The source says the degree of a sākin emphatic consonant follows its preceding vowel. Compare يَقْطَعُونَ (sākin qāf), يَرْزُقُونَ (qāf with ḍammah), and مِرْصَادًا (ṣād with fatḥah). Do not treat all three as sākin examples.
Sukūn concerns the absence of a vowel; tafkhīm/tarqīq concerns the consonant’s sound quality. Those are different matters.
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A sākin emphatic letter follows its context
Tafkhīm and Tarqīq — Alif, Divine-Name Lām and Rāʾ
The source says the degree of a sākin emphatic consonant follows its preceding vowel. Compare يَقْطَعُونَ (sākin qāf), يَرْزُقُونَ (qāf with ḍammah), and مِرْصَادًا (ṣād with fatḥah). Do not treat all three as sākin examples.
Sukūn concerns the absence of a vowel; tafkhīm/tarqīq concerns the consonant’s sound quality. Those are different matters.