Illustration from 1911 Encyclopædia Britannica, article CALCITE.

Original text: Twinned Crystals of Calcite.

i. Twin-plane c (111).—Here there is rotation of one portion with respect to the other through 180° about the principal (trigonal) axis, which is perpendicular to the plane c (111); or the same result may be obtained by reflection across this plane. Fig. 7 shows a prismatic crystal (like fig. 4) twinned in this manner, and fig. 8 represents a twinned scalenohedron v {201}.

ii. Twin-plane e (110).—The principal axes of the two portions are inclined at an angle of 52° 30½′. Repeated twinning on this plane is very common, and the twin-lamellae (fig. 9) to which it gives rise are often to be observed in the grains of calcite of crystalline limestones which have been subjected to pressure. This lamellar twinning is of secondary origin; it may be readily produced artificially by pressure, for example, by pressing a knife into the edge of a cleavage rhombohedron.

iii. Twin-plane r (100).—Here the principal axes of the two portions are nearly at right angles (89° 14′), and one of the directions of cleavage in both portions is parallel to the twin-plane. Fine crystals of prismatic habit twinned according to this law were formerly found in considerable numbers at Wheal Wrey in Cornwall, and of scalenohedral habit at Eyam in Derbyshire and Cleator Moor in Cumberland; those from the last two localities are known as "butterfly twins" or "heart-shaped twins" (fig. 10), according to their shape.

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