BH250-125
Title
BH250-125
Subject
Limestone (Black Limestone – Matheos Formation, Tigray, Northern Ethiopia)
Description
Major Minerals: iron oxides, calcite
BH250-125 is a black limestone sample collected from the Matheos Formation in Tigray, Northern Ethiopia. The Matheos Formation is part of the Tambien Group, composed of siliciclastic and carbonate rocks in northern Ethiopia. It is a fine-grained carbonate sedimentary rock that ranges from dark gray to black in color. Like all limestone, its primary component is micritic calcium carbonate (CaCO₃), mainly in the mineral form calcite (and less commonly aragonite). However, its distinctly dark color sets it apart from more typical light-colored limestones.
The main carbonate phase in this rock is micritic calcite, as evidenced by the vigorous effervescence when hydrochloric acid (HCl) is applied. Under thin section, individual calcite grains are difficult to distinguish due to their extremely fine size.
Minor/Accessory Components: Organic matter: Finely disseminated carbonaceous material
Pyrite (FeS₂): Commonly associated with anoxic depositional conditions
The black coloration of this limestone is primarily attributed to the presence of organic matter and pyrite, both indicative of deposition in low-oxygen (anoxic) marine environments where such materials are preserved.
This sample is consistently one of the most favored among students as its striking appearance confuses them with basalt.
BH250-125 is a black limestone sample collected from the Matheos Formation in Tigray, Northern Ethiopia. The Matheos Formation is part of the Tambien Group, composed of siliciclastic and carbonate rocks in northern Ethiopia. It is a fine-grained carbonate sedimentary rock that ranges from dark gray to black in color. Like all limestone, its primary component is micritic calcium carbonate (CaCO₃), mainly in the mineral form calcite (and less commonly aragonite). However, its distinctly dark color sets it apart from more typical light-colored limestones.
The main carbonate phase in this rock is micritic calcite, as evidenced by the vigorous effervescence when hydrochloric acid (HCl) is applied. Under thin section, individual calcite grains are difficult to distinguish due to their extremely fine size.
Minor/Accessory Components: Organic matter: Finely disseminated carbonaceous material
Pyrite (FeS₂): Commonly associated with anoxic depositional conditions
The black coloration of this limestone is primarily attributed to the presence of organic matter and pyrite, both indicative of deposition in low-oxygen (anoxic) marine environments where such materials are preserved.
This sample is consistently one of the most favored among students as its striking appearance confuses them with basalt.
Coverage
Location: Irin, Tigary, Ethiopia
GPS Coordinates: 13°21'9.43"N, 38°44'7.79"E
GPS Coordinates: 13°21'9.43"N, 38°44'7.79"E
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-125. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-125,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/146.