

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The process I employ is to add a soluble phosphate, or arseniate, to the solution of a salt of the metal, the phosphate or arseniate of which, is required, in the manner hitherto adopted, but only in such limited quantity, that the mixed solution remains acid in its reaction, instead of alkaline, as occurs in the usual method of procedure. If the precipitate is long in appearing, it may occasionally be crystalline; if it comes at once it will be gelatinous, as usual, but in the course of a few hours, sometimes, however, a few days, it will be found crystallized throughout.

The essential features of this process are:—

1st. The maintenance of the precipitated metallic salt in its integrity, which is effected by having the surrounding solution feebly acid.

2nd. Allowing motion to the particles of these gelatinous precipitates, whereby they are amenable to the action of crystallizing force; this is accomplished by keeping a little of the same phosphate in a soluble state in contact with them.

In this manner I have succeeded in crystallizing the following phosphates and arseniates, which occur in this form in the natural state:—

Phosphate of Zinc,—Hopeite, (Zn. O)3 + PO5 + HO 5

Phosphate of Cadmium.

Arseniate of Zinc,—Kottigite.

Arseniate of Lime,—Pharmacolite.

The following crystallized phosphates and arseniates, produced, do not occur as such in a natural state.

Phosphate of Lime (—(CaO)2 + HO) + PO5 + 3HO. This has the same composition as the amorphous precipitate, produced by adding a triphosphate to chloride of calcium, and then a little ammonia, (the precipitate being air-dried); and it is isomorphous with the natural arseniate of lime above, Pharmacolite. It crystallizes in the form of rhombs, and is acid to test paper.

Phosphate of Chromium contains 24 eqs. of water, and has probably the same constitution as Delvauxine, or hydrous phosphate of sesquioxide of iron, the iron being replaced by chromium, its colour is the same as that of chrome-alum, the substance used as the source of the chromium.

Phosphate of Silver. Only crystallized from its solution in acetic acid.

Phosphate of Baryta and Strontia are also easily crystallized. Those salts having formulæ attached, have been analyzed.

On reviewing these salts, it will be noticed that the copper, nickel, cobalt, and iron, phosphates and arseniates, are absent. Indeed, I have not been able to crystallize any of them in this manner; although I am aware that it has been affirmed, that phosphate of nickel has been artificially crystallized. But I find that all these metallic phosphates, etc., are capable of forming double phosphates, etc., with phosphates of magnesia and ammonia. The metal may, I think, be looked upon as substituting one equivalent of magnesia in the common ammoniacal phosphate of magnesia, thus (Met + Mg O + NHO) —PO5 12 HO in place of 2 (Mg O) + NH4 O + PO5 + 12HO.

I also find that phosphate of zinc forms a crystallizable compound with either phosphate of cobalt or nickel. It may be remarked here that the crystalline mineral, Kottigite, an impure arseniate of zinc, always contains a little of both these phosphates.

Lastly, it appears that crystalline precipitates are readily produced by contact of soluble phosphates with solutions of the metals cobalt and nickel, if a salt of ammonia is also present. These precipitates contain ammonia, in small quantity, but it appears to be as an essential element in their composition, and not a mere accidental impurity; its quantity has not yet been determined.

The inferences I would draw from these results are:—

1st. That several of the crystalline, simple, natural phosphates and

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arseniates, have not been produced as such, directly; but that in the first instance compound phosphates or arseniates have formed. Magnesia and ammonia, singly or collectively, being the other members of the term. The magnesia and ammonia being afterwards gradually substituted by the metallic oxide. A continued supply of such metallic oxide to the compound phosphate or arseniate, would almost certainly effect this, the metallic phosphates and arseniates being more insoluble than the alkaline ones.

2nd. This property of some of the metallic phosphates, etc., of combining with phosphate of magnesia and ammonia, to form insoluble compounds, makes it very probable that several of these natural phosphates and arseniates may contain very appreciable quantities of ammonia or magnesia. At any rate, I think, with this property manifested, it would be well to examine rigorously this class of compounds, for either of these substances.

These notes are, of course, merely preliminary, there being several points of interest left undiscussed, which can only be properly represented along with the results of future investigations.