SIMPLE FORMS OF ZIRCON CRYSTALS FROM CRYSTALLINE ROCKS OF THE UKRAINIAN SHIELD AND THEIR MORPHOLOGICAL TYPES

The main basics in geometric crystallography of zircon, developed by many researchers in the 18th - 20th centuries, are briefly described. The data of goniometric study of zircon from crystalline rocks of the Ukrainian Shield (USh) are summarized. They cover zircon predominantly from granites and alkaline rocks of most the USh megablocks. The set of habit simple forms on zircon crystals is small: {111}, {110}, {100}, {221}, {331} and {311}. These forms define two contrasting habits of zircon crystals - prismatic and dipyramidal. Among the prismatic crystals several main morphological types of crystals are distinguished: {110} + {111} – zircon type, {100} + {111} – hyacinth type, {110} +100} + {111}, {110} +{100} + {111} + {311} and {110} + {100} + {311} – intermediate hyacinth-zircon types. Among the dipyramidal crystals two morphological types are contrasting — faceted by {111} dipyramid and {111} + {331} + {221} dipyramid combinations. The simple form {111} is developed on almost all zircon crystals from crystalline rocks of the USh, unless it is completely displaced on the heads of the crystals by the ditetragonal dipyramid {311}. For zircon crystals from syenites, mariupolites, albitites and some pegmatites the {111} is habit form. The simple form prism {110} is also developed on almost all zircon crystals from crystalline rocks of the USh, with the exception of many {111} dipyramidal crystals from syenites of the Zhovtnevy massif and hyacinth type of zircon crystals. It determines the most common morphological type of zircon crystals of prismatic habit – zircon type. The simple form prism {100} is less common on zircon crystals from crystalline rocks of the USh than the form {110}. It determines the hyacinth morphological type of zircon crystals of a prismatic habit. It is characteristic of zircon from granites of the Azov and Middle Dnipro regions. The simple form {311} is well developed on zircon crystals of hyacinth-zircon type from granites. It is almost absent on dipyramidal zircon crystals from alkaline rocks. The simple forms {221} and {331} are well developed only on dipyramidal crystals from syenites, mariupolites, albitites and some pegmatites of the Azov region. They are especially characteristic of zircon crystals of the Azov deposit. The simple form pinacoid {001} is rare and poorly developed; it was found only on zircon crystals of a prismatic habit from carbonatites of the Chernigiv massif and on dipyramidal crystals from syenites of the same massif. Another two dipyramids {101} and {211} can be attributed to reliable simple forms on zircon crystals from crystalline rocks of the USh. However, they are rare and found only on zircon crystals from acid rocks.