Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Yoderite - Mg₂(Al;Fe³⁺)₆Si₄O₁₈(OH)₂

The mineral yoderite was discovered in 1959 by McKie at Mautia Hill, and named in in honor of Hatten Schuyler Yoder. There is a structural kinship between yoderite and kyanite, which explains the common overgrowth of yoderite on kyanite in the natural rock. Like kyanite, yoderite has chains of edge-sharing octahedra A(1) = (MgO, Al)O₅(OH) that extend parallel to its b-axis. In contrast to relations for kyanite, however, these chains are linked by two types of bypyramidal polyhedra, A(2) = (Mg,Al)O₄(OH) and A(3) = (Al,Fe³⁺)O₅, and by isolated tetrahedra occupied exclusively by Si.

Optical properties
• Form: Anhedral Grains.
• Colour: Deep purple to dark violet.
• Relief: High.
• Pleochroism: From purple-violet-blue to brown.
• Interference colors: From I Order deep prussian blue to I order red.

Mautia Hill Geology

Mautia Hill is located approximately 10 km NNE of Kongwa, Fig 1 (central Tanzania), near the western border of the Pan-African Mozambique Belt, the formation of which is attributed to the collision of East and West Gondwana (McWilliams, 1981; Stern, 1994). To the west lies the Archean Tanzania Craton, composed of typical granite-greenstone belts formed between 2.8 and 2.6 Ga; The region to the east of the Tanzania Craton, termed the Western Mozambique Belt (Cutten 2004), is dominated by granitoid gneisses and includes felsic high-temperature granulites.

mautiahills(3).jpg

Fig.1: Simplified tectonic map (after Cutten 2004) of eastern Tanzania, showing the tectonic domains of the Mozambique Belt.

Fig.2: a) Geological map of part of Mautia Hill. b) detailed map of Mautia Hill. Darker shaded regions represent outcrop. (S. P. Johnson in 2000).

Mautia Hill (Fig.2) is a low, elongate hill that measures 3 km by 1 km and is approximately 50 m high; the hill trends WNW–ESE and has predominantly metasedimentary lithologies. Mautia Hill mainly comprises kyanite-bearing orthogneisses and variably deformed marbles, the contact between the two being marked by a 50 m-thick garnet-bearing amphibolite (Fig.2). The highly metasomatized lithologies, which include the yoderite whiteschists and sapphirine calcsilicates, occur along the spine of the hill, which forms the fold hinge of a regional F2 synform (Fig.2).

Mautia Hill is located in a complex regional setting in the Neoproterozoic East African Orogen, close to high-temperature Archean granulites of the Tanzania Craton, and it is only a few tens of kilometers north of Paleoproterozoic high-pressure eclogite facies rocks of the Usagaran Belt. Since other metasomatic rocks, which formed under similar high PT- conditions, have been interpreted to represent the product of continental crust subduction. Fockenberg and Schreyer (1994) have all demonstrated that tectonometamorphism at Mautia Hill was associated with a clockwise, high-pressure, moderate-temperature event, culminating in postpeak-metamorphic isothermal decompression. The PT-t path indicates that the Mautia rocks were buried/subducted to at least 30 km in a compressional/crustal thickening tectonic regime.

Yoderite Schist

The quartz-yoderite-kyanite-talc schist of Mautia hills are composed of coarse interlocking, often curved, flakes of talc with interstitial patches of quartz, exhibiting strain shadows; large irregularly margined grains of yoderite enclose random flakes of talc and some quartz. A constant feature is the presence within each yoderite grain of relicts of kyanite.

Fig.3: Yoderite schist sample from Mautia Hill. In the red box is visible a yello kyanite crystal within a blue yoderite crystals.

The chemical analysis of the quartz-yoderite-kyanite-talc schist of Mautia Hills, show that these rocks are characterized by a virtually complete lack of Ca and alkalis and a high Mg/Fe with MgO = 12.33 Wt%. Such a composition differs markedly from that of any igneous rock or common sediment; three hypotheses have been postulated:

• the rock may result from the metamorphism of a tectonic mixture of quartz with an igneous ultrabasic rock;
• magnesian metasomatims
• isochemical metamorphism of a saponitic bentonite

The first is improbable on the grounds that no source of ultrabasic material is discernible; The second would require magnesian metasomatism of an argillaceous sandstone. The occurrence of an argillaceous sandstone interstratified with a manganiferous sandstone and a limestone is stratigraphically consistent. The third possibility perhaps provides the fewest difficulties; the present composition of the rock is approximately analogous to natural chert-bearing saponitic bentonites, which may be produced under a variety of conditions from basic pyroclastics or by weathering in situ. During metamorphism, montmorillonite decomposes at temperatures less than 500°C. to give rise at this composition to a quartz-talc-pyrophyllite assemblage stable up to 530°C, where cordierite formation begins, accompanied at 575°C by the decomposition of pyrophyllite to cristobalite and mullite. If these two latter reactions are inhibited, as would seem probable, by increased pressure, and pyrophyllite decomposes, without the prior formation of cordierite, to silica and kyanite, the assemblage that apparently existed before the formation of yoderite will have been developed. Whatever may have been the early history of the rock, the assemblage talc-kyanite-quartz was eventually generated, presumably in the presence of excess water vapour. When the temperature subsequently increased, talc and kyanite reacted to produce the hydrous mineral yoderite (McKie, 1958): Talc + Kyanite + H₂O = Yoderite + Quartz

yoderite2017(1).jpg

Panoramic view of yoderite schist thin section. Image by Andy Tindle (Virtual Microscope). PPL image, field of view = 2.5cm.

Panoramic view of yoderite schist thin section. Image by Andy Tindle (Virtual Microscope). XPL image, field of view = 2.5cm.

• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Johnson, S.P & Oliver, G.J.H (1998): A second natural occurence of Yoderite. Journal of Metamorphic Geology, 16 (809-818).
• McKie, D. (1959): Yoderite, a new hydrous magnesium iron aluminosilicate from Mautia Hill. Mineralogical magazine, 32 (282-307)
• Schreyer, W & Yoder, H.S (1968): Yoderite, Synthesis, Stability and interpretation of its natural occurence. Carneige Instituition of Washington Yearbook, 66
• Cutten H., Johnson S. P., De Waele B. (2006) Protolith Ages and Timing of Metasomatism Related to the Formation of Whiteschists at Mautia Hill, Tanzania: Implications for the Assembly of Gondwana, Journal of Geology 114, 683–698.
• PhD thesis, Cutten H. (2004): Development of an ArcView/Access database and its use to decipher the tectonic history of the Mozambique Belt, eastern Africa. Perth, Univ. W. Australia.

Yoderite (blue) on kyanite. Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite. Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)	Yoderite (Blue) crystals. Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)
Yoderite (blue) on kyanite (low interference colors), quartz and Talc (high interference colors). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)	Yoderite (Blue) crystals. Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite (low interference colors), quartz and Talc (high interference colors). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)
Yoderite (Blue) crystals on kyanite. Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite (low interference colors), quartz and Talc (high interference colors). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite (low interference colors), quartz and Talc (high interference colors). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)
Yoderite (blue) on kyanite (low interference colors). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)	Yoderite (Blue) crystals. Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite (low interference colors), quartz and Talc (high interference colors). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)
Yoderite (blue) on kyanite (low interference colors). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite . Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite (low interference colors). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)
Yoderite (blue) on kyanite . Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite (low interference colors). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite . Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)
Yoderite (blue) on kyanite . Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite . Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite (low interference colors). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)
Yoderite (blue) on kyanite . Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite (low interference colors). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite . Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)
Yoderite (blue) on kyanite (low interference colors). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite . Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite . Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)
Yoderite (blue) on kyanite . Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite . Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite (low interference colors). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)
Blue Yoderite, quartz and Talc crystals (high interference colors). Mautia Hill (Tanzania). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)	Quartz and Talc crystals (high interference colors). Mautia Hill (Tanzania). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)	Blue Yoderite on Kyanite, quartz and Talc crystals (high interference colors). Mautia Hill (Tanzania). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)
Blue Yoderite, quartz and Talc crystals (high interference colors). Mautia Hill (Tanzania). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)	Yoderite (Blue), Quartz and Talc crystals (high interference colors). Mautia Hill (Tanzania). Mautia Hill (Tanzania). XPL image, 2x (Field of view = 7mm)	Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)
Yoderite (blue) on kyanite (with Pink Piemontite inclusions). Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)	Yoderite (Blue). Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)	Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)
Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)	Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)	Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)
Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)	Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)	Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)
Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)	Yoderite (Blue) and Talc. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)	Yoderite (Blue) and Talc. Mautia Hill (Tanzania). Mautia Hill (Tanzania). XPL image, 10x (Field of view = 2mm)
Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)	Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)	Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). XPL image, 10x (Field of view = 2mm)