CHAPTER III

THE TITANO-SILICATES AND TITANATES

(_a_) THE TITANO-SILICATES

~Yttrotitanite or Keilhauite.~--A titano-silicate of calcium, aluminium, iron and yttrium metals. The mineral is isomorphous with titanite, CaO,TiO₂,SiO₂ (_q.v._), and is itself probably an isomorphous mixture of titanite with the silicate (Y,Al,Fe)₂SiO₅, where Y = yttrium metals. Its composition will then be represented by the formula _m_ (Y,Al,Fe)₂(SiO₅) + _n_ CaTi(SiO₅).

It is monoclinic, with axial ratios and angles very close to those of titanite. Usual forms--pinakoids _a_ {100} and _c_ {001}, hemi-prism _m_ {110}, hemi-pyramids _n_ {111}, _e_ {1̅11} and _l_ {1̅12}. Cleavage ∥ _n_ distinct. Birefringence weak, +ve. Colour brown to brownish-black. Hardness 6¹⁄₂; sp. gr. 3·52 to 3·77.

The mineral is fusible before the blowpipe, and is decomposed by hydrochloric acid.

It was named by Scheerer in 1844 from its composition, and by Ekeberg in the same year in honour of the Norwegian geologist Keilhau.

~Titanite or Sphene.~--This species, important as an accessory mineral of many rocks, is a titano-silicate of calcium, generally containing small quantities of aluminium and iron. The approximate formula usually given, CaTiSiO₅, is unsatisfactory; some specimens contain as much as 7 per cent. of ferric oxide, others up to 2 per cent. of manganese, whilst the percentage of titanium oxide, TiO₂, varies very considerably (30 to 45 per cent.). Zambonini and Nickolan have independently analysed specimens for which no satisfactory formulæ could be deduced. For specimens containing trivalent metals, Groth considers the mineral to be an isomorphous mixture of CaTiSiO₅ and R´´´₂SiO₅ (see under Yttrotitanite, above); Blomstrand, however, advances the formula 2(R´´R´´´₂O₂,TiO)O,SiO₂, where TiO is basic, and the trivalent metals occur in the divalent group R´´´₂O₂; this formula is also supported by Zambonini.

More recently the problem of the constitution has been attacked by Bruckmoser, using Tschermak’s method of determining the nature of the salts present in silicates. In this method, the mineral is digested with hydrochloric acid, at a temperature not greater than 60°, until decomposition is complete; the silicic acid formed is washed by decantation, and dried in air at a constant temperature; it is weighed at regular intervals until the weight is constant. It is stated that if a curve of times and weights be plotted, a break is observed at the point where drying ceases (for the acid is of course wet) and decomposition begins; the composition at this point, which is taken as the composition of the acid required, can be determined from the weight of the acid, and the weight of anhydrous silica present, which is determined by ignition after the weight has become constant.

Employing this method in the case of titanite, Bruckmoser claims to have obtained the acids H₂Si₂O₅ and H₂Ti₂O₅. He therefore concludes that the constitution of the mineral is represented by the formula Si₂O₅,Ti₂O₅Ca, which presumably may be written Ca(Ti,Si)₂O₅.

Crystal system--monoclinic; _a_ : _b_ : _c_ = 0·7547 : 1 : 0·8543. β = 60° 17´.

Common forms (Des Cloizeaux’s orientation)--the pinakoids _a_ {100} and _c_ {001}, with _m_ {110}, _s_ {021}, _x_ {102}, _n_ {111}, and many others.

(100) ∧ (110) = 38° 14¹⁄₂´; (001) ∧ (1̅01) = 65° 57´; (001) ∧ (011) = 36° 34´.

The habit is very varied, the commonest being the wedge form, elongated ∥ _c_. Twinning is fairly common, especially on the law--Twin plane ∥ _a_, which gives both contact and interpenetrant twins. Cleavage ∥ _m_, fairly distinct. Hardness 5 to 5¹⁄₂; sp. gr. 3·40 to 3·56. Lustre adamantine to resinous. The colour varies very much, doubtless with the content of iron and manganese; it is commonly yellow, green, or brown. Pleochroism is very distinct. The refraction and dispersion are very high, giving the facetted stone a ‘fire’ inferior only to that of diamond. Birefringence positive, strong; the axial angles vary very widely in different specimens.

It is fusible with difficulty before the blowpipe. Hot concentrated hydrochloric acid decomposes it partially, with separation of silica; boiling sulphuric acid, or, better, fused potassium hydrogen sulphate, decomposes it completely.

On account of the high dispersion and refractive index, clear specimens of sphene make very beautiful gems, but the stone is not sufficiently hard to stand much wear.

The mineral was discovered in Chamouni by Pictet in 1787, and was named Pictite by Delamètherie (1797). In 1795 Klaproth analysed a specimen from Passau, and, observing the presence of titanium (which he had just discovered in rutile), proposed the name Titanite. The mineral described by de Saussure (1796) as ‘Schorl rayonnante,’ and afterwards by Hauy (1801) as Sphene (σφήν = a wedge), was shown to be identical in composition with titanite by Cordier, and also by Klaproth (1810); the crystallographic identity was proved by G. Rose (1820).

On account of the difference in colour and composition, a large number of varieties are distinguished. The ordinary yellow and brown varieties are known indifferently as sphene or titanite. _Ligurite_ has an apple-green colour; _Semeline_ is a greenish form named from a fancied resemblance to flax seed. _Lederite_ is a brown variety of tabular habit; _Greenovite_ is rose-coloured, and contains manganese. _Alshedite_ and _Eucolite-Titanite_ are rich in the trivalent metals; _Grothite_ is a brown variety containing a considerable percentage of ferric iron. _Yttrotitanite_, which contains a high proportion of rare earths, is usually treated as a separate species (see above). _Titanomorphite_ and _Leucoxene_ are white amorphous varieties chiefly produced by alteration of rutile and ilmenite.

Titanite is a fairly widespread mineral; as an accessory rock constituent it is common in the massive plutonic rocks in tiny crystals, readily distinguished under the microscope by the high refraction and birefringence, whilst in large embedded crystals it occurs in many granular limestones, and in plutonic acid, as well as in some metamorphic rocks. In good crystals it is found in many parts of Switzerland and the Alps, in Dauphiné, the Tyrol, Piedmont, the Urals, South Norway, and other European localities; it is also widely distributed in the United States and Canada.

The mineral is important as a valuable source of titanium.

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The class of Titano-silicates is a very large one, and might be extended almost at will by the inclusion of the numerous silicates which contain titanium. Owing to the frequency with which small quantities of silica are replaced by titanium dioxide, almost all the commoner silicate minerals contain the latter oxide, so that titanium is one of the most widely distributed of the elements. Relatively very few, however, of the titanium-bearing minerals contain the element in considerable quantities, and only two or three have any importance as commercial sources of titanium compounds.

Only those additional titano-silicates which contain titanium as an important constituent are mentioned below; short accounts will be found in the alphabetical list.

_Johnstrupite_, _Mosandrite_, _Rinkite_, _Rosenbuschite_ and _Tscheffkinite_ are complex titano-silicates containing yttrium or cerium metals.

_Astrophyllite_, _Leucosphenite_, _Molengraafite_, _Neptunite_ and _Rhönite_ are complex titano-silicates free from rare earth elements.

_Benitoite_ is a simple titano-silicate of barium; _Ænigmatite_ and _Narsarsukite_ contain iron and sodium; _Lorenzenite_ has sodium and zirconium. _Schorlomite_ is a titaniferous garnet. A variety of olivine rich in titanium (_Titanium Olivine_) is also known.

(_b_) THE TITANATES

~Yttrocrasite.~[55]--This is a complex titanate of rare earths (chiefly yttria earths) with lime, thoria, and oxides of lead, iron, uranium, etc.; it has a considerable water content. An approximate formula is R´´O,R^{iv}O₂,3R´´´₂O₃,16TiO₂,6H₂O, where R´´ = (Ca,Pb,Fe), R^{iv} = (Th,U), and R´´´₂O₃ = rare earths. No constitutional formula can be given; it will be noticed that the amount of titanium dioxide is considerably more than is required to combine with the bases present (cf. also Delorenzite below). It is radioactive.

[55] Hidden and Warren, _Amer. J. Sci._ 1906, [iv.], ~22~, 515; also _Zeitsch. Kryst. Min._ 1907, ~43~, 18.

Imperfect crystals only were found, apparently belonging to the orthorhombic system. No crystallographic data could be determined.

The mineral is black, closely resembling polycrase and euxenite (_q.v._) in appearance. Hardness 5¹⁄₂-6; sp. gr. 4·80.

It is infusible, and not easily soluble in acids. Hydrofluoric acid decomposes it, and the powdered mineral is also slowly attacked by boiling concentrated sulphuric acid.

It was found in 1904 by Barringer, in Burnet Co., Texas.

~Delorenzite.~[56]--A compound similar to the above, but even richer in titanium dioxide, which amounts to 66 per cent. Tin dioxide is also present, with traces of columbic anhydride. The bases are the yttria earths (almost free from ceria earths), uranium dioxide, and some ferrous oxide, the formula being 2FeO,UO₂,2Y₂O₃,24TiO₂, with a little SnO₂ replacing TiO₂. It is strongly radioactive. Its closest chemical neighbour is yttrocrasite, but in appearance and angles it closely resembles polycrase (_q.v._). Its discoverer, Zambonini, therefore formulates it as a metatitanate with titanium acting also as a base--polycrase is a mixed metatitanate and metacolumbate--thus, 2FeTiO₃ + U(TiO₃)₂ + 2Y₂(TiO₃)₃ + 7(TiO)TiO₃.

[56] Zambonini, _Zeitsch. Kryst. Min._ 1908, ~45~, 76.

The crystals occur in aggregates of numerous individuals in sub-parallel growth. The system is orthorhombic; _a_ : _b_ : _c_ = 0·3375 : 1 : 0·3412. Usual forms--the pinakoids _a_ {100} and _b_ {010} with prism _m_ {110}, dome _d_ {201}, etc. Habit prismatic, elongated ∥ c axis. Hardness 5¹⁄₂-6; sp. gr. about 4·7.

It was found with struvite in a pegmatite at Craveggia, Piedmont, Italy.

~Ilmenite or Menaccanite~ (Specular Iron Ore, Titaniferous Ironstone, etc.).--This is a titanate of iron, usually written FeTiO₃. Its constitution has given rise to very considerable discussion[57]; not only do the relative proportions of iron and titanium vary greatly, but the iron is undoubtedly present in both the ferrous and the ferric states, and in the former state is partly replaced in some specimens by manganese and magnesium. In 1829 Mosander put forward the view that the mineral consisted of FeTiO₃, ferrous titanate, with varying proportions of ferric oxide, the forms and angles of ilmenite being very similar to those of hæmatite, Fe₂O₃. This view was disputed by H. Rose, who concluded that the mineral must have been originally an isomorphous mixture of ferric oxide, Fe₂O₃, and titanic oxide, Ti₂O₃, which on exposure to high temperature in the earth’s crust would change according to the equation

Fe₂O₃ + Ti₂O₃ = 2TiO₂ + 2FeO

so that the proportion of ferrous iron increases with the proportion of titanium dioxide, as is actually found to be the case. This condition, however, is also satisfied by Mosander’s view. The latter view was also supported by Rammelsberg, who pointed out that the presence of magnesium indicated the existence of ferrous iron as a primary constituent. Additional support is lent to this view by the discovery of Pyrophanite, MnTiO₃ (see list), which is found to be isomorphous with ilmenite, so that there can be little doubt that MgTiO₃, which can be only a titanate, would, if it existed in the crystalline form (see Geikielite in list), also be isomorphous with ilmenite. Friedel and Guérin (1876) prepared artificial titanium sesquioxide, Ti₂O₃, and found it to be isomorphous with hæmatite, Fe₂O₃; they concluded that FeFeO₃, FeTiO₃ and TiTiO₃ formed an isomorphous series, and that ilmenite was a mixture of the second with the other two. In 1890 Hamberg pointed out that there was no reason to suppose that hæmatite, Fe₂O₃, contains ferrous iron, _i.e._ has the constitution Fe´´Fe^{iv}O₃, analogous to Fe´´Ti^{iv}O₃, since in corundum, the analogous compound of aluminium, Al₂O₃, divalent aluminium can hardly exist; nevertheless, strict analogy of constitution is not necessary for isomorphism, as shown by the case of potassium nitrate, KNO₃, and aragonite, CaCO₃, so that hæmatite, Fe₂O₃, and ferrous titanate, FeTiO₃, might form solid solutions in varying proportions without the strictly analogous formula FeFeO₃ being true for the former. The balance of opinion inclines to the constitution (_m_FeTiO₃ + _n_Fe₂O₃ in isomorphous mixture) originally proposed by Mosander. The evidence in support of this view has been greatly strengthened by the recent work of Manchot,[58] which has proved the absence of titanium sesquioxide, Ti₂O₃; the mineral is therefore to be regarded as a titanate.

[57] For a full account of the earlier work on the constitution of ilmenite _vide_ Hintze, i. 1858 _et seq._

[58] _Zeitsch. anorg. Chem._ 1912, ~74~, 79.

Crystal system--rhombohedral; in forms and angles very close to hæmatite, but the two differ in symmetry (hæmatite has _t_, 3δ, _c_, 3π; ilmenite has only _t_, _c_).

_c_ = 1·38458; (111) ∧ (100) = 57° 58¹⁄₂´; habit, tabular, thick; or in thin laminæ. Usually in embedded grains or rolled crystals in sand.

Hardness 5 to 6; sp. gr. 4·5 to 5·0, increasing with percentage of ferric oxide. Iron black, opaque; streak black to brownish-red. Lustre sub-metallic. Slightly magnetic.

The mineral is infusible; when powdered, it dissolves slowly in boiling hydrochloric acid, the filtered yellow solution giving the characteristic blue colouration of titanium salts on addition of tinfoil. In fused potassium hydrogen sulphate it dissolves readily. The variation in composition can be judged from the following limits:

TiO₂ Fe₂O₃ FeO 3·5 93·6 3·3 per cent. 52·8 1·2 46·5 „

Ilmenite is a widely distributed mineral. In crystals it occurs chiefly at Kragerö and Arendal in Norway, at Miask in the Ilmen mountains, in Dauphiné, the St. Gothard, etc.; in the massive form at Bay St. Paul, Quebec, and other localities in America; and in sands at Menaccan in Cornwall, Iserwiese in Bohemia, Puy de Dôme, dép. Haute Loire, France, and in Brazil, Australia, and New Zealand.

The mineral was discovered at Menaccan in Cornwall by McGregor, about 1790. He described it as containing iron and a new oxide; the unknown oxide was obtained in 1795 from rutile by Klaproth, who gave the name Titanium to the new metal it contained.

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Short descriptions of the following titanates are also given (see list):

_Davidite_ and _Knopite_; these are complex titanates containing elements of the cerium and yttrium groups.

_Arizonite_ and _Pseudobrookite_--ferric titanates.

_Perovskite_, calcium titanate, and its variety _Hydrotitanite_.

_Pyrophanite_, a manganese titanate isomorphous with ilmenite, and _Senaite_, a species intermediate in composition between these two.

_Geikielite_, the magnesium analogue of ilmenite, with the variety _Picroilmenite_, which is rich in iron.

_Uhligite_, a titanate of zirconium, calcium and aluminium.

_Derbylite_, _Lewisite_ and _Mauzeliite_, an interesting series of titano-antimonates.

_Warwickite_, a boro-titanate.