In spite of the great diversity observed in their macroscopic properties, the nephrites and tremolite-rocks are mineralogically alike in that all are composed essentially of an amphibole of the tremolite-actinolite series. As seen beneath the microscope, however, the structural features of these rocks are by no means constant, and show considerable and complex variations between four pairs of extremes:—
(a) In any representative series of specimens there is a complete range from minute hair-like fibres between 0.01 mm. and 0.05 mm. in length, to relatively coarse prismatic crystals with well developed cross-fracture and sometimes averaging 1 mm. × 0.1 mm.
(b) In almost every specimen the fibres or prisms are aggregated into bundles or tufts in which the arrangement of the individual crystals may vary considerably. In some rocks the fibres comprising a bundle are only sub-parallel and may be oriented obliquely to the direction of elongation of the aggregate itself, while in other cases the individual fibres are parallel both to one another and to the direction of elongation of the bundle.
(c) The tufts of fibres may be mutually parallel throughout the rock (to which they then impart a linear schistosity and nematoblastic structure), or on the other hand they may be
quite unoriented, in which case schistosity is not developed. Many rocks consist partly of unoriented and partly of parallel tufts of fibres.
(d) In typical nephrites the bundles of fibres are twisted and thoroughly felted or interwoven with one another, producing a highly characteristic structure to which the term “nephritic” may be applied. In other specimens the felting is imperfectly developed or even completely absent.
In addition to the essential constituent, the following minerals have been observed as accessories in the nephrites and related rocks examined by the writer: chlorite, serpentine, sphene, diopside, talc, garnet, a carbonate mineral (probably dolomite or magnesite), quartz, chromite, picotite, and hornblende. Of these, chlorite, serpentine, and sphene are fairly frequently met with, while either of the first two minerals may occasionally be sufficiently abundant to rank as an essential constituent of the rock. It is probable that hornblende, chromite, and picotite, whenever present, represent remnants of the original constituents of the parent rocks from which the nephrites have been derived by metamorphism. The remaining minerals appear to constitute, together with tremolite, a paragenetic metamorphic assemblage.
The classification given below is founded essentially upon microstructure. On this basis five divisions are recognised, though these are not sharply defined, transition types being by no means rare.
(a) Non-schistose Nephrites (1800 to 1811*). The non-schistose nephrites are characterised by uniformly small size and unoriented arrangement of the component tufts of fine tremolite fibres, and by perfectly developed nephritic structure, twisting and felting of the tufts being very marked. The tufts usually vary from 0.02 mm. to 0.3 mm. in length, reaching 0.5 mm. in rather rare instances. Occasionally well-defined stout prisms or acicular crystals (up to 1.5 mm. × 0.03 mm.) of coarse tremolite may be present in small amount (e.g., in 1806); these contrast sharply with the exceedingly slender tufted fibres of the surrounding nephritic base, and indicate transition towards the seminephrites. In other specimens (e.g., 1807) gradation towards the schistose nephrites is marked by the presence of minor mutually parallel aggregates composed of rather coarser fibres than the average. Accessory constituents are confined to two sections: 1804, chromite and chrome-diopside; 1809, chlorite.
(b) Schistose Nephrites (Za 5, 1370,† 1812 to 1817, 1822). The distinctive feature of the schistose nephrites is the presence of plentiful parallel tufts of tremolite fibres set in a base composed of finely felted unoriented tufts which in some cases make up only a small percentage of the total composition. This subdivision of typical nephrites into non-schistose and schistose types corresponds with the observations of Finlayson (1909, pp. 367–368), who distinguished between “fissile” and “horny” nephrites and noted the main distinction between the microstructures typical of these two varieties.
[Footnote] * Numbers refer to sections and specimens in the collections of the Geology Department, University of Otago.
[Footnote] † For description of 1370, see Turner, 1933, p. 272.
Though the schistosity is due to parallelism displayed by many of the fibrous aggregates, this regularity of orientation does not necessarily extend to the component fibres within any particular tuft. The individual fibres of a tuft are usually considerably interwoven and only subparallel with one another, while their average orientation is frequently inclined to that of the aggregate itself, i.e., to the direction of schistosity. The length of the oriented aggregates is usually about 0.3 mm. to 0.5 mm., while the felted tufts which make up the base are generally noticeably smaller than this. Rarely (e.g., 1822) a small amount of coarse prismatic tremolite (not more than 5% of the total composition) suggests a transition towards the seminephrites. Accessory minerals were observed in only two sections: 1815, chlorite and magnetite; 1816, colourless diopside and chlorite.
(c) Seminephrites. This group is represented by about twenty specimens (1818–1821, 1823–1837, 1442). While consisting partly of fine-grained interfelted fibrous tufts of tremolite with true nephritic structure, the seminephrites also contain abundant relatively coarsely crystalline tremolite which takes the form of well-defined acicular prisms or relatively large sheaves of parallel unfelted fibres. This coarse material may or may not show approximate parallelism throughout a section. The group of seminephrites as here defined includes also a number of transition rocks (1818–1821) in which the coarse component is developed to only a limited extent, and which therefore grade towards the true nephrites described above. Again, as the interstitial fine-grained nephritic base decreases in amount, these rocks pass in the opposite direction into the group of tremolite-rocks in which truly nephritic material is almost or completely absent. In almost every section examined, the coarse well-defined prismatic crystals are seen to fray out directly into the enclosing felted fibrous base, which itself has obviously originated by shearing and cataclasis of initially coarser crystals. This is especially well shown in 1829 and 1824. The following dimensions are typical: fibrous tufts of the nephritic groundmass, 0.05 mm. to 0.3 mm. in length; sheaf-like aggregates of long fibres and prismatic crystals, 4 mm. × 2 mm. (e.g., 1829); individual coarse acicular crystals, 1 mm. × 0.05 mm. to 3 mm. × 0.25 mm. (1823).
Accessory minerals are much more commonly present in the seminephrites than in the true nephrites, chlorite being present in six out of the twenty specimens examined; in 1825 it makes up between 10% and 15% of the total composition of the rock. Other minor constituents include serpentine (1828), chromite (1831, 1833), picotite (1826), chrome-garnet (1826, 1833, 1834), and diopside (1826).
(d) Tremolite-rocks with Linear Schistosity. Those rocks in which fine-grained truly nephritic fibrous tufts are absent or present in only minor amount are classed as tremolite-rocks. Many of these (1838–1847) are composed essentially of coarsely-fibrous or prismatic tremolite, the crystals of which show marked parallelism imparting nematoblastic texture and distinct linear schistosity to the specimen. There is gradual transition through types such as 1841 and 1845 in which parallelism of the component crystals is less marked than usual, to the second group of tremolite-rocks in which only plane schistosity
is developed. Accessory constituents are present in nine out of ten specimens; they include chlorite (1842–1845), opaque iron-ore (1838, 1843, 1845, 1847), serpentine (1840, 1843), sphene (1845, 1847), talc (1839, 1840), a rhombohedral carbonate (1838), and quartz (1838).
Typically the oriented crystals of tremolite are coarser than in the seminephrites (e.g., 5 mm. to 10 mm. long in 1840, and 2 mm. × 0.5 mm. in 1841), though this is not invariably the case.
(e) Tremolite-rocks with Plane Schistosity. The majority of the tremolite-rocks of this group are composed mainly of fibrous or prismatic crystals of tremolite, which in sections cut parallel to the schistosity lack regular orientation, but at the same time show no trace of the felted and twisted arrangement that gives rise to nephritic structure in other rocks. Coarse, distinct prisms of tremolite are usually plentiful, though a few specimens (e.g., 1852) appear to be composed entirely of relatively small fibres. The rough plane schistosity which is usually evident in hand-specimen is due to a marked tendency for the component crystals to lie with their directions of elongation in parallel planes. Accessory constituents are present in most specimens: 1379, diopside, serpentine, and sphene; 1425, hornblende and sphene; 1443, hornblende and serpentine; 1452, hornblende and sphene; 1849, pennine and chromegarnet; 1850, serpentine; 1851, chromite; 1852 and 1853, hornblende and sphene; 1364 and 1848, accessories absent.
Two of these specimens (1364 and 1379) were collected from boulders in the beds of streams draining from the northern end of the peridotite belt which outcrops along the north-western flank of the Olivine Range, South Westland,* while three others (1425, 1443, 1452) were obtained in situ from a small altered ultrabasic intrusion which outcrops on the northern bank of the Routeburn Stream about two and a-half miles above its junction with the Dart River, Lake Wakatipu district. The remaining six specimens are worked Maori material.
[Footnote] * For full petrographic descriptions see Turner, 1933, pp. 272, 273.