BH250-228
Title
BH250-228
Subject
Benmoriete
Description
Major Minerals: plagioclase, clinopyroxene
Benmoreite is an intermediate volcanic igneous rock of the alkaline magma series and represents a magma composition between mugearite and trachyte. It is typically composed of plagioclase feldspar, alkali feldspar, clinopyroxene, and sometimes amphibole or biotite, with minor iron-titanium oxides. Compared with andesite, benmoreite is richer in sodium and potassium and forms through fractional crystallization of basaltic magma, where early minerals separate from the melt and leave behind a more evolved composition.
Benmoreite is an intermediate volcanic igneous rock of the alkaline magma series and represents a magma composition between mugearite and trachyte. It is typically composed of plagioclase feldspar, alkali feldspar, clinopyroxene, and sometimes amphibole or biotite, with minor iron-titanium oxides. Compared with andesite, benmoreite is richer in sodium and potassium and forms through fractional crystallization of basaltic magma, where early minerals separate from the melt and leave behind a more evolved composition.
In hand sample and thin section, benmoreite may appear fine-grained, porphyritic, or locally flow banded, depending on cooling history. The best-known locations for benmoreitic rocks are associated with Mauna Kea, where late eruptive activity produced hawaiite, mugearite, benmoreite, and trachyte. Similar evolved alkaline lavas may also be found at Hualalai and the older Kohala Volcano. In contrast, Mauna Loa and Kilauea are dominated by basaltic lava and rarely produce benmoreite. The presence of benmoreite in Hawaii is geologically important because it shows that some Hawaiian magma chambers evolved significantly through time, producing a wider range of rock types than the basaltic lavas for which the islands are most famous.
Coverage
Location: Hawaii, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 228. Housed at Carleton College in Minnesota.
Type
Hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-228,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/304.