acmite

Japanese: acmite

...Its chemical composition is NaFe ³⁺ Si ₂ O ₆ , which is a calcium pyroxene Ca(Mg,Fe ²⁺ )Si ₂ O ₆ with Na replacing the Ca and Fe ³⁺ replacing the (Mg,Fe ²⁺ ). A specimen with the same composition and crystal structure but a different crystal habit is called acmite. It has dark green to black columnar or granular crystals, sometimes with pyramidal tips. ...

From [Pyroxene]

...The other major pure clinopyroxenes are CaMgSi ₂ O ₆ (diopside), CaFe ²⁺ Si ₂ O ₆ (hedenbergite, also called hedenbergite), NaFe ³⁺ Si ₂ O ₆ (aegirine or acmite, also called pyroxene), NaAlSi ₂ O ₆ (jadeite), and LiAlSi ₂ O ₆ (spodumene, also called pyroxene)....

*Some of the terminology explanations that mention "acmite" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…化学組成はNaFe^3＋Si₂O₆であり，カルシウム輝石Ca(Mg,Fe^2＋)Si₂O₆のCaをNa，(Mg,Fe^2＋)をFe^3＋が置換したもの。同一組成，同一結晶構造で晶癖が異なるものをアクマイトacmite(錐輝石)という。暗緑色～黒色の柱状または粒状の結晶で，先端はピラミッド状を呈することがある。…

【輝石】より

…その他の主要な純粋の単斜輝石はCaMgSi₂O₆(透輝石)，CaFe^2＋Si₂O₆(ヘデン輝石hedenbergite。灰鉄輝石ともいう)，NaFe^3＋Si₂O₆(エジリンまたはアクマイトacmite。錐輝石ともいう)，NaAlSi₂O₆(ヒスイ輝石)，およびLiAlSi₂O₆(スポジューメンspodumene。…