BH250-227a
Title
Subject
Description
Minor Minerals: oxides
BH250-227a — Ankaramite (Hawaii)
BH250-227a is a striking example of ankaramite, a rare variety of mafic volcanic rock distinguished by its unusually high abundance of large mafic phenocrysts—especially clinopyroxene (diopside/augite) and olivine—set within a much finer-grained volcanic groundmass. This specimen from Hawaii preserves the classic porphyritic texture that makes ankaramites so visually distinctive: large, well-formed crystals are suspended in a dark, fine-grained matrix composed of microlitic plagioclase, pyroxene, volcanic glass or cryptocrystalline material, and minor oxides. In hand sample and thin section, the rock immediately stands out because the phenocrysts appear almost overcrowded relative to the finer matrix, reflecting a magma that began crystallizing significant amounts of mafic minerals prior to eruption.
The rock is composed primarily of diopside, olivine, and plagioclase, with minor opaque oxides. The thin sections beautifully document this mineral assemblage. Large euhedral to subhedral diopside phenocrysts are abundant and display bright second-order interference colors under crossed polars. Many crystals show internal fracturing and minor resorption textures, suggesting that they experienced magmatic transport and decompression prior to final eruption. Some pyroxene grains appear clustered together in glomerocrystic aggregates, indicating that crystals may have accumulated within a magma chamber before eruption. The large, brightly colored grains visible in crossed polars are excellent examples of clinopyroxene birefringence and make this sample especially useful for teaching optical mineralogy.
Olivine phenocrysts are also prominent and occur as larger fractured crystals, some showing irregular embayed margins that may reflect magmatic corrosion or reaction with surrounding melt. In volcanic rocks, olivine commonly crystallizes early because of its high-temperature stability, and its presence indicates a relatively primitive magma derived from the mantle. Some olivine grains may show early alteration along fractures or grain margins, which is common when olivine interacts with later fluids or weathering processes.
The groundmass is dominated by abundant tiny plagioclase microlites, visible in plane-polarized light as densely packed feldspar laths that create a pilotaxitic to intergranular texture. These microlites record rapid cooling after eruption, contrasting sharply with the much slower growth history of the larger phenocrysts. Minor opaque oxides likely represent magnetite or titanomagnetite and record iron-rich crystallization during the later stages of magma evolution.
Ankaramite is a relatively uncommon volcanic rock type and is typically associated with alkaline basaltic magmatism in ocean island settings, continental rifts, and some arc environments. The name originates from the Ankaratra volcanic field of Madagascar, which is regarded as the historical type locality from which the rock name was derived. The term has traditionally been used for olivine- and clinopyroxene-rich porphyritic basalts with unusually high modal abundances of large phenocrysts. Modern petrologists sometimes classify these rocks more broadly within alkaline basalt suites, but the term remains widely used in teaching collections because it highlights this distinctive texture and mineral assemblage.
The Hawaiian occurrence is particularly interesting because Hawaii is dominated by mantle plume volcanism associated with the Hawaiian hotspot. Primitive melts rising from deep mantle sources can preserve high-temperature minerals such as olivine and clinopyroxene, and ankaramites represent some of the more crystal-rich expressions of these magmas. Their abundance of early-formed crystals may reflect magma chamber accumulation, rapid ascent of crystal-rich melts, or eruption of relatively primitive magmas before extensive fractional crystallization occurred.
This sample is especially valuable in teaching because it demonstrates multiple important igneous concepts at once: mantle-derived magma generation, phenocryst formation, crystal accumulation, volcanic groundmass textures, and the relationship between mineral stability and cooling history. The contrast between large phenocrysts and fine-grained matrix makes it an outstanding thin section for introducing students to porphyritic volcanic textures.