Spinel Group Research Articles

The crystalline structure of chrysoberyl

1. Chrysoberyl, BeAl 2 O 4 , is analogous in chemical composition to spinel, MgAl 2 O 4 , and the other members of the spinel group, but it bears no resem­blance to them in its crystalline form. The spinels are cubic, and have a very simple structure, which has been analysed by S. Nishikawa and W. H. Bragg, The same type of structure has been discovered in other compounds, Chryso­beryl belongs to the orthorhombic holohedral class, the axial ratios being a : b : c = 0·4707 : 1 : 0·5823. The “ a ” axis is an axis of pseudo-hexagonal symmetry. The angle (011) : (011) is 60° 26', and the ratio of b to c is very nearly √3 : 1 or 1 : 0·577. The crystal twins in a manner very similar to that of aragonite, three individuals interpenetrating so as to form what appears to be a simple hexagonal crystal. The examination of the structure is interesting for two reasons. Firstly, it is curious that it shows such contrast to that of a spinel. Secondly, the axial ratios are almost identical with those of the olivine group of minerals, and are nearly given by the ratio √ ⅔ : 1 : √3/3.

Societies and Academies

Societies and Academies

A graphic method for the comparison of minerals with four variable components rearming two isomorlohous pairs

Graphic methods are often of the greatest assistance in bringing out the affinities of closely related minerals and particularly of members of isomorphous series. An illustration of the use of such a method in the case of a series involving three variables, all of them mutually replaceab]e, has already been given by the author in dealing with minerals of the tapiolite-mossite-rutile series. Recently, in studying a mineral of the spinel group, occurring in serpentine country near Namban, in the South-Western Division of Western Australia, it was found convenient to use a graphic method to bring out its relationship to the type minerals spinel, hercynite, and chromite, and to the intermediate minerals previously described under the names of picotite, chrompicotite, magnesiochromite, and magnochromite.

XXX. The structure of the spinel group of crystals

XXX. <i>The structure of the spinel group of crystals</i>

The Structure of Magnetite and the Spinels

THE structure of the spinel group of crystals is very interesting. These crystals are cubic, and possess the greatest possible number of symmetries. The composition is given by the formula, R″; R′″2O4, where the divalent metal may be Mg, Fe, Zn, or Mn, and the trivalent metal may be Fe, Mn, Cr, or Al. Magnetite is FeFe2O4.

On the relation of franklinite to the spinel group of minerals

On the relation of franklinite to the spinel group of minerals