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EXPLANATION OF PLATES.
PLATE No. 17.
Inter-trappean Fish Remains.
FIG. 1.-A Pristolepid scale (K29/629). 20.
FIG. 3.-A fragment of a Clupeoid scale (K32/159). ×15.
FIG. 4.-A Labyrinthid scale (K32/147) allied to Polyacanthus C. V.
Inter-trappean Fish Remains.
FIG. 1.-Fish roe (K1/319) from the Inter-trappean of Takli. x4.
FIG. 2.-Ganoid scale (K1/316), probably of Lepidosteus, from the Inter-trappean of Paharsingha. x4.
FIG. 3.-Another Ganoid scale from Paharsingha. X5.
FIG. 10.-Scale of Clupea sp. (K1/317) from the infra-trappean of Dongargaon. x7.
Tirodite, a manganese amphibole from Tirodi, Central
Whilst on a visit to the manganese mines at Tirodi, in the Central Provinces, coarse-bladed prisms of a honey-yellow amphibole were noticed, associated with rather coarse braunite-rock and braunitespessartite-rhodonite-quartz-rock. The manganese-ore at this point is closely penetrated by granite and pegmatite veins and it was obvious that there had been considerable recrystallisation of the manganese-ore here.
The mineral weathers to a greyish-white colour, almost asbestos like in appearance, and is in strong contrast to the general black colour of the associated ore. However, a fair amount of the unweathered pale yellow translucent mineral was collected.
Macroscopic description.-Colour: pale yellow on the prism direction, deep yellow on the basal parting. Lustre vitreous. Sp. gr.: 3-312. Hardness: 6.
Monoclinic ; no prism faces were available for measurement. The prismatic cleavage is perfectly developed, the cleavage angle, measured on the goniometer, being 124° 20'. A parting occurs, making an angle with c of about 74°, and parallel to b.
Optical properties.-The optical characteristics of the mineral are graphically represented in fig. 1. The refractive indices were determined by the immersion method; 2V on the universal stage.
Plane of the optic axes in the plane of symmetry. Z c=21°, Z in the obtuse angle between a and c.
Dispersion > v
Pleochroism X-pale yellow to colourless.
Z=pale yellow to straw yellow.
The mineral is often zoned.
Analysis. For analysis a small, apparently pure, piece was selected and crushed down to <0-5 mm. size, then hand-picked under a lens. It was obvious that there were occasional thin films of braunite and quartz along cleavage faces and such fragments as showed contamination were rejected. Fortunately, on crushing, the thin plates of quartz appeared to have separated quite readily from the amphibole. The result of the analysis is shown in Table 1, with also analyses of the nearest known amphiboles, richterite, cummingtonite and dannemorite as quoted by Dana, System of Mineralogy'.
The only impurity of any importance in the Tirodi mineral is a little braunite, but, in consequence of the very careful hand-picking, this could not possibly amount to more than 3 per cent., and is almost certainly much less. Allowing for this, the MnO content is still over 5 per cent. The other constituents are definitely in the
tirodite molecule and not mechanically admixed impurities. The mineral is a remarkably pure magnesian-manganese amphibole with
an unusually high percentage of magnesia. It differs from richterite, cummingtonite and dannemorite in the notably higher percentage of MgO, from cummingtonite and dannemorite in the very low FeO, and from richterite in the low CaO and alkalies.
Fermor [Memoirs, Geol. Surv. India, 39, pt. 1, p. 145 (1909)], in noting the occurrence of manganese amphiboles in India, divided them into two divisions according to their colour: bluish amphiboles were classed under winchite; yellow amphiboles, in the absence of more precise determinations, were classed under dannemorite. He regarded them all as of metamorphic origin. Tirodite, if anything, is closer to richterite than to the other amphiboles, but apart from the differences in composition tirodite has slightly higher indices of refraction, 2V, and extinction than the data quoted by Larsen [Bull.