Hardness: 5.5 - 6.
D = 2.485 ± 0.015 gm./cm3.
Luster: vitreous. Streak: white.
Isotropic or very weakly anisotropic, n 1.508 - 1.511. δ 0.001
All leucite is twinned, appearing as cross-hatched lamellae parallel to {110}, just visible in thin section with the weak birefringence.
low temperature polymorph:
a 13.09, c 13.75 Å
Z = 16, Space Group I 41/a
high temperature polymorphs:
a 13.50, c 13.59 Å
Z = 16, Space Group I 41/acd
a 13.55 Å
Z = 16, Space Group I a3d
Leucite was described by Blumenbachs (1791), attributing the name to Werner, who had previously described the mineral as “white garnet”. The type locality is Vesuvius, Italy, and the name is from the Greek, leukos, meaning white.
The term pseudoleucite refers to cloudy spheroids, consisting of granular or fibrous, radiating intergrowths of potassium feldspar, granular nepheline, and commonly zeolites. Because of the common trapezohedron shape, they are generally considered to have crystallized as leucite, which then reacted with the magma or with subsolidus fluids, to form the later phases.
Using differential thermal analysis (DTA) heating curves, Faust (1963) showed that the transition is actually characterized by two endothermic peaks, suggesting that there is an intermediate form. Lange et al. (1986) used differential scanning calorimeter (DSC) scans to show that the transitions occur over a span of 122 to 176 degrees. They assigned the space group, I41/acd to the intermediate phase, because it could be related to the cubic (Ia3d) form by pseudomerohedral twinning and to the low temperature phase (I41/a) by merohedral twinning. The crystallography of these twins are described by Mazzi et al. (1976), and by Palmer et al. (1988) and Putnis (1992), who illustrate twins with transmission electron micrographs. Transition temperatures vary from sample to sample, but the transition of cubic leucite (point group, m3m) to tetragonal (point group, 4/mmm) is at about 665°C. The transition to the 4/m form occurs at about 630°C. Palmer et al. (1989) measured the cell parameters with temperature, to illustrate thermal contraction and phase changes with decreasing temperature.
Framework sites are occupied mainly by Si and Al, but some Fe occurs in most samples. It is most likely that this iron occurs as Fe3+ in tetrahedral sites, because iron leucite KFe3+Si2O6 is easily synthesized (Gupta and Yagi 1980, Palmer et al. 1997). The Si content, expressed as Si/(Si+Al+Fe), is near 0.667 for many samples, but some are higher. For example, Si in leucite from Wyoming is 0.69 (33.0 Si per unit cell), which is expressed as the K-feldspar component by some workers. The substitution of Na for K is limited, although some analyses indicate amounts up to 2.4 atoms per unit cell. It is not clear how much substitution can occur at liquidus temperatures, but replacement of K by Na can be extensive at subsolidus temperatures from altering fluids. Up to complete replacement of leucite by analcime occurs in many areas, for example, Roccamonfina, Italy (Luhr and Giannetti, 1989).
Trachybasalt-basanite-tephrite-leucitite series
Leucite phenocrysts occur with plagioclase and titanaugite in trachybasalt lavas on Tristan da Cunha (Baker et al. 1964). These lavas with SiO2 about 48 wt %, Na2O 4-5%, and K2O 3.4-3.9% are midway in the fractionation series alkali basalt, trachyandesite, and trachyte that have been erupted from the islands’ volcanoes.
Leucite basanite has been erupted from Holocene vents (e.g. Nyiragongo) in the Virunga Mountains, Democratic Republic of Congo-Uganda-Rwanda (Holmes and Harwood 1937, Bell and Powell 1969, Sahama 1973), from numerous vents in the Eifel District of western Germany (Duda and Schminke 1978), and from several volcanic centers in the Roman Province (Washington 1906, Savelli 1967, Luhr and Giannetti 1989), and Somma-Vesuvius, Italy (Rittmann 1933). Even with the compositional variation between eruptive areas, leucite basanite lavas have SiO2 between 46 and 48%, Na2O about 3%, and K2O about 4%. Phenocryst minerals in these lavas are commonly leucite, labradorite, olivine, and diopsidic augite, with leucite also crystallizing in the groundmass.
Leucite-bearing tephritic to phonolitic lavas have been erupted in the volcanic fields of the Roman Province, Italy. For example, the composition of lavas and tuffs from Roccamonfina has SiO2 in the range 50 to 56%, Na2O about 2.4%, and K2O nearly 9% (Appleton 1972, Luhr and Gianetti 1987). A typical mode is leucite phenocrysts 40%, labradorite 20%, sanidine 20% augite and minor olivine 15%, and various accessory minerals including nepheline 5%. In a different setting a series of leucite-bearing mafic lavas, tuffs and breccia of mostly leucite shonkinite and leucite tephrite, from Mt. Mouriah volcano, northeast Java, have been described by Rittmann (1951). Pyroclastic kamafugite near L’Aquila, Abruzzo, in the Umbrium-Latjum ultralkaline district of central Italy contains diopside, leucite, haüyne, Mg-mica, andraditic garnet, apatite, magnetite, kalsilite and olivine (Stoppa et al. 2002).
Leucitite is a volcanic rock, strongly undersaturated with respect to silica, in which leucite is the sole or dominant felsic mineral (feldspars do not exceed 10 modal per cent). Leucite occurs both as phenocrysts, accompanied by diopsidic augite and olivine, and in the groundmass. The phenocryst abundances of leucite in leucitite in the Roman Province have a considerable range. In flows of the Colli Albani volcanic field leucite and augite occur in roughly equal proportions, while some ejecta blocks consist of about 90% leucite. Also in the Alban Hill the 561 ka Trigoria-Tor de' Cenci tuff resulted from a pyroclastic flow from an unusually energetic eruption (Palladino et al. 2001). The magma (SiO2, 42-45 wt. %) had crystallized only leucite, clinopyroxene, and phogopite before the eruption. Lavas erupted in the Toro-Ankole and Bufumbira province are olivine leucitite with ultramafic compositions (SiO2 between 35-40% and K2O about 9 weight percent).
An unusual rock suite of ultrapotassic, but silica saturated, lavas are generally known by such names as wyomingite, as well as many others of local origin. These occur in small volumes in the Leucite Hills, Wyoming, USA (Carmichael 1967, Lange et al. 2000), and west Kimberly, New South Wales, Australia (Cundari 1973). SiO2 contents range between 50-55, and K2O up to 12%. Wyomingite is porphyritic with phenocrysts of phlogopite, microphenoscryts of diopsidic augite in a groundmass of leucite. Some varieties contain sanidine in the form of large phenocrysts. The lavas of West Kimberly contain less K2O (about 8-9%) than those of the Leucite Hills, and therefore, lack phlogopite phenocrysts; otherwise mineral assemblages are similar.
Preservation of leucite crystallized in sub-surface magma is very rare, and only occurs in unusual circumstances. In Proterozoic lamproite dikes in the Sisiuit area, central West Greenland (Thy et al. 1987) and at Napolean Bay, Baffin Island, Canada, leucite crystallized early, and was preserved as microphenocrysts in the chilled dike margins (Hogarth 1997). More commonly pseudomorphs after leucite with euhedral outlines consist of tabular kalsilite interleaved with sanidine. These pseudomorphs are similar to those in the Sisimiut lamproite dikes of Greenland, illustrated by Thy et al. (1987). Ejecta blocks from the 1944 eruption of Mt. Vesuvius are highly crystalline rocks consisting of leucite, clinopyroxene, plagioclase, olivine, apatite, oxides and glass (Fulignati et al. 2000). These rocks formed by sidewall accumulation of crystals from the potassic tephritic-phonolitic magma resident in the chamber, and were ejected by the phreatomagmatic eruptions.
Pseudomorphs of leucite
There are four kinds of pseudomorphs of leucite retaining an outline characteristic of sections through trapezohedral crystals consisting of (a) intergrowths of granular nepheline and potassium feldspar, with analcime in some cases, to which the term pseudoleucite refers, (b) tabular kalsilite interleaved with sanidine, (c) granular analcime, and (d) aluminous clays and hydroxides.
Pseudoleucite. Pseudoleucite was first described by Hussak (1890) from Serra de Tingua, Brazil, and has since been found in many other localities, including the Bearpaw (Zies and Chayes 1960) and Highwood Mountains (Larsen and Buie 1938) of Montana; Magnet Cove, Arkansas, USA (Knight 1906); Spotted Fawn Creek, Yukon Territory, Canada (Knight 1906), (Tempelman-Kluit 1969); Loch Borolan Laccolith, Scotland (Shand 1939); Laacher See district, Germany; Tzu Shin Shan, Shansi, China (Yagi 1954); Tezharsk, Armenia (Yagi and Gupta 1977); Vesuvius area, Italy (Rittmann 1933); Lugingol Massif, Mongolia (Kononova et al. 1982); and the Sakun Massif in the western Aldan area, Russia (Kononova et al. 1997). In descriptions of these and other occurrences the usage of the term has not been uniformly consistent. The clearest examples are those in volcanic and hypabyssal rocks, where the pseudomorph boundaries and shapes are well defined. Potassium feldspar and nepheline intergrowths in some plutonic rocks are less well defined, having rounded margins which merge into the other rock constituents. In some localities, e.g. Tzu Chin Shan (Yagi 1954), pseudoleucite “crystals” have an internal structure, a narrow outer zone of randomly oriented orthoclase and nepheline covers an inner zone in which crystals are oriented with their longer dimensions perpendicular to the pseudomorph boundary. Gittins et al. (1980) argue that some intergrowths of nepheline and potassium, to which the name pseudoleucite has also been applied, are interstitial to other minerals of the rock or have grown on nepheline grains during late stage magmatic crystallization and are not related to leucite crystallization.
Over the past century the several processes, proposed for the origin of pseudoleucite, have been the subject of on-going discussion, which has been reviewed by Guta and Yagi (1980) and Gittins et al. (1980). In all cases the assumption has been that the intergrowths with good crystal form initially crystallized as leucite and were subsequently replaced. Knight (1906) suggested that after the magma had solidified and cooled below the stability limit of leucite, it was transformed into the stable assemblage of nepheline and potassium feldspar. The principal argument against this hypothesis is that the original leucite would have had to be considerably more Na-rich than any phenocrysts known. Fudali (1963) showed that at 1 atm leucite can accommodate up to 40 wt. % of NaAlSi2O6 and up to 27 wt. % at P(H2O) = 1 Kbar at 800°C. These compositions are unstable at lower temperatures and breakdown to nepheline and potassium feldspar. Roux and MacKenzie (1978) found that extensive solid solution of NaAlSi2O6 in leucite is metastable or unstable. They suggest that potassium-rich analcime may be the precursor to pseudoleucite. In fact Larsen and Buie (1938) found such an analcime in the Highwood Mountains. However, similar analcime compositions from Roccamonfina have resulted from analcime replacement of leucite (Luhr and Kyser 1989).
Based on the phase diagram of the system nepheline-kalsilite-SiO2 (Schairer and Bowen 1935), Bowen and Ellestad (1937) suggested that pseudoleucite forms at the reaction point, leucite + L = K-feldspar + nepheline. The main argument against this mechanism is that such reactions show dissolution of the reacting phase resulting in embayments in the crystal outline. However, it is the only mechanism that accounts for pseudoleucite formation at liquidus temperatures, thus accounting for pseudoleucite is young lava flows.
Taylor and MacKenzie (1975) proposed that ion exchange between early-formed leucite and sodium-rich glass or sodium-rich vapour under subsolidus conditions may account for most occurrences of pseudoleucite. Given the ease of Na-replacement of leucite by Na-containing fluids (Barrer and Hinds 1953), the suggestion that pseudoleucite formation occurs by subsolidus reaction is receiving increasing support, see for example Ryka (1998). Edgar (1978), using sub-solidus phase relations along the join NaAlSi2O6-KAlSi2O6 at 1 kb P(H2O), suggests that pseudoleucite may form in the analcime+nepheline+feldspar field at temperatures below 500°C without the necessity of Na-bearing fluids.
Kalsilite-potassium feldspar intergrowths. In the lamproite dikes of southern Baffin Island, Canada (Hogarth 1997) and Sisimuit area, West Greenland (Thy et al. 1987) leucite microphencrysts not quenched in the chilled margins are replaced by sandine and kalsilite. These pseudomorphs consist of tabular kalsilite interleaved with sanidine, and commonly display several different orientations per “crystal”. Hogarth (1997) suggests that these packets may present different domains in the original tetragonal leucite. Similar intergrowths with a texture, termed sympletitic, occur in the layered rocks of the southern Sakun Massif, western Aldan area, on the Siberian platform, Russia (Kononova et al. 1997).
Analcime pseudomorphs. In Italy analcime replacement of leucite is widespread in the potassic lavas in the region around Lago di Vico (northwest of Rome). Cundari and Graziani (1964) have documented the process in tephritic lavas, some of which contain partially replaced leucite phenocrysts. Luhr and Giannetti (1987) describe analcime that has completely replaced centimeter-sized leucite phenocrysts in a 385 Ka leucite tephrite lava flow from the Quaternary volcano, Roccamonfina (northwest of Naples). These analcime pseudomorphs are milky white apparently from cracking of crystals, caused by the nearly 10% volume increase in the conversion of leucite to analcime, and are richer in K than crystals from other occurrences. Karlsson and Clayton (1991), among others, have proposed that most igneous-appearing analcime is replaced leucite.
Clay pseudomorphs. Other pseudomorphs of leucite are known such as those replaced by kaolinite and aluminum hydroxides including gibbsite (Cassedanne and Menezes 1989). Such pseudomorphs likely form by low temperature hydrothermal alteration. Pseudoleucite from Loucná, Hory Mtns., Czech Republic (Pivec and Ulrych 1982) consists of potassium feldspar and illite.
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