Tridymite, inversion

Tridymite is another form of crystalline sdica stable between 870 and 1,470°C at atmospheric pressure. It is found in volcanic rocks and has been identified in many stony meteorites. Tridymite also exists in various forms. It has six different modifications that undergo thermal inversions from one to another. Its density at 200°C is about 2.22 g/cm. The hexagonal unit cell contains four Si02 units. The Si—O bond distance is 1.52A. 

The first detailed data on the inversions of S1O2 were published in the classical studies by Fenner (1913, 1914) who used sodium tungstate as the mineralizer. As proved later, the use of the mineralizer affected the conclusions which had assumed tridymite to be a stable phase with a precisely defined stability region. The stability ranges of the Si02 phases, according to Fenner, are represented in the schematic equilibrium diagram in Fig. 4. 

High-temperature electrolysis of tridymite brings about migration of impurities towards the cathode, while tridymite in the vicinity of the anode is converted to quartz below 1050 °C and to cristobalite above 1050 °C this temperature is approximately equal to that of quartz-cristobalite inversion, according to data of other authors. 

The concepts ofinstabilityoftridymitein the pureSi02 system have not generally been accepted. Hill and Roy (1958) prepared, under hydrothermal conditions, pure stable and metastable tridymite (S and M) exhibiting the following sequence of inversions ... 

The composition of talc is situated close to the eutectic, so that the amount of liquid phase suddenly formed would deform the product. However, in a talc-day mixture, one may choose a proportion of components providing an amount of meJt just suitable for satisfactory sintering (about 30%) and for inhibition of the proto-enstatite-clinoenstatite inversion. This amount corresponds to approximately 10% clay. The composition of such a mix which roughly corresponds to commercial steatite compositions is indicated in Fig. 189 by point A. The diagram then allows equilibrium amounts of the melt to be evaluated for various temperatures, as shown in the diagram in Fig. 190. The shape of curve A indicates that about 30 % of melt is formed rapidly at 1345 °C. This is the temperature of the tridymite-enstatite-cordierite eutectic. The amount of melt increases with temperature, so that the sintering interval is comparatively narrow. 

The following sequence of inversions takes place during gradual heating up of low-temperature quartz at 573 °C, jS-quartz is very rapidly inverted to a-quartz, which is stable up to about 1025 °C when it has high purity (content of impurities < < 10 %). Itis then converted to cristobalite. If the quartz contains more impurities (solid solutions), then at about 870 °C a-quartz is converted to a-tridymite which in turn is transformed to a-cristobalite above 1470 °C, Non-equilibrium fusion of quartz at temperatures above 1400 —1450 °C was often observed cristobalite is then formed secondarily from this melt. 

During cooling, the sequence of inversions is different tridymite and cristobalite are not reconverted to quartz under normal conditions however, they are subject to very rapid displacive transformations to y-tridymite and jS-cristobalite respectively. The high-temperature a-forms cannot be undercooled. 

The thermal expansion curves of the individual Si02 modifications are plotted in Fig. 2, which also illustrates the discontinuous change in the specimen size occurring at the inversion temperature. The high-temperature quartz exhibits a quite rare anomaly, namely a negative coefficient of expansion in all crystallographic directions. On its expansion curve, tridymite likewise exhibits a peak followed by contraction. 

The expansion of tridymite was studied in detail by Austin fl954) his results plotted in Fig. 3 indicate a decrease in expansion coefficient to a value close to zero at 1000 C. The extremes on the curve correspond to inversions of tridymite at 117 C and 163 0 respectively the further three deflections on the curve are ascribed to the other three tridymite modifications considered below. 

Silica crystallizes from sodium silicates in three forms, cristobalite, tridymite, and quartz. The inversion temperatures are 1470 and 870°C. Cristobalite, the high-temperature modification, melts at 1713°C. The cristobalite liquidus decreases from the melting point of SiOz to the inversion point (located at 88.7 wt % Si02) between cristobalite and tridymite. The tridymite liquidus then descends from this point and meets the liquidus curve of quartz at 75.5 wt % Si02 (870 10°C). The tridymite liquidus extends metastable below 870 to 793°C ending at the disilicate-... 

Tridymite also undergoes a series of inversions, the number and type of which vary from one specimen to another. Because the number of modifications is greater than with quartz or cristobalite, Roman numerals are now used to distinguish them and have replaced the older a - / notation. Thus, a typical specimen of tridymite undergoes the following inversions ... 

Some specimens of tridymite have been reported to exhibit as many as five inversions. In contrast to conversions, the inversions referred to above occur rapidly and are reversible. 

The variation in the coefficient of thermal expansion of quartz and cristobalite with temperature reveals very strikingly the a-P inversions, and is best seen by plotting the percentage volume expansion for each mineral against the temperature (Figure 9). Owing to the difficulty of obtaining pure tridymite its expansion... 

Conversion. A change in crystalline structure on heating that is not immediately reversible on cooling. The most important example in ceramics is the conversion of quartz at high temperature into cristobalite and tridymite (cf. inversion). 

The room-temperature form, a-quartz (sp. gr. 2.65) undergoes a reversible crystalline change to p-quartz at 573°C this inversion is accompanied by a linear expansion of 0.45 %. At 870 C quartz ceases to be stable but, in the absence of fluxes, does not alter until a much higher temperature is reached, when it is converted into cristobalite and/or tridymite, depending on the temperature and nature of the fluxes present. 

More recent research has indicated a further inversion at about 250 C and has suggested that tridymite can be produced only in the presence of foreign ions, which enter the crystal lattice and cause disorder in the structure. 

When standard tridymite is cooled below 380 C, several phase inversions occur with various changes in symmetry. These tend to produce a large shrinki e and therefore a high thermal coefficient of expansion between 0 -200 C, almost 400 X lO K-h... 

While tridymite and cristobalite may exist for indefinite periods of time at room temperature, tire low-temperature alpha-quartz is believed to be the form of silica truly stable at these temperatures. A considerable expansion accompanies the conversion of a-cristobalite to /J-cristobalite at 220-275°C, and a sudden expansion of -2.2% occurs during the inversion of a-quartz at 573°C. The conversion of quartz to tridymite at 870°C is accompanied by an expansion of -15%. 

The integral of the peak in the length change curve (at approximately 575°C) was shown to decrease systematically with firing temperature. This peak suggests the a-quartz to j8-quartz inversion. This inversion, unlike the quartz-tridymite conversion, is rapid and instantaneously reversible. A decrease in the anomaly with increased firing temperature is likely a result of a more complete conversion of quartz to tridymite above STO C. 

The low-temperature inversions associated with tridymite and cristobalite are as follows -tridymite jS -tridymite —- - j82-tridymite... 

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