alkaholic [æl'kəhol'ik] n. A person (usually, one with a post-secondary degree in geology) addicted to alkaline igneous rocks and their constituent minerals.
Hogsford Encyclopedic Dictionary of Science
So, what exactly are alkaline rocks? In a broad sense, these are the rocks that form from magmas and fluids so enriched in alkalis that they precipitate sodium- and potassium-bearing minerals not usually found in "normal" rocks (like feldspathoids, aegirine and sodic amphiboles). Magmas rapidly chilled to glassy rocks during volcanic eruptions may not actually contain any of these minerals, but will certainly turn up alkali-rich components in their normative composition, i.e. chemical analysis recalculated to a standard set of components which approximate real minerals: normative nepheline = NaAlSiO4, normative aegirine (a.k.a. acmite) = NaFeSi2O6, etc. In addition to alkalis, these rocks contain elevated levels of those trace elements whose ionic radius is either too large or too small in comparison with more common elements in the same oxidation state. This difference in radius precludes extensive substitution of over- and undersized trace elements in the structure of common rock-forming minerals (olivine, plagioclase, pyroxenes, micas, etc.), making them incompatible with respect to these minerals. Actually, the term "incompatible" is somewhat misleading here because some alkaline magmas and fluids contain such high levels of these elements that they form their own mineral phases and, thus, become perfectly compatible in these specific magma compositions. Alkaline rocks may comprise several volumetric per cent or more of minerals containing essential Zr, rare-earth elements (REE), Nb, Sr, Ba, or Li: eudialyte, loparite, lamprophyllite, noonkanbahite, baratovite, etc. These minerals are either exceedingly rare or not present at all in other rock types. Different groups of alkaline rocks are enriched in specific incompatible trace elements and may also show either prevalence of Na over K or vice versa. For example, lamproites are rich in K, Sr and Ba, but generally poor in Nb. Needless to say, these chemical differences are reflected in the mineralogical makeup of alkaline rocks.
Alkaline igneous rocks crystallize from magmas derived by small-degree partial melting of ultramafic rocks (mostly, peridotites) in the Earth's upper mantle. Some types of kimberlite contain the so-called "ultradeep" diamonds (i.e. diamonds hosting inclusions of minerals stable at pressures above 13 GPa) and may potentially descend from melts generated in the transition zone or even in the lower mantle. Small melt-to-rock ratios drive enrichment of alkaline magmas in alkalis and trace elements incompatible with respect to the mantle rocks. Certain alkaline magmas have chemistries too unusual to be accounted for by melting of unmodified peridotite (e.g., contain high K, Ba or CO2 contents). Their sources are believed to have been modified (metasomatized) by melts or fluids rising from yet deeper levels in the mantle. The driving forces behind mantle metasomatism are not very well understood. Possible sources of metasomatic melts/fluids include mantle plumes, subducted oceanic crust and delaminated subcontinental lithosphere.
During their ascent from the mantle, alkaline magmas evolve, i.e. their chemical composition, physical properties (density, dynamic viscosity, etc.) and solid-to-liquid ratio change in response to changes in pressure, temperature, composition of the surrounding rocks, etc. Alkaline magmas may undergo crystal fractionation, thermogravitational diffusion, liquid immiscibility and other processes that will produce derivative magmas quite distinct from the primary (initial) magma. For example, the astonishing diversity of alkaline igneous rocks exposed at Lovozero (Kola, Russia) can almost all be traced to the same type of parental magma. This is not to say, of course, that this 650-km2 layer cake of nepheline syenites and foidolites crystallized from a single blob of phonolitic melt. Quite to the contrary, there may have been as many as six or more discrete intrusive phases, each with its own trace-element budget and isotope signature. The earliest products of magma differentiation may not be what comes to mind when you think of alkali-rich rocks. Thermogravitational diffusion and fractionation of Mg-Fe silicates give rise to ultramafic rocks just like the ones produced from other magma types (e.g., tholeiitic basaltic). A careful study of mineral compositions, "melt" and fluid inclusions and whole-rock trace-element geochemistry will be needed to recognize the alkaline affinity of these rocks.
Some evolutinary processes (e.g., fractionation of olivine) enhance magma's enrichment in alkalis and incompatible elements, while others (e.g., fluidization and vapor discharge) cause its depletion in certain major and trace elements. Speaking of fluids: alkaline magmas commonly contain significant amounts of dissolved water, CO2 and fluorine. The relative proportion of these so-called volatile components cannot exceed a certain critical level beyond which they will unmix from the magma and form a discrete vapor or fluid phase. This critical saturation level may be reached if the pressure drops, or the proportion of volatiles in the magma rises due to its interaction with wall-rock, or precipitation of volatile-free minerals (olivine, feldspars, etc.). Fluids released by alkaline magmas emplaced in the crust react with their surroundings and produce various metasomatic rocks, including fenites, albitites, aegirinites, charoitites (see below), etc. This diversity arises not only from variations in the chemistry of fluids, but also from different protolith compositions. Any rock, including alkaline igneous parageneses themselves, can serve as a protolith (i.e. precursor rock subjected to metasomatism). Fluids percolating along fractures and structural unconformities in the surrounding rocks gradually cool and give rise to hydrothermal veins. Alkaline metasomatic and hydrothermal rocks may contain most of the minerals found in alkaline igneous rocks (nepheline, feldspars, aegirine, biotite, etc.), but, in addition, carry H2O-rich and carbonate minerals peculiar to them (e.g., pectolite, zeolites and water-soluble Na carbonates).
Given such an eventful evolutionary history and that, during their ascent, alkaline magmas become exposed to a wide range of temperatures, pressures and diverse chemical environments, it is hardly a wonder that alkaline rocks are among the most complex and challenging geological materials known. Kimberlites, for example, may contain diamonds and diamond-hosted minerals from the Earth's transition zone, ultramafic and eclogite xenoliths from subcontinental lithosphere, crystals precipitated from the kimberlitic magma at mantle depths (macrocrysts), fragments of lower- and upper-crustal rocks, low-pressure phenocrysts, groundmass minerals, and products of their subsolidus alteration. Discrete intrusions of alkaline rocks and carbonatites (some as small as a few tens of square meters in exposed area) may comprise dozens of different minerals. Amphibole-diopside silicocarbonatite at Afrikanda (Kola, Russia) consists of more than 50 minerals that represent several distinct stages in the evolution of carbonatitic magma and its derivative fluids. Some hydrothermally altered agpaitic pegmatite veins in the Khibiny Mts, also in Kola, contain as many as 78 mineral species – from ubiquitous nepheline and aegirine to such oddities as carbocernaite (Na,Ca)(Sr,REE,Ba)[CO3]2, epididymite Na[BeSi3O7](OH) and franconite Na2Nb4O11·9H2O (Pekov & Podlesnyi, 2004; Yakovenchuk et al., 2005). In the New World, the small alkaline intrusion of Mont Saint-Hilaire east of Montréal (Québec, Canada) has an unbeatable reputation for its mineral riches, but the Ilímaussaq complex in southern Greenland is not too far behind. Quite a bit has been written on both these localities (see here), but some aspects of their petrology and geochemistry will probably keep us looking for clues for many decades to come. Large intrusions of alkaline rocks similar to those exposed at Khibiny or Ilímaussaq are also known in South America, Africa and Asia (e.g., Poços de Caldas in Brazil, Pilansberg in South Africa and Saima in China), but they are yet to be examined in adequate detail.
Can you recognize an alkaline rock when you see one? Unfortunately, it is not always easy, especially when it comes to volcanics. Phonolites, for example, can be confused with a variety of feldspar-bearing extrusive rocks, and alkali basalt often looks just like any other basalt. You will need to examine your rock under the polarizing microscope and determine its chemical composition to put a name to it. Identification of some rock types requires even more advanced methods of analysis. For instance, carbonatites, lamproites and ultramafic lamprophyres sometimes disguise as kimberlites, and it takes an experienced petrologist with an in-depth understanding of the mineralogy and geochemistry of these complex rocks to tell them apart. Secondary processes (like weathering) may obscure the primary texture and mineralogy of a rock and alter its chemical budget, rendering it virtually unrecognizable. If the rock is phaneritic (i.e. consists of crystals sufficiently large to be identified) and fresh, it may contain a mineral or combination of minerals that make its identification straightforward. For example, association of nepheline with aegirine(-augite) is characteristic of nephelinolites. On the other hand, the absence of certain minerals may also be important for identification; e.g., neither nepheline nor aegirine occur in kimberlites and lamproites. Many phaneritic alkaline rocks contain variable volumetric proportions of feldspathoids and feldspar-group minerals, which can be estimated and then used to read the rock's name off the APF diagram. Those volcanic and shallow intrusive rocks, whose mineral composition cannot be reliably determined, need to be analyzed for major elements, and their chemical analyses recast into normative compositions (see above). The normative contents of nepheline, olivine and other minerals are used in conjunction with the TAS diagram to find the name that suits the rock best.
Given the relative rarity of alkaline and kindred rocks, one might wonder if it is really necessary to study them in any great detail. Well, there is actually a plenty of reasons why we should. First of all, these rocks provide a wealth of information on how magmas of different chemical composition evolve and crystallize (see above). Also, alkaline rocks are the principal source of diversified mantle material (xenoliths and xenocrysts), which is widely used for the modeling of mantle petrology and structure. Another source are the so-called massif-type peridotites (i.e. tectonic slices of the upper mantle), but they are not as mineralogically diverse as xenoliths and sample a much smaller portion of the mantle. Today, petrologists can detect heterogeneities in the lithosphere by studying the chemistry and mineralogy of mantle debris entrained in kimberlites and similar rocks. Some alkaline rocks (especially, feldspathoid syenites and their associated pegmatites) contain unique mineral assemblages and even "endemic" (i.e. one-of-a-kind) mineral species. That is probably why the very first State Mineralogical Reserve in history was founded at Miass in the Il'meny Mountains (Russia), one of the classical localities of alkaline rocks. Unfortunately, the number of such protected sites is very limited and one day we might well be facing the need for a "Red Book of Minerals". Last, but not least, alkaline and related rocks are an important (and, in some cases, the only) source of a wide spectrum of mineral resources. For example, the bulk of niobium production comes from pyrochlore mineralization in carbonatites. Phoscorites are mined for zirconium, copper and iron ores, phosphate and vermiculite. The ijolite-urtite suite at Khibiny hosts the largest igneous deposit of apatite (major source of fertilizer-grade P2O5) in the world. Kimberlites and lamproites are the principal source of gem-quality diamonds; other gemstones extracted from alkaline rocks are pyrope, peridot and chromian diopside (a.k.a. "Siberian emerald"). I could go on with examples, but my main point is that our earthly existence might not have been quite as comfortable and enhanced, were it not for these extraordinary rocks.
For my wife Katya and me, our obsession... oops, I mean fascination with alkaline rocks began with a field trip to the Murun massif in eastern Siberia back in 1990. We both had just completed our third year at St. Petersburg State University and were eager to explore some off-the-beaten-path geology, preferably as far away from home as possible. Murun is such an enchanting place that it would have been impossible not to fall in love with it. Our interest in alkaline rocks was also inspired by the enthusiasm of our teacher and, subsequently M.Sc. thesis advisor, Dr. Mikhail Evdokimov. “Uncle Misha”, as he was colloquially known among students, knew all there was to know about the Murun rocks and had a ready explanation even for the most perplexing things we saw in the field. In addition, Murun is the birthplace of charoite, a K-Na-Ca silicate sporting a fibrous, swirly, radiating, massive, schistose and many other kinds of texture. Its color varies from saturated purple to lavender-gray and bronzy-brown. Unlike diamonds, rubies and other “conventional” gemstones, whose popularity is driven largely by their lore and aggressive marketing, charoite is a truly unique gemstone found in only a single place in the entire world. It is a relatively “young” mineral, as it was formally described only in 1978 (Rogova et al., Zapiski Vsesoyuznogo Mineralogicheskogo Obshchestva, 107, 94-100); prior to the 1970s, it had been mistaken for a purple variety of amphibole. Rocks containing charoite (charoitites) probably formed by fluid-induced metamorphism (metasomatism) at the contact of Precambrian carbonate strata with much younger intrusions of K-rich alkaline rocks. Metamorphism of this type (called fenitization) is certainly not unique to Murun, but charoitites are. In addition to charoite, they contain minerals typical of fenites (microcline, aegirine, apatite, carbonates), and a plethora of exotic minerals, which are known either only from Murun (like gray frankamenite) or from just a handful of localities worldwide (e.g., honey-yellow tinaksite). How and why such a unique paragenesis of minerals formed here, in this magnificent corner of Siberian taiga, remains enigmatic, despite all the research papers and student theses written on that topic. To add to the mystery, the crystal structure of charoite has not been deciphered to date. But, maybe, it was meant that way?.. After all, the name “charoite” derives from the Russian verb charovat’ meaning to charm or to bewitch. To my knowledge, the best collection of charoitites is exhibited in the Department of Mineralogy at St. Petersburg University, and the best reference on the subject is Mikhail Evdokimov's article in the World of Stones magazine (1995, no. 7, pp. 3-11).
Need more info on specific rock types or localities? Check out this list of references.