Contributions to the chemistry of tapiolite - manganotapiolite, a new mineral

Abstract

Microprobe determinations and X-ray diffraction studies of tapiolite specimens from the granitic pegmatites of the Eräjärvi area, in Orivesi, southern Finland, revealed a new manganesedominant analogue of tapiolite. A small crystal, 1.5 mm long and 0.5 mm thick, from a narrow lithium pegmatite dyke shows some parts richer in Mn than Fe. The name manganotapiolite is used for this Mn-dominant member and ferrotapiolite for the common Fedominant member of the tapiolite group. The tapiolite crystal studied is prismatic, showing poorly developed {100} faces. Manganotapiolite is dark brown, strongly pleochroic from yellowish brown to reddish brown, but thicker fragments are opaque. In reflected light the colour is grey with a weak bireflectance changing from light brownish grey to grey. Internal reflections are strong with red brown colour. The reflectance values measured in

FERROTAPIOLITE AS A PSEUDOMORPH OF STIBIOTANTALITE FROM THE LASTOVICKY LEPIDOLITE PEGMATITE, CZECH REPUBLIC; AN EXAMPLE OF HYDROTHERMAL ALTERATION AT CONSTANT Ta/(Ta + Nb)

“…It is noteworthy that ferrotapiolite with the second highest Sb content (1.20 wt.% Sb 2 O 3 , found in a single analyzed spot), from the Tiainen pegmatite, in Finland, contains numerous microscopic inclusions of native antimony (Lahti et al 1983). Consequently, the presence of microscopic inclusions of Sb-rich minerals in ferrotapiolite (native antimony at the Tiainen pegmatite, relics of stibiotantalite at Laštovičky), the nonexistence of SbTaO 4 having the rutile structure and the strong stereoactive character of Sb 3+ , deforming octahedral sites (see Ercit & Foord 1995), suggest that the high contents of Sb 2 O 3 found in ferrotapiolite (Lahti et al 1983, Novák & Č erný 1998a probably reflect the presence of microscopic inclusions of a Sbrich mineral (primary stibiotantalite at Laštovičky), hidden below the polished surface of the analyzed spots.…”

“…Consequently, the presence of microscopic inclusions of Sb-rich minerals in ferrotapiolite (native antimony at the Tiainen pegmatite, relics of stibiotantalite at Laštovičky), the nonexistence of SbTaO 4 having the rutile structure and the strong stereoactive character of Sb 3+ , deforming octahedral sites (see Ercit & Foord 1995), suggest that the high contents of Sb 2 O 3 found in ferrotapiolite (Lahti et al 1983, Novák & Č erný 1998a probably reflect the presence of microscopic inclusions of a Sbrich mineral (primary stibiotantalite at Laštovičky), hidden below the polished surface of the analyzed spots. It seems likely that Sb does not enter the ferrotapiolite structure, or if so, only in a minor quantity.…”

Studies on tapiolite from Arehalli pegmatite, Hassan District, Karnataka

“…A comparison of observed crystallographic parameters, including cell volume, of the investigated tapiolite (a o = 4.7582Å and c o = 9.2186Å, with a cell volume 208.711 Å) reveals a tendency of the studied Analysed by Chemistry Group, AMD, Hyderabad. Wise and Cerny, 1996); and (11) Tiainen pegmatite, Erajarvi, Finland (Lahti et al 1983). mineral towards the ordered phase.…”

Hidden story of tapiolite

“…It can be seen that the equation of Zema et al (2006) fails for compositions that are rich in Mn (Wise and Č erný, 1996;Lahti et al, 1983) and Fe 3+ and Ti (Kinast et al, 2002). This is shown spectacularly by the sample of Kinast et al (2002) in which the fully ordered heated sample is calculated to have a degree of order which is 23% less than its natural, partially ordered equivalent.…”

“…Where measured values of Q were available, agreement between the measured and calculated values is generally good; however, some cases deserve further comment. The structural states of type tapiolite-(Mn) (Lahti et al, 1983) and the manganoan tapiolite-(Fe) of Č ech (1973) were not measured; however, as highly Mn-enriched compositions, both samples formed late in the crystallization history of their highly evolved granitic pegmatites and should similarly be expected to be highly to maximally ordered (see above). Indeed the results suggest maximal cation order for both.…”