40 AUSTRALASIAN ANTARCTIC EXPEDITION. 



4. CRYSTALLOBLASTIC ORDER. 



The crystalloblastic order, as defined by Grubenmann*, is determined by the form 

 development of the crystal grains. A mineral placed in the crystalloblastic order will 

 assert its crystal form against all minerals that follow it in the sequence. The order is 

 based on a fundamentally different conception to that on which the order of 

 crystallisation of minerals in igneous rocks is based. In igneous rocks a definite order 

 is obtained based on the principles of solubility and mass action which prevail in a 

 rock magma. When, however, a set of conditions prevail which will stamp the special 

 metamorphic characters on a rock mass, the whole alteration takes place while the 

 rock remains solid. The new minerals resulting from the new set of conditions will 

 arise at practically the same time. They will grow together, and any one mineral may 

 be included in, or surrounded by, any other mineral. In doing so some minerals will 

 exert their crystalline form against the adjacent grains of other minerals. For example, 

 we may find felspar included in epidote, and we may also find epidote included in felspar. 

 The felspar included in epidote will be rounded in outline while the epidote included in 

 felspar will very likely possess crystalline boundaries. Epidote, therefore, stands 

 above felspar in the crystalloblastic orderf. 



The crystalloblastic order is of value because the minerals possessing crystalline 

 form in schists are those which possess the greatest crystallisation force. The speed of 

 crystallisation may also be a factor in assisting or hindering crystalline form. 



Grubenmann's teaching in this manner is not accepted by Leith and MeadJ. These 

 authors are inclined to imagine that crystal habit or crystal dimensions influence the 

 development of the new minerals, and believe that the mineral constituents are not of 

 equal rank and do show a definite order of crystallisation. It is to be pointed out that 

 Leith and Mead make no attempt to discriminate between sets of physico-chemical 

 conditions grouped together in Van Hise's zone of anamorphism, and, therefore, do 

 not recognise that a metamorphic rock may carry the impress of two or more sets of 

 conditions. They have not appreciated the fact that Grubenmann has attempted 

 to form a crystalloblastic order for each defined set of conditions, i.e., for each zone. 

 The crystal habits of the new minerals are but a reflection of these superimposed 

 conditions, and the size of crystals in metamorphic rocks is a variable factor without 

 important genetic connection. They have not shown that their order of crystallisation 

 means anything more than an order of application of successive sets of metamorphic 

 conditions. 



The crystalloblastic sequence, as far as it can be observed in this suite of rocks, is 

 magnetite, sphene, epidote, hornblende and lawsonite, biotite and chlorite, felspar and 

 quartz. 



* Op. oit., voL I., p. 73. 



t The expression of this interpretation of Grubenmann's crystalloblastio order is necessary, because the term has been 

 in other meaning by Lahee in his paper " Cryetalloblastic Order and Minerals," in the Journal of Geology, vol. 22 pp. 

 >15. Lahee (p. 514) has confused order of origin with the crystalloblaatic order. 

 J " Metamorphic Geology," C. K. Leith & W. J. Mead, New York, 1915, p. 187. 



